# About force and motion in Physics

1. Oct 19, 2003

### Johnny Leong

Question: Team A and B are competing in a tug-of-war. Team A is winning. Assuming that the mass of the rope is negligible, which of the following statements are correct. Explain your answer.
i. The force exerted by A on B is greater than the force exerted by B on A.
Ans. I think this is right. Because A is winning.
ii. The frictional force exerted by A on the ground is greater than that exerted by B on the ground.
Ans. I think this is not correct. Because not A exerts friction on the ground, the direction should be reversed.
iii. The frictional force exerted by the ground on A is greater than that exerted by the ground on B.
Ans. I think this is correct. Because A exerts more force, the friction exerts on A is also greater.

The rope will break if the force exerted on it is greater than 5000 N. Will the rope break in the tug-of-war if two teams pull on it, each with a force of 3000 N?
Ans. I think the rope will not break. Because the force of each team is still below 5000 N, even they exert 3000 N but they are in different directions. The main point is that the resultant force acted on the rope is still below 5000 N.

Last edited: Oct 20, 2003
2. Oct 19, 2003

### HallsofIvy

Staff Emeritus
Question: Team A and B are competing in a tug-of-war. Team A is winning. Assuming that the mass of the rope is negligible, which of the following statements are correct. Explain your answer.
No, the force exerted by A on B is exactly the same a the force exerted by B on A. (Equal and opposite reaction)

Why would you think A is not exerting friction on the ground? Suppose A were a bunch of enormous strong men who were sitting on ice and the other end of the rope were attached to a tree. The tree doesn't pull at all, the men pull with great force. Who moves towards whom?

You got this one right! But your reason makes no sense. The friction the ground exerts on A is exactly the same as the friction A exerts on the ground!

Once again, correct answer, bad reason. If both teams were exerting 3000N in the SAME direction, there would be no tension in the rope- it would go flying very fast in that direction but there would be no tension! The reason the rope won't break is this: suppose one team were exerting 3000 N force on the rope and the rope were tied to a tree. There would be 3000N tension in the rope (so it doesn't break) but the fact that the rope doesn't move means that the tree is exerting 3000 N force on the rope also. This is exactly the situation you have in the problem. One team is exerting 3000 N force on the rope. The fact that the other team is also exerting 3000 N on it just tells us that the rope is not moving in one or the other direction. The tension in the rope is still 3000 N- less than 5000.