The Second Law is concerned with relating acceleration to mass and net force.
Newton's second law of motion explains how an object will change velocity if it is pushed or pulled upon.
Firstly, this law states that if you do place a force on an object, it will accelerate (change its velocity), and it will change its velocity in the direction of the force. So, a force aimed in a positive direction will create a positive change in velocity (a positive acceleration). And a force aimed in a negative direction will create a negative change in velocity (a negative acceleration).
Secondly, this acceleration is directly proportional to the force. For example, if you are pushing on an object, causing it to accelerate, and then you push, say, three times harder, the acceleration will be three times greater.
Thirdly, this acceleration is inversely proportional to the mass of the object. For example, if you are pushing equally on two objects, and one of the objects has five times more mass than the other, it will accelerate at one fifth the acceleration of the other.
A Closer Look At The Proportions
Acceleration is directly proportional to the applied net force.
Below is an animation that illustrates the direct proportion between force and acceleration. There are some notes about this under the animation.