^Newton’s second law
NSL defines force. External force is required to accelerate a body (change speed or direction or both).
When a body is seen from inertial frame of reference, the time rate of change of linear momentum is equal to the net external force acting on the body & takes place in the direction of net force
As all the three laws can be explained only from second law, that is why the 2nd law is called real law of motion.