Physics Discussion Thread

gakakkad · Post by **gakakkad** » 24 May 2015 21:10

SriKumar wrote:Problem 1:
Mom. Iner_solid_Sphere = (2/5)*M*r^2; M.I._hollow_Sphere = (2/3)*m*r^2.

Applied Torque = Force x radius
Force seeking to rotate the sphere = mass*g*cos(theta) where theta is the angle of inclination.
Forcce equilibium equation in rotational system= Torque = I* alpha (where alpha = omega/t).

For solid sphere: M*g*cos(theta) = (2/5)* M*r^2/2 *alpha; alpha = 2.5*g*cos(theta)/r^2
For hollow sphere: m*g*cos(theta) = (2/3) m*r^2 * alpha; alpha = 1.5 g*cos(theta)/r^2

The solid sphere should reach the bottom faster because it has a higher acceleration (if both start from rest).
Problem 2: I am assuming a sliding mass (i.e. no car, no wheels, no friction, no rolling anything...just two masses sliding down an inclined plane).
They should reach at the same time because force equilibrium eqn is: mass*g*cos(theta) = mass* acc.
Mass cancels out; so acceleration for both is the same (and both start from rest).
PRoblem 3:
No idea why they move, I did check my cereal box for ingredients, and it contains some metals- a lot of Fe, some Zn, Cu, Mg..... Which might be ferro (or dia) magnetic. Just remembered that water is diamagnetic....need to think this one out (there's the irregular shape of the cereal, surface tension of water and the force exerted by magnet on the water...)

Problem 4:
Pure guess here....tap water has all sorts of charged ions and they are being affected by the statically-charged comb. I have no idea whether the charge in the comb is positive or negative (which would actually tell us the nature of the charged ions in the tap water).

one thing to ponder here is that if there is a hollow sphere and solid sphere ,both with the same mass m why does the hollow sphere have greater moment of inertia...

gakakkad · Post by **gakakkad** » 24 May 2015 21:12

prasannasimha wrote:
sanjaykumar wrote:So the observer travelling alongside the photon, at c, will see it travelling at c? Or will he now see a photon at rest mass. If the photon really (whatever that means) has rest mass 0, will it disappear?

So Einstein's theory of relativity is seemingly incomplete just as was Newton's.
No it doesn't. the whole premise of Einsteins work was the constancy of the speed of light in all directions for a given observer.

The error is mixing Newtonian and relativistic equations.The rest mass of a Photon is "Zero"
http://math.ucr.edu/home/baez/physics/P ... _mass.html

a corollary problem..lasers in opposite direction...would relative velocity of a photon in one laser w.r.t relative velocity of the other laser be 2c ?

member_22733 · Post by **member_22733** » 24 May 2015 23:21

You cannot have an inertial reference frame that has a photon at rest as it violates Maxwell's equations.

SriKumar · Post by **SriKumar** » 25 May 2015 04:29

gakakkad wrote: one thing to ponder here is that if there is a hollow sphere and solid sphere ,both with the same mass m why does the hollow sphere have greater moment of inertia...

..interesting question...hadn't thought about this. It would be very unusual to find a solid and a hollow sphere of the same mass and radius (!). I see moment of inertia as the degree of difficulty in getting an oject to rotate i.e. a rotational 'mass' If something requires more torque, it has a higher MoI. In a hollow sphere of the same mass as a solid sphere (dont know how this is possible except to use materials of different densities for hollow and solid spheres), the hollow sphere has more 'mass' (or material) further away from the center of the sphere. Example, MoI of disk = 0.5*m*r^2; MoI of ring (which is a 'hollow disk') = m*r^2. I rationalize this as: the ring has more material further away from the center, and would require more torque to accelerate to a certain rpm; hence higher MoI. Of course, if a mathematical approach is preferred: dI = r^2*dm (where dm is the delta mass, and r is the distance from the center of rotation). The longer the 'r' at which delta mass 'dm' is located, the larger the MoI (which would be the case for a ring, hollow sphere compared to a dsik, solid sphere). My 2 cents onlee...usually my 'explanations' just raise more questions...but this has not stopped me from creating some comedy at my expense. Ultimately AmberG. shall opine...

SriKumar · Post by **SriKumar** » 25 May 2015 05:26

Amber G. wrote:^^^..So when all is said and done..in practice (if one does an experiment in real life with a real car - or model car(s)) will one see a difference? (Hint: easy to perform an experiment and easy to see )
(Again: all problems above are interesting in a way that one can easily perform an actual experiment).

