Force = (mass) x (acceleration)
Acceleration = (force) / (mass) ==>
This says that with the same force,
acceleration is inversely proportional to mass. That means that with the
same force, greater mass has smaller acceleration. You know that, because
when you push on a pencil and a school bus with equal force, the pencil has
greater acceleration.
The acceleration of the 7kg object is three times the acceleration
of the 21kg object.
The acceleration of gravity on Mars is 3.711 m/s².