Study Forces
Standard - 4.2
---A force is any push or pull that causes an object to move, stop or change speed or direction.
---The greater the force, the greater the change in motion will be.  The more massive an object, the less effect a given force will have on the object.
---Friction is the resistance to motion created by two object moving against each other. Friction creates heat.
---Unless acted on by a force, objects in motion tend to stay in motion and objects at rest remain at rest.

READ ABOUT  FORCES, FRICTION AND INERTIA

A force is a push or a pull that causes an object to move, stop, or change speed or direction.  Every time you push or pull something you apply a force. Forces transfer energy.
When you throw a ball, the force of your arm pushes the ball into the air. Energy is transferred from your arm to the ball.
An object will stay in place until a force sets it in motion. 
The pull of gravity is a force that might set an object (or a skier) in motion. The pull or push of a magnet is another force that can move objects.  The force you exert with your legs or arms can set a ball in motion.
An object will continue to move until a force slows or stops it.  The force stopping the object's motion might be an obvious one - the ground!
Friction is a force that slows or stops motion.  Friction is the resistance to motion created by two objects rubbing against each other (the sled and the snow, for instance). Even air causes friction.  Friction creates heat.
The effect of a force depends on the object's mass.  More massive objects  have more "inertia".  This means a force will have less effect on a more massive object.  For example, it's easier to push a light plastic ball than a more massive boulder. It's also harder to stop a boulder when it is rolling down a slope. The more massive boulder has more "inertia".