Newton's Laws of Motion
An object in motion tends to stay in motion while an object at rest tends to stay at rest unless acted upon by an outside force.
An object moving at a constant rate and an object standing still both have equal forces and a net force of zero. If their balanced forces become unbalanced, they will have a net force of zero and either speed up or slow down. Objects moving at a constant rate have trouble changing directions. That is called inertia- "The resistance an object has to a change in its motion. Objects with larger mass have more inertia that smaller objects, which are easier to move around.
An object acted upon by an unbalanced force will accelerate in the direction of that force, in direct proportion to the strength of the force, and in inverse proportion to the mass of the object.
Usually expressed my the equation F=ma, Newton's second law stated that you need more force to move an object with more mass than it takes to move an object with less mass. An example of this is that it takes more force to push a cart full of bricks that it is to push the same cart while its empty.
For every action force, there is an equal but opposite reaction force.
This means that in every interaction, there are two forces acting on the objects in the interactions. An example of this is when you ride a bike, you push on the pedals moving the bike forward and the earth moves backward along with the bike.