Berkeley Review Physics Chapter 4 #20 (conservation of torque?)

[lightblueskies]

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In the passage you have an experiment where clay balls are being rolled at each other, and you're asked to ignore friction.

Question 20 asks: "If the experiment were done using super rubber balls instead of clay balls, what quantities would be conserved in collisions?"

a) Kinetic energy only
b) Momentum only
c) Kinetic energy and momentum
d) Kinetic energy, momentum, and torque


They give the answer as C, and say that "if the balls were spinning when they collided, then both rotational kinetic energy and angular momentum would be conserved as well, but not torque."

However, I don't understand this - I thought it should be D. Torque is a force applied to rotate something about its axis. There are absolutely no forces at play in this experiment, even if the balls are spinning when they hit each other. How can torque not be conserved? Torque should be zero at all times, since there is no force that acts on the balls after they are launched. Obviously my logic is faulting somewhere - please explain. Thank you.

 

[Maxxxx]

I'm not sure what the experiment is exactly, but if the 2 balls collided off center (not head on, but scraped each other on their sides), then there will be some torque at collision. So torque went from 0 to some positive number then back to 0, clearly not conserved.

 

[lightblueskies]

I'm not sure what the experiment is exactly, but if the 2 balls collided off center (not head on, but scraped each other on their sides), then there will be some torque at collision. So torque went from 0 to some positive number then back to 0, clearly not conserved.

Torque is a FORCE applied to rotate some object. Any spin by one ball caused by scraping will result in an equal yet opposite spin on the other ball in this plane, so torque should be conserved.

 

[butters221]

I was looking for a more complex answer, but perhaps when they say the "balls are being rolled at each other," they are implying they balls are going directly at one another (if I wrote this question I would probably wonder why someone would assume the balls were not moving directly at each other).