Newton's Second Law of Motion says that force is directly proportional to acceleration (the rate of change of momentum) for a constant mass. Meanwhile, acceleration is inversely proportional to mass. For example, when you throw a ball thrown onto the ground, it exerts a downward force; the ground, in response, exerts an upward force causing the ball to bounce. This law is useful for measuring forces. If you know two of the factors, you can calculate the third. You also know that if an object is accelerating, there must be a force acting on it.