# Q & A: Newton's 3d law

Q:
Hey so I am taking the IB Physics HL course, I just wanted to ask that, when we drop a ball, it bounces back, but why? I know the conservation of momentum and Newton's third law, but if the force of the floor on the ball is the same as the force of the ball on the floor, then why does the ball bounce back? like shouldn't all forces cancel out? it sounds a bit stupid but it has been in my mind for a long time
- Vanshaj Agarwal (age 16)
50480
A:

Newton's 3d law doesn't say that all the forces on any one object add up to zero. If that were true, nothing would accelerate. It says that all the forces on everything add up to zero, so that if something accelerates one way, something else accelerates the other way.

Obviously there's a force between the ball and the floor, otherwise the ball would keep falling right through the floor. The general form of Newton's laws says nothing about whether that force is big enough to make the ball bounce back. Some balls do, some don't.

Mike W.

(published on 01/15/2018)