AmberG ji, mere paas abhi chote chote model car jaise khilone tho hain nahin..... Lagta hai mujhe Toys'r'us jaake kuch plastic cars khareed ne padenge aur uske baad bhauthik shastra ka yeh prayog karna hoga.

In my defence, I did try the cereal on water with magnet experiment. Indeed there was an attraction, which was strange to behold....the force was much more than what one would expect to get from traces of Fe in the cereal; but interestingly, I found that my finger also seemed to posses a similar attractive power. I tried to see if a plastic pen would similarly attract cereal...that was inconclusive. I am assuming that it has to do something with surface tension...the cereal makers try to keep the cereal from getting soggy, so they cover the surface with some hydrophobic compound (??!!). I'll give it another shot...

member_28108 · Post by **member_28108** » 25 May 2015 06:03

a corollary problem..lasers in opposite direction...would relative velocity of a photon in one laser w.r.t relative velocity of the other laser be 2c ?

The relative velocity is still c (c+c)/(1+c2 /c2 )=c

That is the essence of the Michelson Morley experiment and which Einstein explained in special relativity. Also strictly a photon has no resting frame of reference.

member_28108 · Post by **member_28108** » 25 May 2015 06:15

one thing to ponder here is that if there is a hollow sphere and solid sphere ,both with the same mass m why does the hollow sphere have greater moment of inertia...

The mass distribution difference accounts for this.The hollow sphere has its entire mass at the given radius of the sphere whereas for a solid sphere of the same mass the density will be lower and distributed across the entire sphere. In a hollow sphere the entire mass is concentrated at the given radius making it harder to change the angular momentum.
It is something like how if you twirl a ball and string around your finger/arm the ball speeds up as you get closer to the finger/arm. The "effective mass distribution" for the hollow sphere is greater than that of the solid sphere and thus the moment is different.

vina · Post by **vina** » 25 May 2015 11:52

Bored at work and eating lunch at my desk. So will give it a shot.

Amber G. wrote: 4 problems:

Motion 1 - There are two identical sized spheres, one solid other hollow but they look the same from outside and made of the same material. The solid one is, naturally heavier. You roll both the balls on an inclined plane. Which ball (if any) will roll faster? (By how much?)

(Extra credit: Does radius have any effect? If so in which way? How about substance (heavier vs lighter) it is made out of?

They will roll down at the same speed. There has to be friction for it to roll, otherwise, the spheres will simply slide down. Since they are rolling without slipping the max limits are the velocity of sphere = angular velocity * radius . Radii are the same, same angular velocity as well (other wise , with the max limit of rolling friction of a given surface it will start sliding) . So same- same ..

Weight will have an effect. Radius won't. Reaction due to friction (which is dependent on weight) will have an upper limit . If body is heavier than that limit it starts sliding down.

Motion 2 - Two identical cars (it is easier to use model cars, if an experiment is done) roll down a hill. (Car engines as well as brakes are not working - just rolling down due to gravity. One of the car has a heavy object in the trunk, other's trunk is empty. So the first car -- including the load -- is heavier. Which car (if any) will reach the bottom of the hill first?

Refer to question 1. Both reach bottom at same time (as long as the weight is not so heavy as that the car starts sliding and then it is not rolling anymore, then the answers will be different)

Electricity 1 - One has a piece of breakfast cereal floating floating in water. Use a strong magnet (<modern rare earth magnets work best>) and move it near (without touching) and you will see that you can move the cereal. Why? (Perform the experiment)

Water is diamagnetic. So a strong magnet will repel it. So a teeny weeny cereal bit floating on the surface will move

Electricity 2 - Start a very narrow stream of water
and bring a elector statically charged comb near it. (Instead of comb one can use a plastic cup or a balloon - just rub it against dry hair ) . Why does water bend? (see the picture below)

(Extra credit - bring a strong magnet (described above) near.. does water bend? which direction? Why?)

Enjoy-- Hope there is some discussion...

refer to previous answer. Water will be repelled by magnet (Stronger the better to see it). Bends away.

gakakkad · Post by **gakakkad** » 25 May 2015 13:15

>>They will roll down at the same speed. There has to be friction for it to roll, otherwise, the spheres will simply slide down. Since they are rolling without slipping the max limits are the velocity of sphere = angular velocity * radius . Radii are the same, same angular velocity as well (other wise , with the max limit of rolling friction of a given surface it will start sliding) . So same- same ..

both the balls are rolling to begin with..

gakakkad · Post by **gakakkad** » 25 May 2015 13:25

what would be the minimum coefficient of friction between solid ball of mass m and inclined plane , that would be necessary for a ball at an incline of theta , and mass m to roll without sliding ?

hint :torque due to friction cannot be less than total torque when it rolls

gakakkad · Post by **gakakkad** » 25 May 2015 13:32

>>Water is diamagnetic. So a strong magnet will repel it. So a teeny weeny cereal bit floating on the surface will move

the cereal egg-spriment can aalso be done without paani...

because cereal has iron...which is ferro magnetic

...

crushing cereals makes the experiment easy..

