# Motion is zero but acceleration is not zero?

So In my last Physics Exam I had one of these question which annoyed me a lot,

"Is Motion is zero but acceleration is not zero ? If no , Why not ?

I answered that it is impossible with some line of reasoning . They gave me zero marks . They gave me a example .

"Imagine there is a ball lying on the ground on earth.The ball is standing still with no motion.But ther is still an acceleration vector pointing downwards from the ball with a magnitude of $$9.8$$ ms$$-2$$.So there is acceleration without motion."

I can just put this on my test next time and get full marks , but I still have many doughts about this reasoning.

For example , If the ball has zero motion , the ball is not moving . And we know acceleration is defined by velocity and velocity is defined by motion.So,If there is zero motion ,there is zero velocity , If there is zero velocity ,then there is zero acceleration.My reasoning is that , there is indeed a force on it acting on the ball by the earth , but because the ball is on the ground ,there is a normal opposite force action on the ball ,canceling the original force.So the net force is zero .We know that $$a$$ = $$F$$ / $$m$$ .So If there is no force ,then there should be no acceleration.

Is there a flaw in my reasoning?

• Sounds like your professor doesn't understand Newton's second law uses the net force, and the resting ball situation includes a normal force that balances gravity. Nov 9, 2020 at 10:13

When you throw an object upwards, at topmost point the the object is momentarily at rest but acceleration is non zero throughout the motion . If we neglect air resistance , the value of acceleration comes out to be $$9.8m/s^2$$.