# Speed of Force or 'Transfer of Momentum'

Discussion in 'Physics & Math' started by hansda, Feb 14, 2013.

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1. ### hansdaValued Senior Member

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In the "Newton's Cradle", force/momentum applied by the first ball is transferred to the last ball.

Is this transfer simultaneous or there is some time delay, if there are many balls between the first and last ball?

3. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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Yes, of course there is a time delay. I beleive if the balls are touching then the energy is transfered at the speed of sound.

5. ### Q-reeusValued Senior Member

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Take the case of just one ball being lifted and let go. As the impact of first ball is a shock event it must generate a quite complex multiply reflecting and dispersive elastic wave-train within itself as well as the next ball in line, and then so on down the array. Seems almost miraculous that net result is almost - but not quite - complete transfer of momentum and KE to the final ball. Speed of transfer should be an appreciable fraction of longitudinal sound speed in a rod of the same material, but could not equal it, since for starters the pressure waves will travel at anything but a nice straight uniformly longitudinal line within the balls. There's some serious math out there if you want to dig deep.

7. ### hansdaValued Senior Member

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What about 'conservation of momentum' during this period of time-delay?

Does the sound carry momentum also?

8. ### hansdaValued Senior Member

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Why miraculous?

"Newton's Cradle" is an experiment for conservation of momentum and energy.

9. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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The conservation of momentum applies. The time delay is from the intial hit of the first ball to the last ball moving away. The energy is being transported through the balls as a pressure wave which moves at the speed of sound, .

The KE of the first ball is transfered through the stationary balls at the speed of sound. THE SPEED OF SOUND, that does not mean the energy is sound.

Maybe this will help.

10. ### hansdaValued Senior Member

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Suppose there are many balls between the first and last ball. The distance between first and last ball is say one sound-hour(speed of sound x one hour).

In this case after the first impact all the balls will remain stationary for one hour(being the time delay). So, momentum of all the masses are zero in this period of one-hour(rest).

11. ### Q-reeusValued Senior Member

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I did say 'seems' deliberately. Try pondering how complex the elastic wave propagation must be within each ball - radiating initially from a tiny contact patch and then bouncing around from the spherical surface boundary. That almost complete cancellation of these multiply reflecting waves - many vibrational modes - happens successively from ball to ball is imo an amazing fact. But it's never perfect, which brings to the next point.
Sort of - it's foremost appeal seems to be as executive toy. Notice though that eventually it does run down to zero motion. Partly from frictional loss at the contact patches, partly from acoustic and windage loss, partly from elastic hysteresis, and partly from elastic energy stored in the balls as modes that do not assist propagation - imperfect destructive interference as pulse moves from ball to ball.
So sure it 'demonstrates conservation of energy and momentum' - but also that perpetual motion of the third kind is impossible.

12. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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Yes I know, they apparently assume that the reader is not retarded.

So your question is; if you had a newtons cradle that was > 10,000 miles long when would the last ball move? Never.

Is your goal here is to ask questions and never learn anything? I hope so because you are doing a 'bang up' job!

13. ### hansdaValued Senior Member

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I think you are not getting the implications of this 'time-delay' factor.

This factor can imply,

1) There may be a time-delay in Planck's scale between an action and its reaction.

2) The acoustic pulse can carry momentum.

If there is no loss of energy and momentum, the last ball will move after the time delay.

Acoustic Pulse do carry momentum. See here.

14. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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What implications do you think there are?

Sure if there is no loss of energy and momentum after the energy is transmitted to the last ball it will move.

Yup.

15. ### AlphaNumericFully ionizedRegistered Senior Member

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Sound oscillations can carry momentum, as they are oscillations in the position and velocity of the material's atoms and obviously they can carry momentum. Sound wave momentum is therefore a macroscopic ensemble effect of lots of little momentum transfers.

This is particularly illustrated by phonons, which are oscillations within a material which are quantised and behave pretty much exactly how particles so, except this is the coherent behaviour of many particles.

16. ### hansdaValued Senior Member

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Following three implications can be made from this 'time-delay' factor.

1) "Speed of force or 'transfer of momentum/energy' from first ball to last ball, can be considered as speed of sound".

2) "Sound wave can carry momentum along with its energy."

3) "There may be a time-delay in Planck's scale between an action and its reaction."

-Newton's Third Law of Motion states that, "To every action there is always an equal and opposite reaction.". These action and reaction are instantaneous but not simultaneous. This can be observed from Newton's Cradle. At the instant when action happens from first ball to the second ball, there is no reaction from the second ball to the first ball. This reaction from second ball to the first ball happens at a later instant of time after the reverse oscillation of the last ball. From this fact it can be concluded that, "There is a time delay in Planck's scale between an action and its reaction".

17. ### Motor Daddy☼☼☼☼☼☼☼☼☼☼☼Valued Senior Member

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I was thinking about Newton's Third Law as you've posted above, and I asked myself "how much action does a bowling ball have at 12 O'Clock?" I'm having a conflict and I'd like to resolve that conflict.

18. ### hansdaValued Senior Member

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Force of re-action at 12 O'Clock position(attachment point) on stationary bowling ball is mg[where m is mass of the ball and g is gravitational acceleration]. Its direction is upward to balance gravitational force.

19. ### Motor Daddy☼☼☼☼☼☼☼☼☼☼☼Valued Senior Member

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I'm not sure what the term "action" is meant to imply. Is it simply force, or is it more than force? I agree if "action" is another term for force. If it means something different than force than please explain.

20. ### hansdaValued Senior Member

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Say there are two masses m1, m2 and m1 applies a force to m2.

So, at the contact point of m1 and m2, force experienced by m2 is action and force experienced by m1 is reaction.

Action is the cause and reaction is the effect.

21. ### Motor Daddy☼☼☼☼☼☼☼☼☼☼☼Valued Senior Member

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How does a mass apply a force to another mass? Can you give a simple realistic example?

22. ### hansdaValued Senior Member

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See the link at post #1.