But if you have a net force applied to one side of this object then you would have a net acceleration going in the same direction and what Newton's second law of motion tells us is that the acceleration will be proportional to the force applied or the force applied is proportional to that acceleration. and the constant to proportionality...or to figure out what you multiply the acceleration by to get the force, or what to divide the force by to get the acceleration... ...is called mass. that is an object's mass