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Rest and Motion

# Rest and Motion

When a body remains in one position for a long time, it is said to be at rest.

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### Rest

When a body remains in one position for a long time, it is said to be at rest. For example, the chairs of the dining table are at rest unless and until they are moved, and the flower vase, table, and the blackboard in the class room are all at the position of rest.

### Motion

The act, process or state of the change in place or positionof a body with respect to time and relative to the observer is said to be motion.
For example the blades of a rotating fan, the hands of a working wall clock, a moving car, a spinning top and satellites are all in motion.

#### Rest and motion are relative terms

A body seems to be at rest with respect to one object, but may appear to be in motion with respect to some other object.
For example, a person on a railway platform is at rest with respect to another person on the same platform, but is in motion with reference to a person looking at him from a train crossing that platform. Similarly, a passenger sitting in the train will appear at rest to another passenger on the same train.

### Types of Motion

#### Rectilinear motion

Rectilinear motion is the motion of an object that moves in a straight line.
For example, a train moving on a track, a parade, coins tossed in the air are all in rectilinear motion.

#### Circular motion

Circular motion is the motion of an object that moves at a fixed distance from a fixed point. Here, all objects rotate in circular motion. So, circular motion is motion in which the body traverses a circular path. The hands of a clock, a merry-go-round, the blades of a fan, the wheel of a moving vehicle, satellites, a spinning top, are all good examples of circular motion.

#### Periodic motion

Periodic motion is motion that repeats itself at regular intervals of time. Every body executing circular motion can be said to be executing periodic motion. For example, the pendulum of a wall clock moves at regular intervals, the bells in a church, a bouncing ball, a vibrating string and a swinging cradle are all in periodic 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 a moving car displays both rectilinear and circular motion.
• In a sewing machine, the needle is in periodic motion whereas the wheels of the sewing machine are in circular motion. So a sewing machine displays circular and periodic motions.