9th Class Science Time and Motion Motion

         Introduction

In everyday life we always come across some objects which are at rest and some objects in motion. The branch of Physics, which deals with the behavior of moving objects, is known as mechanics. Mechanics is further divided into two sections namely Kinematics and Dynamics. Kinematics deals with the study of motion without taking into account the cause of motion, while Dynamics is concerned with the cause of motion, namely force. This chapter covers only the different aspects of motion without considering the cause of motion.

         Motion

A body is said to be in motion if its position changes continuously, with respect to a stationary object, taken as reference point, with the passage of time.

When we are sitting in a moving bus, we observe the continuous changes of position with respect to stationary object like houses, trees, lamp posts, etc. We say that the bus is moving or that the bus is in motion.

          Motion and rest are Relative Terms

Suppose you are travelling in a train, which is in motion.

Observe 1: Is there any change in your position with respect to your co passengers? Answer: No

Observe 2: Is there any change of scene you view through the window?

Answer: Yes

Thus, we can say that an object is in, motion, if it changes its position continuously with respect to its surroundings in a given time. Since the position of desk and benches does not changes with time, hence we can say that they are not in motion, i.e. they are stationary. You must have observed sky at night: the position of stars changes with the passage of time, whereas the position of house and our surrounding remains the same. But in reality the earth is also moving, that means all the objects on the surface of the earth are also in motion. Thus, an object which appears to be at rest, may actually be in motion. Therefore, motion and rest are relative terms. Hence, to describe the motion of an object we have to specify how its position changes with respect to a fixed point called the reference point or origin.

Without frame of reference we cannot specify whether an object is in motion or at rest. A frame of reference is another object or scene with respect to which we compare an object's position.

          Types of Motion

• Translatory motion
• Circular motion
• Rotatory motion and
• Vibratory motion

              Translatory Motion

The continuous change of position of an object from one point to another in space is called the translatory motion. This motion may be linear or curvilinear.

              Linear Motion

The motion of an object along a straight line is called linear or rectilinear motion.

A car moving on a straight road

                              Rectilinear Motion                

           Curvilinear Motion

Motion along a curved path is called curvilinear motion..

A car negotiating a curve or turn..

            Circular Motion

The motion of an object on a circular path is called circular motion.

An athlete running on a circular track.

             Rotatory Motion

The motion in which objects rotate on a given fixed given point is called the rotatory motion.

Motion of a top, motion of a fan in the celling etc.

             Vibratory Motion

To and fro motion of the body about the mean position is called the vibratory motion.

The motion of the pendulum about the mean position.

Suppose an auto, starting from an auto stand X, travels 1000 m to reach Anand Vihar Bus terminal. The distance covered by an auto is 1000 m. Now if the same auto returns to the auto stand X from Anand Vihar bus terminal with another passenger, then what is the distance covered by an auto during the return trip? The distance covered during the return trip is 1000 m. But the total distance covered by the an auto during the trip from X to Anand Vihar Bus terminal and then back to X is 1000 m + 1000 m = 2000 m.

Thus, the distance traveled by a body is the actual length of the path covered by a moving body, irrespective of the direction in which the body travels.

The shortest distance between the initial and final position of a moving object in a particular direction is called its displacement. It is path independent.

(i) Distance is a scalar quantity: (It has magnitude only, and has no specified directions).

(ii) Displacement is a vector quantity (It has magnitude as well as a specified direction).