MOTION AND TIME Chapter 13 Class 7 (C.B.S.E.)

Learning Objectives–

1.    To understand motion and its types

2.    To define speed

3.    To understand uniform and non-uniform motions

4.    To know the concept of time

5.     To know units of time and speed

6.  To know common modern and ancient time-measuring devices

                        7.  To know state of rest and motion

8.  To plot different distance-time relationships on graph

Introduction– In this chapter we will learn about motion, its types, fast and slow motion, speed, uniform and non-uniform motions, time and its measurement, simple pendulum, ancient time measuring devices, unit of time and speed, measuring speed and distance time graphs.

Motion and its types– The change of place with respect to time is called motion. The main types of motion are-

(a) Linear/Rectilinear/Straight line motion- The motion which occurs on a straight pathway is called rectilinear motion. In it position changes but not the direction. e. g- Motion of a car on a straight road, an apple falling from tree etc.

(b)Circular motion- The motion which occurs on circular pathway is called circular motion. In it position as well as direction change. e.g.- Motion of a fan , Motion of a child in a merry –go-round etc.

(c)  Periodic motion- The motion which repeats itself after a fixed interval of time is called periodic motion. E.g.- Motion of arms of a clock, motion of the Earth around the Sun etc.

(d)Oscillatory motion- The to and fro motion along the same pathway is called oscillatory motion. E.g.- Motion of pendulum of clock, Motion of a child on a swing etc.

                                              Wall Clock with Pendulum

The slow and fast motions are relative terms to each other.  When an object covers more distance in less time than the other object, it is called fast and other object will be called slow. If time duration is kept fixed, the slower object will be behind the faster object.

Let us take an example

Car 1……………………………………Car 1 (after 10 Minutes)

Car 2……………………………………………………….Car 2 (after 10 Minutes)

Obviously,   Car 2 is moving faster than Car 1.

Speed- The slow or fast moving object can be described easily by its speed. The speed of slow moving object will be less as compared to the faster one.

The ratio of distance covered by an object and time taken in it is called speed of an object. Simply, distance by time is called speed.

Or,                s=d/t

Basic or S.I. Unit of Speed is meter per second (m/s). The higher unit is Kilometer per hour (Km/h).

When an object moves along a straight pathway with constant speed is said to be in uniform motion. In uniform motion, the actual speed and average speed will be same.

In daily life hardly any object moving with uniform motion for long duration. The motion which changes its speed on a straight pathway is said to be in non-uniform motion. In non-uniform motion, the actual speed and average speed are different.   

Average speed– When speed is different in different time of travel average speed is calculated. It is calculated by total distance traveled divided by total time taken.  

The interval between two instances is called time. e.g.- The interval between one sunrise to the next sunrise is a time called a day. The interval between one rotation to the next by the Earth on its axis is a time called a solar day. The interval between one new moon to the next is a time called a month. The interval between one revolution to the next by the earth is a time called a year. So, a day, a solar day, a month, a year denotes time. For measuring shorter time intervals second, minute and hour are used.

Time is measured by devices like watch and wall clock. Both these use periodic motion to measure the time. In a wall clock the pendulum shows periodic motion to measure time. Let’s understand the motion of a simple pendulum.

                                                 

                   Table clock                                           Digital Clocks

                                                      

In ancient time the Sun dial, The Sand Clock and water clock were used to know the time.

                                                
Jantar Mantar: Delhi (Photographs Courtesy, NCERT), Sand Clock, Water Clock

Simple Pendulum–

First of all Galileo Galilei (A.D. 1564-1642) did experiments on simple pendulum. A simple pendulum is a small metallic ball suspended by a thread from a rigid point. The small metallic ball is called bob of the pendulum.

When the bob is released from after pulling it slightly to one side, it starts moving to one side. The to and fro motion of the simple pendulum which repeats itself after a fixed interval of time is called periodic or oscillatory motion. The pendulum completes one oscillation when it moves from A–> O–>B–>O–>A. or O–>A–>O–>B–>O  or O–>B–>O–>A–>O or B–> O–>A–>O–>B. The time taken by simple pendulum to complete one oscillation is called ‘time period’.  

                                              Length of string = 100 cm or 1m

The basic unit of time is called second. It symbol is ‘s’ or ‘sec’. Minute (m) and hour (hr) are larger units of time. To measure time smaller than second ‘atomic clocks’ are used. Atomic clock measures 10 millionth of a second. There are clocks which measures time in microseconds (one millionth of a second) and nanoseconds (one billionth of a second). Centuries, millenniums, millions and billions of years are also used to express historical and evolutionary events.

Speed is the ration of distance and time. Therefore, the basic unit of speed is m/s. It can also be written in m/min or km/hr. The symbol of any unit is always written in singular. e. g- 21 km and not 21 kms.

 If the value of distance covered by an object and time taken is known to us, the value of speed can be deduced by the formula-

We can find distance if the value of speed and time is known to us.                            

Similarly we can find time if the value of speed and distance is known to us.           

On the dashboard of vehicles like car, bus two meter dials in front of driver remain fitted. One of them is a speedometer and another one is an odometer. Speedometer records the speed in km/h and odometer records the distance traveled in km.

                                          Speedometer                      Odometer

 Distance-Time Graph

We use different types of graphs to represent data or values in an organized way. The graph shows relationship between two groups of measurements or quantities.

In a cricket match the runs scored by a team in each over in an innings can be easily shown by a bar graph.    

A Pie chart is a graphical representation in form of divisions or different percentages. What is the percentage of anything out of 100 %. So, pie chart is a division of whole.

The line graphs show relationships in form of a line. The distance-time graphs are line graphs. When the speed of vehicle is constant then the distance time graph will be a straight line. When the speed of vehicle keeps on changing then the distance time graphs can be of various shapes. 

How to Draw Line graph-The X-axis (horizontal) and Y-Axis (vertical) of a graph paper are used to put the data. The first coordinates have all the positive values; so, it is taken to plot the graph. On X-Axis the first kind of measurement data is kept and on Y-Axis second kind of measurement data is kept. On matching the values on X-axis and Y-axis, a relationship in form of line is obtained.

Example 1– A vehicle is running 2 Km in each minute. The graph will show a constant speed as like the data.

S.N.	Time (In Per Half hour)	Distance (In Km)
1	2:00 PM	00 Km
2	2:30 PM	20 Km
3	3:00 PM	40 Km
4	3:30 PM	60 Km
5	4:00 PM	80 Km

S.N. of Experiment	Time taken  To complete 20 Oscillations (in s)	Calculation	Time Period ( in s)
1.	44 s	44 s/ 20	2.2 s
2.	56 s	56 s/ 20	2.8 s
3.	68 s	68 s/ 20	3.4 s
4.

S.N.	Distance covered by the ball (in m)	Time taken  (in s)	Speed=distance/Time taken             = m/s
1.	4.2 m	8 s	4.2m/8s =0.52 m/s
2.	3.8 m	6 s	3.8 m/6s=0.63 m/s
3.