7th Class Science Time and Motion Rest and Motion

       Rest and Motion

As we know, in this universe nothing is fixed at a single place. The galaxies are moving away from each other and universe is spreading. Therefore, everything is changing its position with respect to the time. Rest and Motion is the branch of Physics that deals with the position of a body. We will study the two different physical quantities "motion and rest" to describe the position of a body.

In our day to day life, we often come across the terms rest and motion. In actual, in this universe nothing is in the rest. Earth moves around the Sun; Sun moves around the centre of the galaxy. Galaxies are moving away from each other and so on. The objects, which we see as in rest, actual are in motion. As the Earth is moving around the Sun, objects on the Earth are also moving along.

However we use the terms rest and motion in physics and in our day to day life.

The rest and motion are actually two different physical quantities, which are used to describe the position of an object with respect to the time and a reference point. If the object's position has changed with respect to the reference point, the object is said to be in the motion and if the object's position has not changed with respect to the reference point, the object is said to be in the rest.

Note: A reference point is essential to describe whether the object is in motion or in the rest and different reference point may give different result.

Let us describe the position of a person who is traveling from Delhi to Shimla by train. If we take Delhi station as a reference point, then he is in the motion; as in the beginning he was on the station and now he is away of the station. And if we take window of the train as a reference point, then the person is still at the same distance from the window as on Delhi station. So the person is in the rest. Therefore, whether an object is in rest or in motion depends on the reference point that the observer choose to describe object's position.

Let us take one more example:

Suppose an object is at the distance of 5 m in east direction from a pole fixed on the ground. The object is said to be in motion:

• If the object's distance from the pole increases or decreases.
• The distance remains fixed and direction of the object changes.

And the object is in the rest:

If neither distance nor direction of the object from the pole changes.

          Classification of Motion

There are four types of motion. Let us study them in detail  

          Rectilinear Motion

When a body travels on a straight path, the body is said to be in rectilinear motion.

           Circular Motion

When a body travels in a circular path , the body is said to be in circular motion.

           Rotatory Motion

When a body moves around a fixed axis, it is said to be in rotatory motion.

           Periodic Motion

When a body repeats its motion, after a regular intervals of time, it is said to be in periodic motion.

       When a body moves around a fixed axis, the body is in..........       

(a) Rectilinear motion

(b) Circular motion

(c) Rotatory motion

(d) Periodic motion

(e) None of these

Answer: (c)

         Motion of a car on a straight path is an example of.............

(a) Rectilinear motion

(b) Circular motion

(c) Rotatory motion

(d) Periodic motion

(e) None of these

Answer: (a)