Force

The formula for force is:
 
F = ma
 
The formula for the force of gravity is:
 
Fg = GMm / r ²
 
G is a constant, M and m are the masses of two bodies, r is the distance between their centers.
 

 
Falling Bodies

What is the acceleration of a falling body?

The force acting on a falling body is the force of gravity. For a falling body, we can write:


F = Fg

ma = GMm / r ²
 
a = GM / r ²




Springs

One end of a spring is attached to a wall. The other end of the spring is attached to a block of mass m. The block is on a flat, frictionless surface. The block is pulled out a little distance. This stretches the spring. The block is then released. The stretched spring contracts, pulling the block back to where it started. What is the acceleration of the block?

A restoring force is created in a stretched spring. The formula for restoring force is:


Fr = −kx


The force acting on the block is the restoring force of the spring. We can write:


F = Fr

ma = −kx

a = −kx / m