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coconut62

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## Homework Statement

Q13

All external forces on a body cancel out.

Which statement must be correct?

A The body does not move.

B The momentum of the body remains unchanged.

C The speed of the body remains unchanged.

D The total energy of the body remains unchanged.

## Homework Equations

## The Attempt at a Solution

I know that B is the best answer and that A and D is wrong, but I don't know why C is incorrect.

If you cancel out every forces on the body, surely the body will continue its motion with the same velocity

__in a straight line__, and since the direction of motion is constant, then certainly the speed should be the same too.

Now consider a circular object being applied a torque. My question is, since the lines of action do not meet, can you say that the forces "cancel out"?

If you say that it

__does__cancel out, then considering the changing of the velocity of the circular object, B would be wrong.

If you say it

__doesn't__cancel out, that means torques/moments are not considered in this question at all, so what is left is just bodies traveling in a straight line whose speed can't possibly change when there's no external force. So does this support my claim that C is correct?

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