MOTION
AND TIME Chapter 13 Class VII
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.
Slow and Fast motion-
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)
At
Rest
In Motion
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.
Speed=Total distance
covered/Total time taken
Or,
s=d/t
Basic
or S.I. Unit of Speed is meter per second (m/s). The higher unit is Kilometre
per hour (Km/h).
Uniform and Non-uniform motions-
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.
Measurement of Time-
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
Wrist Watch Wall Clock with Pendulum
In
ancient time the Sun dial, The Sand Clock and water clock were used to know the
time.
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’.
A Simple Pendulum Different Positions of
the bob of
an oscillating
simple pendulum
Activity - To Calculate the time
period of a simple pendulum
Length
of string = 100 cm or 1m
S. No.
|
Time taken for 20 oscillations
|
Time period
|
1
|
28 s
|
1.4
s
|
2
|
42 s
|
2.1
s
|
3
|
38 s
|
1.9
s
|
Units of Time and Speed
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.
Measuring speed
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-
Speed = Distance/Time
We
can find distance if the value of speed and time is known to us.
Distance= speed X Time
Similarly
we can find time if the value of speed and distance is known to us.
Time= Distance/Speed
S.N.
|
Name of Animal
|
Speed in km/h
|
Speed in m/s
|
|
1
|
Falcon
|
320
|
320x 1000
60 x 60
|
88.8
|
2
|
Cheetah
|
112
|
-
|
31.1
|
3
|
Rabbit
|
56
|
-
|
15.5
|
4
|
Squirrel
|
19
|
-
|
5.2
|
5
|
Human
|
40
|
-
|
11.1
|
6
|
Giant Tortoise
|
0.27
|
-
|
0.075
|
7
|
Snail
|
0.05
|
-
|
0.013
|
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
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 Minute)
|
Distance (In Km)
|
1
|
0
|
00
|
2
|
1
|
02
|
2
|
2
|
04
|
3
|
3
|
06
|
4
|
4
|
08
|
5
|
5
|
10
|
6
|
6
|
12
|
7
|
7
|
14
|
The scales used for representing motion of a car can be-
Time: 1min= 1cm on X-Axis
Distance: 2km= 1cm on Y-Axis
Example 2 –A bus starts journey at 2
P.M. and covers a distance of 80 Km in 2 Hours at constant speed.
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
|
There
are basic types of line graphs in four situations-
(a)
At rest
(b) Constant speed
(c)
Acceleration (d) Retardation (De-acceleration)
Points to Remember-
1. Motion is change of place with respect to
time. Linear, Circular, Periodic and oscillatory are main examples of motion.
2. The distance moved by an object per unit
time is called speed.
3. Speed = Distance/ Time
Unit= m/s
4. By comparing the speed of two moving
objects we can say which is faster than other.
5. Measurement
of time occurs by modern devices like watches and clocks. They are based on oscillatory
motion of simple pendulum.
6. Ancient devices to measure time are sundial
and water clock.
7. The
time taken to complete one oscillation is called its time period.
8. Speedometer measures speed in Km/h and
Odometer measures distance covered by the vehicle.
9. Distance time graphs show the motion of
object. For an object moving with constant speed the distance time graph will
be a straight line.
NAME OF ACTIVITY- To Know the Time Period of a Simple Pendulum
REQUIREMENTS-Metallic bob or Small
stone, String or thread, Stop watch or Wrist watch
THEORY-
A simple pendulum has a string and a bob suspended from any rigid point. It is
used to calculate the time period.
PROCEDURE-
A simple pendulum of length 1m (100 cm)
was firmly suspended from a rigid
point. The bob is taken at one of extreme position away from the mean position
of simple pendulum and released carefully without pushing. The number of
oscillations completed in 20 seconds was recorded.
A
simple pendulum Different
Positions of the bob of
an
oscillating simple pendulum
Where, A= Extreme Point A
B= Extreme Point B
O=
Mean Position
The experiment is repeated 3
times. The time period of each oscillation can be calculated by dividing the total
time with 20 i.e. number of oscillations we have assumed to be completed in
each case.
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.
|
Fourth experiment is for you.
CONCLUSION-The measurement of time
period of an oscillation can be done
by dividing total time taken in completing 20 oscillations (supposed).
Note-
Number of oscillations per second is called frequency.
On increasing
the length of string/ thread the time period
increases and
frequency reduces. But, on reducing the length
of string the time period decreases and
frequency increases.
NAME OF ACTIVITY- To measure the speed
REQUIREMENTS – A ball, Measuring tape
and Stop watch or Wrist watch
THEORY- Speed is the ratio of distance
to the time. Speed =Distance/ Time. It’s unit is m/s.
