A body is said to be at rest if its position does not change with time with respect to an observer (or a reference point). For example, the chairs of the dining table are at rest with respect to the dining table; the passengers sitting in a moving bus are said to be at rest with respect to the driver of the same bus, because their positions do not change with respect to the driver.
A body is said to be in motion if its position changes with time with respect to an observer (or a reference point). The passengers sitting in a moving bus are said to be in motion with respect to an observer standing outside the bus, Similarly, the blades of a rotating fan, the hands of a working wall clock, a spinning top and satellites are all in motion with resect to a fixed axis .
Rest and motion are relative terms. A body may seem to be at rest with respect to one object, but may appear to be in motion with respect to another object. If you consider a passenger in a moving train, he is at rest with respect to his co-passengers, but is in motion with respect to an observer standing on the ground.
Types of Motion
There are different types of motion. They are rectilinear motion, circular motion and periodic motion. Rectilinear motion is the motion of a body along a straight line. For example, a train moving on a straight track and a stone falling from a height are in rectilinear motion. Circular motion is the motion of a body that moves along a circular path. Examples of bodies in circular motion are the rotating blades of a fan, a child sitting on a merry-go-round, and the tips of the hands of a clock.
Periodic motion is the motion that repeats itself at regular intervals of time. Every object executing uniform circular motion can be said to be executing periodic motion. The motion of the pendulum in a pendulum clock, the motion of a swinging cradle and the motion of the needle in a sewing machine are some examples of periodic motion.
Sometimes an object can display combinations of different types of motion. For example, a moving car which moves straight on the road displays rectilinear motion but at the same time the wheels of the car which are moving in circles display circular motion. So the wheels of a moving car execute both rectilinear and circular motions. In a sewing machine, the tip of the needle is in rectilinear motion as well as in periodic motion.