Motion

Introduction

     In the physical world, one of the most common phenomena is motion. The branch of Physics, which deals with the behaviour 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 and Rest

     Imagine you are travelling in a moving train. Do you observe any change in your position with respect to your co-passengers? Is there any change of scene you view through the window? The change of scene indicates that the train is moving. That is, an object is said to be in motion if it changes its position with respect to its surroundings in a given time.

Types of Motion

     In translatory motion the particle moves from one point in space to another. This motion may be along a straight line or along a curved path. The motion along a straight line is called rectilinear motion.

Distance and Displacement

     Suppose a bus starting from a terminus A travels 15000 m to reach terminus B. Then the distance covered by the bus is 15000 m. Now if the bus returns to the terminus A, then what is the distance covered by the bus during the return trip? The distance covered is 15000 m.

Uniform Motion and Non-uniform Motion

     The distances covered by car A and car B with respect to time is given below.

Speed

     In an athletic meet different participants start running at the same time and cover the same distance. The person who takes the minimum time to cover the distance will be judged as the winner.

Distance-Time Table and Distance-Time Graph

     Mr.X is traveling from terminus A to terminus B in a bus and records his observation.

Velocity

     Velocity is defined as the distance covered by a moving object in a particular direction in unit time or speed in a particular direction.

Acceleration

     All of us know that a car moving on road does not have a uniform velocity. Either the speed or the direction changes. Whenever a vehicle is speeding i.e., when the speed is increased we say that the vehicle is accelerating.

Uniform Velocity and Non-uniform Velocity

     Imagine that two athletes Tom and Harry are running with a uniform speed of 5 m/s on a straight line and circular track respectively. For a layman both Tom and Harry are moving with uniform velocity but for a physicist only Tom is running with uniform velocity because there is no change in his speed and direction of motion.

Uniform Circular Motion

     In the example discussed under the topic uniform and non-uniform motion we have classified motion along circular track as an example of non-uniform motion.

Velocity-Time Graph

     The variation of velocity with time can be represented graphically to calculate acceleration exactly like we calculated speed from distance-time graph.

Equations of Motion

     The variable quantities in a uniformly accelerated rectilinear motion are time, speed, distance covered and acceleration. Simple relations exist between these quantities. These relations are expressed in terms of equations called equations of motion.

Summary

     Motion: Whenever a body changes its position continuously with respect to the position of other bodies around it then it is said to be in motion.