Newton's 2nd Law

Equation= Fnet=m x a (Net force= mass x acceleration)

The main idea of Newton's Second law consists of acceleration, mass, and net forces. With the second law equation you can determine an object's acceleration relative to Earth or another object. To have acceleration you must have a net force. A net force is a force that does not have an equal force in the opposite direction to balance it out. An easy way to find the net force in an object is to draw a force diagram for the object's forces. If there is not the same amount of force in opposite directions, then the object will have a net force, which means acceleration. If there is only one force acting on an object in the horizontal direction, that is its net force. If there are two, and they are pointing in the same direction, you add them and that is your net force. If there are two and they are facing opposite directions, you subtract them and that's your net force. If you can find the net force, and you know the mass, this is an easy equation to find your system's acceleration, or vice-versa.

Simple application
When 500N force is acting on a 50kg box
500N=50kg x a
When a 50kg box has 6ms^-2 acceleration
F= 50kg x 6ms^-2
F= 300N
When 100N is causing 10ms^-2 acceleration of an object
100N= m x 10ms^-2
m= 10kg