1. The problem statement, all variables and given/known data 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. 2. Relevant equations 3. 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?