UlanBatori · Post by **UlanBatori** » 25 May 2015 19:13

Apologies 4 the intrusion into this fine dhaga.
Pls see the paper here. It is on the life and work of Dr. Jagdish Chandra Bose. He was a pioneer in Millimeter Wave technology, and (correctly, but a few years too soon) predicted that this would be the right way for long-distance wireless communication.

The innovation and persistence, and sheer brilliant insights, are truly magnificent. Stunning, how little this is known in India, and how all this spirit seems to not have been amplified. For instance, how do you develop a capacitor bank with hundreds of identical, parallel plates that are very thin?
you grab a few TimeTables from the Railway Station!!
Today desi varsities might sit around

that Millimeter Wave Capacitors cost $60 Lakhs and are under ITAR, etc etc.

My complete no-nonsense question is: Is anyone in India (or elsewhere, but I am less interested except Yoo-Ess) interested in setting up a facility to (a) duplicate and (b) build on the lab /experiments of Dr. JCB? If so, pls indicate so for further discussion, we can take the discussion elsewhere.

Basically, a university research group in US wants to see if such a collaboration with Indian universit(y/ies) can be developed. They have the applications and architecture models to go along a roadmap to some grand things, but the stumbling block is mmwave tech. IOW, all Electrical Engg expertise will be in India, the application tech may come from elsewhere. Serious inquiries only. Thnx

Amber G. · Post by **Amber G.** » 26 May 2015 20:09

vina wrote:Bored at work and eating lunch at my desk. So will give it a shot.

...They will roll down at the same speed. ... So same- same ..

and ..

SriKumar wrote:
Amber G. wrote:^^^..So when all is said and done..in practice (if one does an experiment in real life with a real car - or model car(s)) will one see a difference? (Hint: easy to perform an experiment and easy to see )
(Again: all problems above are interesting in a way that one can easily perform an actual experiment).
AmberG ji, mere paas abhi chote chote model car jaise khilone tho hain nahin..... Lagta hai mujhe Toys'r'us jaake kuch plastic cars khareed ne padenge aur uske baad bhauthik shastra ka yeh prayog karna hoga.
.

Good idea, meanwhile let me put a video from a popular MIT physics professor doing the similar things.. worth watching..(some probably have seen such demo's in their into physics class)
(It is a 15 minute demo, trying out all sort of things rolling down)
https://www.youtube.com/watch?v=cB8GNQuyMPc

Please do watch it.

(Basically For sliding surfaces - The smoothness of the surface matters but the total weight makes no difference.

When things have wheels or it roll, part of the energy is used by rotating part. Again weight/radius/length makes no difference but geometry of car/object does make difference which one can easily notice. Hollow sphere will take more time than a solid sphere.

Two cars (with identical type wheels, but one has extra weight in the body part) will roll down at different rates. (As said before, if you don't believe, just try it

).. Heavy car will win the race.

Neshant · Post by **Neshant** » 29 May 2015 09:42

TSJones · Post by **TSJones** » 04 Jun 2015 12:29

TSJones wrote:Yes, he has wonderful voice and some great musical ideas, too! Thanks, for posting that video.

Did you know that the stars sing to us? Yes indeed, they are a cosmic choir and sing in all sorts of voices.

I was reading in Astronomy magazine a few months ago about the Kepler space telescope. It was originally built to find planets as they orbit around their stars. Kepler watches closely for the star to "darken" as the planet orbits around the star in front of Kepler's view. Astronomers also noted that Kepler could monitor the stars vibrations at the same time. These vibrations are somewhat similar to seismic waves that scientists study here on Earth. What happens is that as stars burn fuel they eject large fountains of gas that fall back down on the "surface" of the sun and create sound waves that travel around the inner structure of the star. Using these waves or vibrations scientists can determine how old the star is (how much hydrogen has been converted to helium) as well as various other data that also reveals information about the planets that orbit the star. They also note that as a star ages its inner core spins faster and it emits a higher vibration. So we have all sorts of humming and singing going on in the cosmos. One scientist has even converted these vibrations into an acoustical recording. There is a link on the internet to listen to it.

