Newton's Cradle: Speed of Force Q&A

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SUMMARY

The discussion centers on the propagation speed of kinetic energy in a Newton's cradle scenario. It is established that the force communication between the balls occurs via pressure waves, which travel at the speed of sound in the material, approximately 5-10 km/s, rather than at the speed of light. Therefore, the last ball in a hypothetical one light-year long Newton's cradle will not move instantaneously; it will only respond after the pressure wave reaches it, confirming that kinetic energy does not propagate instantaneously.

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The Divine Zephyr
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Hey I'm new here and I would like to propose a question about the speed of kinetic energy. If let's say we had a Newton's cradle one light-year long, will the last ball fly up as soon as the first ball hits the second? Assume all the balls are in perfect contact with each other. Does this happen instantly or at light or sub-light speed? I do not think the speed is limited, but what do you people think?

-tdz
 
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Kinetic energy does have a "speed."

Futhermore, the balls communicate force via pressure. Pressure variations propagate at the speed of sound, which is different for different media, and is always less than the speed of light.

So no, the ball at the far end won't move until the pressure wave reaches it, which will take some time.

- Warren
 
I think the the Newtons cradle example, the shockwave will travel through the material at the speed of sound (about 5-10 km/s).

Originally posted by chroot
Kinetic energy does have a "speed."
Don't you mean 'does not'?
 

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