What force slows objects down and keeps them from moving?

What do each of the letters correctly represent?

In a football game, two players tackle each other so hard that they both fly in opposite directions after they hit each other.  What law of motion is this an example of?

An object in motion will stay in motion and an object at rest will stay at rest unless acted upon by an outside force.  Which law does this describe?

What are the units for FORCE?

If there is a big mass, it would need a stronger force to accelerate it.  Which law does this describe?