This was a very heavy revelation to me. To think that there was a heavenly choir so-to-speak blasting out their voices to me from 1000's of light years away was overwhelming at the time. I could not put down the magazine all weekend. I carried it with me everywhere I went. I read and re-read all the details so that I could properly understand it (as much as I was able). To be honest, I am still thrilled about it. And oh yes, does our very own star, Sol, sing to us? Yes, indeed! Although it is nothing too "loud". Just a mild mannered "hum" to accompany the other louder members of the choir. He's a very modest fellow, you know. And thank goodness for that!

I also discovered why gold is so rare. Scientists now think that gold is only created when two stars are closely orbiting each other (scientists tell us these stars are real screamers in the choir) and then collide into each other creating the very rare minerals such as gold. That's the theory anyway. Again. thanks for the video and the mixture of art and science.

I think I will now go to youtube and play Mandolin U Srinivas and read Astronomy magazine again.

Lead Kepler mission scientist who had been demoted by NASA wins the Shaw prize:

http://spacenews.com/keplers-shaw-prize ... dog-house/

Awarded for discovery of thousands of planets

and studying solar interiors

.

It seems management however, did not like his style.

Amber G. · Post by **Amber G.** » 05 Jun 2015 03:03

TSJones wrote: Awarded for discovery of thousands of planets ...

Minor trivia item. (which I have bragged about in the past) ... A minor planet (Number 21431 see wiki or a catalog of minor planets), discovered in 1998 by MIT's Linclon Lab was is named with a name "amber.." ..after someone who is... let us just say I know fairly well)

TSJones · Post by **TSJones** » 05 Jun 2015 03:45

Amber G. wrote:
TSJones wrote: Awarded for discovery of thousands of planets ...
Minor trivia item. (which I have bragged about in the past) ... A minor planet (Number 21431 see wiki or a catalog of minor planets), discovered in 1998 by MIT's Linclon Lab was is named with a name "amber.." ..after someone who is... let us just say I know fairly well)

That is awesome!

gakakkad · Post by **gakakkad** » 05 Jun 2015 09:23

that must have been for winning the Intel science fair ,no ? in those days even astronomy olympiad winners were incentivized by getting planets named after em...

Amber G. · Post by **Amber G.** » 08 Jun 2015 18:45

^^^ Yes a ISTS finalist.

Amber G. · Post by **Amber G.** » 08 Jun 2015 19:29

Comments regarding the problems posted <here>. Some discussion has already taken place before.

As said before fun part is to do the experiments and discuss scientific principles..

Motion 1 - There are two identical sized spheres, one solid other hollow but they look the same from outside and made of the same material. The solid one is, naturally heavier. You roll both the balls on an inclined plane. Which ball (if any) will roll faster? (By how much?)

(Extra credit: Does radius have any effect? If so in which way? How about substance (heavier vs lighter) it is made out of?

If things slide and if we do not count the friction, the time does not depend on the mass. (Just how smooth the surface is - surprising friction also (almost) does not depend on the contact area.)

If things "roll" than some energy is used up in rotation. Hence a solid ball (or a cylinder) will roll faster than a hollow one. (Other factors like, radius, size, mass have no practical effec

Motion 2 - Two identical cars (it is easier to use model cars, if an experiment is done) roll down a hill. (Car engines as well as brakes are not working - just rolling down due to gravity. One of the car has a heavy object in the trunk, other's trunk is empty. So the first car -- including the load -- is heavier. Which car (if any) will reach the bottom of the hill first?

Again, for a car, with wheels, there is energy in wheels rotation, so for identical wheels, a heavier car
will roll-down faster. Very easy to verify by doing actual experiment.

Electricity 1 - Obvious thing to check, most cereals have iron. One can not "lift" it up by a magnet
because the quantity is too small but if the cereal is floating, one can move it. Easy to verify by just
crushing the cereal in fine powder. Some powder will stick to magnet.

(For a really strong magnet, one can observer this even for a plastic or floating paper. The water "dips down" - magnet repels water (and humans, frogs etc) though this force is 100xx weaker than what one sees with iron (attracts). So one needs a real powerful magnet. You can see the "dipping down" by looking at a reflection of a graph paper by the surface of the water, for example.
(Things like iron is attracted by magnet but bismuth, water etc are repelled by a magnet -- we do not notice it because this force is too small unless one has a VERY powerful magnet.

Electricity 2 - Start a very narrow stream of water
and bring a elector statically charged comb near it. (Instead of comb one can use a plastic cup or a balloon - just rub it against dry hair ) . Why does water bend? (see the picture below)

Many, if not most, get it wrong. (I have seen internet sources, textbooks etc getting it wrong).
Popular (and wrong) answer is -- "water molecule has electric dipole moment" (h2O - see wiki for example)

(If one calculates, the dipole part will NOT produce any notifiable bending of water stream - besides one can select a liquid which does not have dipole moment and still exhibit this bending)

The reason is ions. Some ions break-away by the electrostatic force leaving rest of the water stream
charged, hence the bending.

