Class 7 Science Motion and Time Chapter Notes

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Motion and Time

3.1 Introduction

In our daily life, we see various objects in motion. The walking man, the running cars, the water flowing in lakes, flying aeroplanes etc. are all examples of objects in motion.

3.2 Rest and Motion

Rest: A body is said to be in rest if it does not change its position with respect to time with respect to another body. Example-A book lying on a table.

Motion: A body is said to be in motion if it changes its position with respect to time with respect to another body. Example-A train moving on rails, a man walking on road.

3.3 Physical quantities

Physical quantity : The quantity which can be measured is called physical quantity. In Physics, we study about physical quantity.
Types of physical quantity
i) Fundamental physical quantity
ii) Derived physical quantity

Fundamental physical quantity : The physical quantities which are independent of other physical quantities are called fundamental physical quantities. Example : Lenth, Mass, Time etc.

Derived physical quantity: The physical quantities which depend on fundamental physical quantities are called derived physical quantities. Example : Speed, Energy etc. All physical quantities can be categorised into scalars and vectors.

Scalars: The quantities which have magnitude only are called scalars. Example : Distance, Speed, Energy etc.

Vectors: The quantities which have magnitude as well as directions both are called vectors. Examples : Displacement, Velocity, Force etc. A vector is represented by putting an arrow over symbol of the phycal quantity like velocity is represented as v r , force is represented as Fr, etc

3.4 Speed
We know that some vehicles move faster than others. The distance moved by objects in a given interval of time can help us to decide which one is faster or slower.
Speed can be defined as the distance covered by an object in a unit time. Thus Speed = Total distance covered/ Total time taken
The S.I. unit of speed is m/s and it is a scalar quantity.

3.5 Velocity
The velocity of an object is the ratio of total displacement to the total time taken. Thus
Velocity =Total displacement/Total time taken
The S.I. unit of velocity is similar to that of speed as m/s but velocity is a vector quantity, because it also determines the direction of motion.

3.5 Acceleration
The acceleration of a moving object is defined as the rate of change in its velocity with time. Thus

The S.I. unit of acceleration is m/s2 and it is a vector quantity

3.6 Types of motion :

(a) Uniform Motion : If an object is moving along a straighty line with constant speed/velocity, it is said to be in uniform motion.
for Uniform motion : Velocity = constant

So, for uniform motion acceleration is 0.

Equation of motion for uniform motion :
Distance/Displacement = speed × time = velocity × time
Note: For uniform motion along a straight line, speed and velocity are same. Similary, distance
and displacement are also same for the above mentioned case.

(b) Non-uniform motion : If the velocity of an object keeps on changing with time, the motion is said to be non-uniform. Since, the velocity of the object is not constant, therefore, it possesses acceleration. So, depending on the acceleration, motion can be further of two types:

(i) Uniformly accelerated motion : Where the acceleration of the body remains constant.

(ii) Non-uniformly accelerated miotion : Where the acceleration of the body keeps on changing with time.

Equations of motion for uniformly accelerated body :
(a) Horizontal motion 

Joint all the points on the graph. This is the distnace time graph for the motion of the car. If the distance-time graph is a straight line, it indicates that the object is moving with a constant speed. However, if the speed of the object keeps changing, the graph can be of any shape.

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