- #1

- 58

- 0

## Main Question or Discussion Point

Hi all,

I am thinking about a problem and am currently quite confused about it. Suppose I have 2 identical balls on the x-axis, tied together by a loose inextensible string. When I hit the ball on the right to the right, the string will eventually get taut and the ball on the left will get pulled along. My question is, will the string become loose after that, and will the two balls rebounce towards each other?

My answer is yes, but apparently I am wrong. I think of it as something like an elastic collision. When the string becomes taut, it is as if the two balls collided, and thus you can use conservation of momentum and the equation for the relative velocities. However, apparently I am wrong. My friend says that the ball will not rebounce, because the string is inextensible. If it is inextensible, it cannot store energy and thus cannot pull the balls back together.

But I am not sure about this explanation. Does this mean that if a string is inextensible, it cannot exert tension? If it can still exert tension, shouldn't the balls bounce back towards each other, because the tension will pull them back?

Also, say I tie the ball to a wall this time, and I kick the ball out really hard. The ball will rebounce back. Why is it that if I tie it to an identical ball, it will not? How do you know when it will bounce back and when it will not? Also, if the balls are not identical and are of different mass, what will happen?

Sorry if I am spamming a lot of questions. I just got quite confused here :S

I am thinking about a problem and am currently quite confused about it. Suppose I have 2 identical balls on the x-axis, tied together by a loose inextensible string. When I hit the ball on the right to the right, the string will eventually get taut and the ball on the left will get pulled along. My question is, will the string become loose after that, and will the two balls rebounce towards each other?

My answer is yes, but apparently I am wrong. I think of it as something like an elastic collision. When the string becomes taut, it is as if the two balls collided, and thus you can use conservation of momentum and the equation for the relative velocities. However, apparently I am wrong. My friend says that the ball will not rebounce, because the string is inextensible. If it is inextensible, it cannot store energy and thus cannot pull the balls back together.

But I am not sure about this explanation. Does this mean that if a string is inextensible, it cannot exert tension? If it can still exert tension, shouldn't the balls bounce back towards each other, because the tension will pull them back?

Also, say I tie the ball to a wall this time, and I kick the ball out really hard. The ball will rebounce back. Why is it that if I tie it to an identical ball, it will not? How do you know when it will bounce back and when it will not? Also, if the balls are not identical and are of different mass, what will happen?

Sorry if I am spamming a lot of questions. I just got quite confused here :S