Neshant · Post by **Neshant** » 25 Jun 2015 13:48

Amber G. wrote: Electricity 2 - Start a very narrow stream of water
and bring a elector statically charged comb near it. (Instead of comb one can use a plastic cup or a balloon - just rub it against dry hair ) . Why does water bend? (see the picture below)

(Extra credit - bring a strong magnet (described above) near.. does water bend? which direction? Why?)

This is my guess :

Electrostatic force. A very large voltage is built up in the comb when you rub it. It can be as high as a few thousand volts. This static electric charge attracts the ions in the water causing it to bend towards (or is it away?) from the comb. This would not happen if it was a stream of deionized water.

I'm not sure if the water stream will bend in response to a magnet. If it did, my guess would be the ions are being repelled by the magnet.

Vayutuvan · Post by **Vayutuvan** » 28 Jun 2015 03:05

Does time dilation destroy quantum superposition?

Why do we not see everyday objects in quantum superpositions? The answer to that long-standing question may partly lie with gravity. So says a group of physicists in Austria, which has shown theoretically that a feature of Einstein's general relativity, known as time dilation, can render quantum states classical. The researchers say that even the Earth's puny gravitational field may be strong enough for the effect to be measurable in a laboratory within a few years.

SaiK · Post by **SaiK** » 30 Jun 2015 05:02

does that lead to universe/multiverse is spherical?

TSJones · Post by **TSJones** » 30 Jun 2015 07:07

SaiK wrote:does that lead to universe/multiverse is spherical?

To an infinite universe what is spherical?

When you write "multiverse", it could be "infiverse" (infinite universes) so to speak.

In an article in the June issue of Astronomy magazine Bob Berman writes that there is no set bedrock reality to space. It's tentative. That is to say the distance from point A to point B depends upon your point of view. Or as Einstein teaches to us "reference frame".

What is your reference frame? Well, it all depends. it is relative to the speed you are going or to the amount of gravity you are being subjected to at the time.

IOWs, here on earth the distance from point A to point B may be one light year in distance. But to another being on a very heavy gravitational body, the distance may only be a fourth of a light year or a third. It just all depends on the reference frame.

So to a being on a neutron star reference frame, the universe may not be all that far away from it.

Truly, we live in an infinite universe.

What is interesting to think about is what would the reference frame be like for a photon traveling at the speed of light?

rsingh · Post by **rsingh** » 04 Aug 2015 19:44

What happens if you put two bullet in the gun(assumimg it is possible) one after another. You fire the gun. First bullet fires the second. Doest it mean that second bullet has double vilocity then the first one?

TSJones · Post by **TSJones** » 04 Aug 2015 22:38

it depends.

on earth the bullets would be limited by atmospheric friction.

in space, they would not.

speed of light is the limit.

we use the earths orbital speed and gravity pull in assisting rockets to launch their payloads to other planets. as well as using other planets' gravity and orbital speed.

the New Horizons mission to Pluto for example. that puppy was MOVING ALONG I tell you. and still is.

the Indian sat to Mars used earth's gravity and orbit to fire its rocket and break through to Mars. very similar principle. it just kept boosting and boosting until it broke free of earth's gravitational pull into a greater and greater oval orbit and Mars grabbed it and it then the sat used its remaining fuel to break for a permanent Mars elliptical orbit. google Holmann transfers,

pretty slick, huh?

Viv S · Post by **Viv S** » 04 Aug 2015 22:54

rsingh wrote:What happens if you put two bullet in the gun(assumimg it is possible) one after another. You fire the gun. First bullet fires the second. Doest it mean that second bullet has double vilocity then the first one?

This is easier to visualize with pool balls. Next you'll have to definite the elasticity of the collision. Momentum will be conserved. If the collision is perfectly elastic (no K.E. lost to environment), the two balls will exchange velocities.

In your 'bullet' question -

1. The first bullet will come to a stop;
2. Second bullet will gain a velocity equal to the initial velocity of the first bullet.

TSJones · Post by **TSJones** » 04 Aug 2015 23:00

Viv S wrote:
rsingh wrote:What happens if you put two bullet in the gun(assumimg it is possible) one after another. You fire the gun. First bullet fires the second. Doest it mean that second bullet has double vilocity then the first one?
This is easier to visualize with pool balls. Next you'll have to definite the elasticity of the collision. Momentum will be conserved. If the collision is perfectly elastic (no K.E. lost to environment), the two balls will exchange velocities.

