Newton's first law explains what happens to an object when no forces act on it it either remains at rest or moves in a straight line with constant speed Newton's sec- ond law answers the question of what happens to an object when one or more forces act on it. a Imagine performing an experiment in which you push a block of mass m across frictionless, horizontal surface when you exert some horizontal force F on the block, it moves with some acceleration a If you apply a force twice as great the on same block, experimental results show that the acceleration of the block doubles, if you increase the applied force to 3F, the acceleration unples, and so on From such observations, we conclude tha the acceleration of an object proportional to the force acting on it F 3 a This idea was first introduced in Section we discussed the direction of the acceleration of an object we also know from the preceding section that the magnitude of the acceleration of an object us inversely proportional to its mass. Ia 1/m. These experimental observations are summarized in Newton's second law: When viewed from an inertial reference frame, the accelerauon of an object is directly proportional to the net force acting on it and inversely proportional to mass