The "second law" says how much a force changes how a thing moves. When there is a net outside force on an object, its velocity (speed and direction of motion) will change. How fast the velocity changes is called the acceleration. Newton's second law says that bigger forces produce more acceleration. But objects with a lot of stuff in them (mass) are harder to push, so they do not accelerate as much. Another way of saying this is that the net force on an object equals the rate of change of its momentum. The momentum measures how much mass is in a thing, how fast it is going, and which direction it is going. So forces change the momentum, but how much they can change the speed and direction of motion still depends on mass.