In your 'bullet' question -

1. The first bullet will come to a stop;
2. Second bullet will gain a velocity equal to the initial velocity of the first bullet.

again, it depends. if the second bullet does not touch the first bullet when it fires, there should be no loss of velocity for the first bullet, but if the bullets were touching each other, newton's law would apply.

SriKumar · Post by **SriKumar** » 05 Aug 2015 04:38

Viv S wrote:
rsingh wrote:What happens if you put two bullet in the gun(assumimg it is possible) one after another. You fire the gun. First bullet fires the second. Doest it mean that second bullet has double vilocity then the first one?
This is easier to visualize with pool balls. Next you'll have to definite the elasticity of the collision. Momentum will be conserved. If the collision is perfectly elastic (no K.E. lost to environment), the two balls will exchange velocities..

The pool ball analogy is not fully applicable to a bullet (if indeed we are talking about an actual bullet and not just a slug). A bullet = cartridge+chemical+slug. A bullet contains a chemical which imparts a bullet its its velocity. The second bullet too has this source of chemical (potential) energy to propel it, in addition to the kinetic energy of the first bulllet.

The answer depends on how long after getting impacted by the first bullet does the second bullet fire/ignite. During this (delta) time the second bullet will be accelerating (and the first one will accelerate less- if still inside the barrel), at the end of which the chemical will ignite and propel the second bullet even faster.

If the second bullet is merely a slug of the same mass as the first one, then I agree with your pool ball analogy and result . (P.S: This is quite similar to the converyor-belt and ball problem posted by Amber.G earlier).

rsingh · Post by **rsingh** » 05 Aug 2015 16:08

Thanks Gurus.

Theo_Fidel · Post by **Theo_Fidel** » 05 Aug 2015 20:21

TSJones wrote:What is interesting to think about is what would the reference frame be like for a photon traveling at the speed of light?

Ah! then you would trully understand the universe. What happens to time for instance. And as CS says that is not permitted. There are clear limits on what we are allowed to know. What is the reference frame from inside a black hole. Again not permitted to know. Smolin says that time might stop and even flow backwards. From its frame of reference a black hole might be a gigantic instant explosion similar to a trully immense super nova. But from the universes frame it is in excruciating slow motion matter destryer.

Amber G. · Post by **Amber G.** » 05 Aug 2015 20:22

..hmm..actually the first freshman physics course I taught, not as a TA but as a faculty where I had more freedom , I had a lab where bullet(s) were fired into a hanging wooden plank (like a simple pendulum) to teach kids basic laws of physics -- conservation of momentum and energy! (Second Amendment allows guns in Physics Lab -- and actually kids, even those who were not from Texas, enjoyed and learned.

If one thinks the right way, it is easy to think what will happen. (And confirm it by actually doing an experiment!). Of course, you have to define exactly the mechanism (because the result may vary depending on what exactly you mean by "two bullets" etc).. but you apply two principles..

Take the reference earth where gun initially was fired.
- 1. - Conservation of Momentum

-- Total momentum is zero before and after the event. In practice this means include the recoil of gun, and motion of two bullets. (the momentum lost in smoke particles, air-molecules (sound) etc will be negligible). Since gun's mass is >> bullet(s) ==> the recoil velocity will be small. (though not negligible).

2- Total Energy will be conserved.
In practice this means -
-- Total energy added is two bullets explosive (chemical energy), and it will be converted into:
-- Sound (big dhamaka etc) - negligible
-- Smoke particles moving - negligible
-- Gun causing pain in shoulder of the shooter etc (recoil) -- again very small less than a few %..
-- Damage caused to the bullets body (if they hit each other - small fragments of bullet flying away etc) -- again IN PRACTICE fairy small may be about 1-5% or so..

So mostly (90+%) energy is in bullets KE (1/2 mv^2).

Now how much energy (chemical energy) was induced -- Typical for example my physics lab gun was about 50 Jules.. (A typical 9mm Luger or .45 Colt - pistol is about 500 J, typical Rifle is about 2KJ (to may be 6K))

So what will happen? In nutshell -- (results may vary a little, but if I understand the question correctly ) .. Assuming if only one bullet fires, the velocity is v (typical v=1000 m/s for rifles)

If only one bullet "fire" - (as Viv S said) - the first bullet will come to rest, the second bullet will travel with velocity v
If the second bullet also "fired" (inside the muzzle causing all the recoil passed to the gun)
- two bullets each traveling with velocity = v ! (Second bullet will NOT go faster than v...

