newton's laws

Newton’s First Law (Inertia): In the absence of a net force, an object at rest stays at rest and an object in motion stays in motion at a constant speed in a straight line forever

This means: Things keep doing what they’re doing unless there’s a leftover force in some direction. If all forces balance, it keeps on keepin’ on.

This also means: When F=0, a=o (Follows from F=ma.) If acceleration is zero, it cannot be speeding up, slowing down, or changing directions. Its velocity is constant!!

Newton’s Second Law (F=ma): 

A net force is a fancy name for the sum of forces

Net force = mass x acceleration.

or a = sum of all forces / mass of object

This means: If there is a leftover force, things will change. It will speed up, slow down, or change directions. That is, it will NOT have constant velocity. Recall that velocity includes direction!

For every action there is an equal and opposite reaction.

This means: If two objects hit each other, they hit with the same force no matter what their masses or accelerations are. Always. These forces cannot cancel out even though they are the same amount and opposite directions because they aren’t on the same object. 

Example:1:  Earth pulls object down, object pulls Earth up

          2:  Horse pulls on apple cart, apple cart pulls on horse

         3.   Apple cart wheel pushes ground back, ground pushes apple cart forward

** key is to only look at the interaction between 2 objects **