You’re correct, it’s gravity
Answer:
D.
Explanation: Both have shapes determined by gravitational forces.
An object in motion changes position
Answer:
Mass of second object will be 15 kg
Explanation:
We have given mass of first object = 12 kg
Acceleration 
According to second law of motion we know that force F = MA
So force 
As the same force is applied to the second object of acceleration 
So force = ma

m = 15 kg
So mass of second object will be 15 kg
If the runner is running in a circular track then yes when something or someone is moving in a circular motion at a constant speed they are indeed accelerating. They’re accelerating because the direction of the velocity vector is changing