(By the way the range of the bullet depends VERY little on the muzzle velocity -- strange as it may seem but a fact known to Newton -- so a 1000 m/s or a 2000 m/s bullet (identical in other respect) will have almost the same range -- but this is OT here, though very important)

(what if the first bullet hit the second bullet (outside the muzzle - so expanding gas did not pass recoil to the gun) and ignited the second bullet? (Answer is left as an exercise

)

You don't have to take my word for this.. just do the experiment

-- just be careful if you are not familiar with guns)

rsingh · Post by **rsingh** » 06 Aug 2015 22:04

So what is difference between two bullet case and multi stage rocket. In rocket booster provide acceleration to the rest of rocket and disconnects itself. Then second stage and so on. In bullet case first bullet hits the second bullet which is at rest. Ok. What happens if we have a super bullet which is assemled as rockets. Trigger hits the first bullet, supper bullet is fired out of gun and then second bullet is charged (after micrseconds). In this case super bulket has to behave like rocket........doubling,trpling the original vilocity. Forgive me if Imade some stupid paqisque assumption.......I am an aam abdul afterall. Salam

SriKumar · Post by **SriKumar** » 07 Aug 2015 03:24

Amber G wrote: what if the first bullet hit the second bullet (outside the muzzle - so expanding gas did not pass recoil to the gun) and ignited the second bullet? (Answer is left as an exercise )

Life is getting complicated.....
Here are some assumptions made for the discussion below: the structure of the bullet = casing+chemical+Slug is assumed as in this picture. https://en.wikipedia.org/wiki/Bullet In this situation, the casing provides some (a small amount) of 'cover' to the slug but not much.

My position is that a bullet shot from a gun barrel accelerates because of the gas pressure developed by the chemical that ignites (sublimates?) and creates a large volume of gas very quickly. This gas is 'trapped' inside the bore of the barrel and the slug. And since the slug can move and the gun cannot (since the gun is held by a person = large mass relative to the slug), it is the slug that moves- in response to air pressure inside the gun bore. The air pressure accelerating the slug will drop to zero (or close) the moment the slug exits the barrel.

Now, if a bullet were to be ignited outside a barrel, the expanding gas would eject the slug out quickly (i.e using very little energy) and then expend the rest of the energy 'harmlessly'. So, the energy contribution to the slug from its own chemical would be 'small'. A bullet ignited by another bullet hitting it from behind would get most of its velocity from the kinetic energy of the bullet behind it.

Viv S · Post by **Viv S** » 07 Aug 2015 03:42

Amber G. wrote:If only one bullet "fire" - (as Viv S said) - the first bullet will come to rest, the second bullet will travel with velocity v
If the second bullet also "fired" (inside the muzzle causing all the recoil passed to the gun)
- two bullets each traveling with velocity = v ! (Second bullet will NOT go faster than v...

We'll perhaps also have to cater for the KE absorbed by the second bullet to trigger the charge, i.e function normally performed by the hammer.

1. Subtract that from the first bullet's K.E (say new effective velocity v').
2. Transfer that v' to second bullet and assume first bullet comes to rest.
3. Split energy released by explosion evenly between the two. (Would it be even, if one of the two has been imparted a momentum?)
4. Get velocity from new KE for each.

(This assumes that there is a negligible delay between the charge exploding and the impact of the first bullet.)

Amber G. · Post by **Amber G.** » 08 Aug 2015 08:52

rsingh wrote:So what is difference between two bullet case and multi stage rocket. ...

A multi-stage (or any kind) rocket is more like a "gun" which is firing lots of bullets (instead of lead the bullets are gas molecule discharged at high velocity) and the 'recoil moves the gun(rocket).

Hope this makes sense...

(Yes, a "rocket" uses MUCH MUCH more fuel than a bullet of the same weigh as the rocket..
It will be MUCH more economical if we used a cannon to fire a rocket to moon - the trouble is we can not give very high-velocity because the friction will slow/burn the rocket before it can go very high.. this is why we use multi-stage rockets so that we can keep giving it a "push" though we use up MANY MANY times the fuel).. This is why if we had a space station (of big cannon on the moon) we can save a lot on fuel if we fire rockets from there..

***

In firing an ordinary gun, all most all the energy (converted from chemical energy) is passed on to KE of the bullet. Yes there is some heat (bullet/gun gets a little hotter), some sound (noise), and recoil (gun moves backwards ).. but energy given/lost by a striker (or bullet hitting the other bullet inside muzzle) etc is virtually negligible .... so virtually all the chemical energy (say 50 to a few KJ, depending on gun) is converted into KE of the moving bullet.

For all practical purposes, it is expanding gas (inclosed in muzzle of the gun with finite volume) pushes the bullet.

