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?

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    $\begingroup$ 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. $\endgroup$ Nov 9, 2020 at 10:13

1 Answer 1


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 $9.8m/s^2$.

The example given by the examiner doesn't make sense as net force on the ball lying is zero i.e the gravitational force (mg) is balanced by normal reaction of the earth. Hence the net acceleration of the ball is 0 in this case.


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