Astronomy 101: From the Sun and Moon to Wormholes and Warp Drive, Key Theories, Discoveries, and Facts about the Universe (Adams 101 Series)

by Carolyn Collins Petersen

Newton’s Three Laws of Motion Isaac Newton carefully worked out the math describing an object’s motion in space. On the basis of this, he postulated three laws: An object at rest will stay at rest. If it’s in motion, it will stay in motion unless it’s acted upon by another object or force. Acceleration occurs when a force acts on a mass. The greater the mass, the more force is needed to accelerate the object. Physics and astronomy students learn this as F = ma, where m stands for the amount of mass and a stands for acceleration. In the case of an apple falling to the ground, you plug in the mass of the apple and its acceleration, and that gives you the force (in units called, appropriately, Newtons). For every action there is an equal and opposite reaction. If one ball hits another ball in a bowling alley, the first ball is pushed back in the opposite direction as hard as the second ball is pushed forward.