The LAWS OF MOTION formed the basis of our understanding of the motions of objects on earth and even the planets in space. They explain the relations between the forces acting on a body and the motion of the body. The first law is inertia. This states that an object will continue to stay at rest or in uniform motion in a straight line unless acted on by an external force. The second law states that force is equal to mass times acceleration of an object. The third states that for every force action, there is an equal and opposite reaction. These laws also eventually led to the LAW OF UNIVERSAL GRAVITATION. Isaac Newton, who discovered this law, observed an apple falling straight down from a tree, and he thought that if gravity pulled the apple down from the tree, it can pull on planets too. He also knew that if there was no other force pulling the moon towards the Earth, then the moon would fly away from the Earth in a straight line tangent to its orbit, so there must be gravity interacting between planets. This led to the explanation of the motions of planets and the ebbs and flows of rivers and tides all effected by gravity.