Gravity, energy, velocity time, distance and acceleration are all terms, and concepts that any serious martial artist must become familiar with.  These concepts are closely related and are necessary for the production of force.   

Isaac Newton while still in his twenties invented the branch of mathematics known as calculus, discovered the laws of gravity and found that white light is a combination of colors.  However, it is Newton's laws of motion that we as martial artist should be concerned with.

Newton's first law of motion is reasonable and in agreement with our everyday experience.  Even young kids know that a body at rest will remain at rest until some external force causes it to move.  Thus, Newton's first law of motion can be stated as: 

Every body perseveres in its state of rest or in uniform motion in a straight line unless it is compelled to change that state by forces impressed there on. 

Newton's third law states that: 

To every action there is always opposed an equal reaction, or the mutual actions of two bodies upon each other are always equal, and are directly opposite.

Everyone knows this third law as:  For every action there is an equal and opposite reaction.