Newton's Laws
Newton's 1st Law: Inertia
An object's tendency to keep doing what it is doing.
"An object in motion will remain in motion, and an object at rest will remain at rest, unless acted upon by a force"
An object's mass directly affects the object's inertia, or its tendency to keep or stop moving.
Newton's 2nd Law of Motion
"Net force is equal to mass times acceleration"
Net force is the added total force after you take away all forces that may cancel each other out.
Equilibrium: An object that is in equilibrium can still be moving, but its velocity won't be changing.
Gravity
The most common case of a net force making something move is the gravitational force.
Gravity is 9.81 m/s^2
When we try to calculate the net force of an object, we often need to take into account more than just gravity.
Newton's 3rd Law of Motion
"For every action, there is an equal and opposite reaction"
Normal force changes its magnitude and is the force that is always perpendicular to whatever surface your object is sitting on.
Basically, things can move because there's more going on than just the action and reaction forces.
Free Body Diagrams
Basically, you draw a rough line of an object, put a dot in the middle, and then draw and label arrows, to represent all the forces.
We also need to decide which direction is positive, and usually upwards is positive.