***
So one will not be wrong, (if one does an experiment) if one just assumes that two bullets with (twice the gun powder) fired will act as if two bullets fired one after another (both moving with velocity v)..
(It is as if you have 2x massive bullet with 2x gun powder)

Hope this is helpful

ArmenT · Post by **ArmenT** » 08 Aug 2015 22:31

Amber G. wrote:..hmm..actually the first freshman physics course I taught, not as a TA but as a faculty where I had more freedom , I had a lab where bullet(s) were fired into a hanging wooden plank (like a simple pendulum) to teach kids basic laws of physics -- conservation of momentum and energy! (Second Amendment allows guns in Physics Lab -- and actually kids, even those who were not from Texas, enjoyed and learned.

Good ol' ballistic pendulum experiment. This was first proposed by a gent named Benjamin Robins, who was a mathematician with an interest in artillery and firearms, and also studied the flight paths of rockets. I wrote about this experiment here.

ArmenT · Post by **ArmenT** » 08 Aug 2015 22:45

rsingh wrote:What happens if you put two bullet in the gun(assumimg it is possible) one after another. You fire the gun. First bullet fires the second. Doest it mean that second bullet has double vilocity then the first one?

I see that others have posted a lot of theoretical replies above as to what would happen in this scenario, but allow me to put a bit of practical reality into this:

The situation where there is a bullet in the barrel and a second one is loaded behind it is often caused by what is known as a squib load. If you are ever in this situation, NEVER fire that second cartridge. Why? At best, you may suffer a bulged barrel, at worst, there is a good chance of the gun barrel exploding and injuring yourself and those around you. See the above link for details.

Physics: it's all fun in theory, until someone loses an eye.

Amber G. · Post by **Amber G.** » 09 Aug 2015 22:46

ArmenT wrote: Physics: it's all fun in theory, until someone loses an eye.

..Actually in my VHO, if you understand physics (real physics), it is not only fun but it will save you from harm...If some one "loses an eye".. it is certain that the person misunderstood very basic physics..

(Example, if one jumps down from Kutub Minar and breaks a bone.. Knowledge of gravity (or physics) is not to be blamed. In fact if the jumper knew basic physics, (and hence knew that PE (mgh) converted into KE (1/2mv^2)= energy absorbed and contributing to breaking of bone), he will NOT jump down at all whenever h is greater than some value)

SriKumar · Post by **SriKumar** » 10 Aug 2015 04:52

Amber G. wrote:
(Example, if one jumps down from Kutub Minar and breaks a bone.. Knowledge of gravity (or physics) is not to be blamed. In fact if the jumper knew basic physics, (and hence knew that PE (mgh) converted into KE (1/2mv^2)= energy absorbed and contributing to breaking of bone), he will NOT jump down at all whenever h is greater than some value)

Oh brother...one more question ...

kya karen....chalo ek try marte hain...
Height of kutub minar (S.I. units): 72.6 m
This is probably too high for a 'safe landing', so the reverse problem would be solved, i.e. what is a height from which a fall would break a bone.
breaking force of a bone: 4000 N (for a femur:
http://www.livescience.com/6040-brute-f ... unch.html- the value may not be accurate)

Potential energy of a person jumping from a height 'h': m*g*h.
Comparing this to the velocity attained (0.5*m*v^2); v= sqrt(2*g*h).

Initial velocity: v1= -sqrt(2*g*h).
Final velocity: 0
deceleration: (0-sqrt(2gh))/delta t
'delta t'= time in which the person comes to rest
'delta t' arbitrarily set to 0.1 second.
deceleration 'a' = sqrt(2gh)/0.1 = 10*sqrt(gh).
force = mass * deceleration
Assumption: average mass of tourist visiting Kutub minar = 75 kg

force = 750*10sqrt(2gh).

Assumption 1: person lands on his feet:
- effect: force will be distributed between the two limbs; no cushioning effect
ASsumption2: person lands on his ...well, posterior:
-effect: force will be borne by spine alone. Some cushioning provided by the ...well, the posterior.

Set: breaking force = force due to deceleration

Assumption 1: Since the force will be divided between the two femurs, twice the force can be assumed:
2*4000 = 75*10*sqrt(gh)
h= (1/g)*[(8000/(750)]^2 = 11.6 m (g=9.8 m/s)

Calculations with assumption # 2 TBD (degree of cushioning effect not known, can vary from person to person).

Jesting aside, the value of force sustained by the body is _very_ sensitive to the 'delta t' i.e. time to come to a stop.
Answer: The 'first floor' of Kutub minar is much higher than 10 meters, hence any fall from K Minar is guaranteed to break both femurs.
all in jest