Mechanics is a branch of science and technology where we deal with various kinds of motions of bodies. Based on the observations for the motions of bodies, Newton put forth three laws, often called the Newton's laws of motion. In the present chapter, we shall discuss about them. While discussing the Newton's laws of motion, we shall assume that the atmosphere around us is not imposing any force on the moving bodies. That is, we shall assume that the atmosphere is not present. For expressing the motion, we require some reference frame and a coordinate system.

Inertial reference frame

For describing motion of a body, we require a reference frame. Newton considered an absolute reference frame and a universal time. For an absolute reference frame, its origin as well as its axes are absolutely fixed. Distances (displacements) of bodies are measured with reference to this reference frame.

The reference frame in which Newton's laws of motion are valid is known as an inertial reference frame. In fact such a reference frame does not exist, as no material body is at absolute rest and therefore we cannot have an absolute reference frame. Even a reference frame attached to the earth is not an absolute one, as the earth revolves around the sun, as well as it spins about its own axis. Further, sun also moves in our galaxy, the Milky Way. In fact, no body in the universe is at the absolute rest.