PROCEDURE- The ball was moved on a flat
surface having marking of the length. In each case the distance covered before
stoppage and time taken was noted. By the formula of speed, the speed was
calculated.
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.
|
Third activity is for you.
CHECK YOUR PROGRESS- Time=
35 Minutes Marks= 3 x 5= 15
Q.1 What is speed?
What is its S.I. unit?
Q.2 Differentiate
between uniform and non-uniform motions.
Q.3 What is
oscillation? Show the oscillation positions of a simple pendulum.
Q.4 If a simple
pendulum completes 58 oscillations in 20 s, what is the time period?
Q.5 Draw distance-time
graphs for-
(i) A vehicle in rest
(ii) A vehicle in constant motion
CO-CURRICULAR PROJECTS-
1. Calculating the speed of 4 students in S.I. unit and changing in Km/h.
2. Preparation of a sundial and it's working.
3. Development periods of ancient and modern time measuring devices.
4. Drawing of distance time graphs in various situations of real life.
EXERCISE QUESTIONS SOLVED
Q.1 Classify the following as motion along
a straight line, circular or oscillatory motion:
(i) Motion of your hands while running.
(ii) Motion of a horse pulling a cart on a
straight road.
(iii) Motion of a child in a
merry-go-round.
(iv) Motion of a child in see saw
(v) Motion of the hammer of an electric bell
Ans-
(i) Oscillatory motion (ii) Straight line motion
(iii) Circular motion (iv) Oscillatory motion
(v) Oscillatory motion (vi) Straight line motion
Q.2 Which of the following are not correct?
(i) The basic unit of time is second.
(ii) Every object moves with a constant
speed.
(iii) Distances between two cities are
measured in kilometers.
(iv) The time period of a given pendulum
is not constant.
(v) The speed of a train is expressed in
m/h.
Ans-
(ii), (iv) and (v) are not correct.
Q.3 A simple pendulum takes 32 s to
complete 20 oscillations. What is the time period of the pendulum?
Ans-
Given that, Number of oscillations = 20
Time taken = 32 s
So, Time period = Time taken/Number
of oscillations
=
32/20 =1.6 s
Q.4 The distance between two stations is
240 km. A train takes 4 hours to cover this distance. Calculate the speed of
the train.
Ans-Given
that, the distance covered = 240 km.
Time taken = 4 h
So, Speed = Total distance covered/Total
time taken
= 240/4 =60 km/h
Q.5 The odometer of a car reads 57321.0 km
when the clock shows the time 08:30 AM. What is the distance moved by the car,
if at 8:50 AM, the odometer reading has changed to 57336.0 km? Calculate the
speed of the car in km/min during this time. Express the speed in km/h also.
Ans-
Given that, reading of odometer at 08:30 AM = 57321.0 km
reading of odometer at
08:30 AM = 57336.0 km
So, Total distance covered by the
car= 57336.0-57321.0 km
= 15 km
Total time taken = 08:50 AM – 08:30
AM= 20 min
= 1/60 x 20 h
=1/3 h
So, Speed of the Car =15 km/20 min = 0.75
km/min
Or, speed of the car in km/h = 15 km/(1/3h) = 15 x 3 km/h
= 45km/h
Q.6 Salma takes 15 minutes from her house
to reach her school on a bicycle. If the bicycle has a speed of 2 m/s,
calculate the distance between her house and the school.
Ans-
Given that, Time taken = 15 min = 15 x 60 =900 s
Speed of
bicycle = 2 m/s
˚˳ ˚ Speed =Total distance
covered/ Total time taken
Or, Total
distance covered = Speed x total time taken
= 2 m/s x 900 s
=1800 m = 1.8 km
Q.7 Show the shape of the distance-time
graph for the motion in the following cases: (i) A car moving with a constant
speed.
(ii) A car parked on a side road.
Ans-
(i) A car moving with a constant speed covers equal distance in equal interval
of time. So, the distance-time graph for such motion looks like as this.
Figure
Uniform /Constant Speed
(ii) A
car parked on a side road will not change its position with change in time. So,
the distance-time graph for a parked non-moving car looks like as this.
Figure Object at rest
Q.8 Which
of the following relations is correct?
(i) Speed = Distance x Time
(ii) Speed = Distance / Time
(iii) Speed = Time / Distance
(iv) Speed = 1/Distance x Time
Ans- (ii) Speed = Distance/ Time
Q.9
The basic unit of speed is:
(i) km / min (ii) m /min
(iii) Km / h (iv) m
/s
Ans-(iv) m /s
Q.10
A car moves with a speed of 40 km/h for 15 minutes and then with a speed of 60
km/h for the next 15 minutes. The total distance covered by the car is:
(i) 100 km
(ii) 25 km
(iii) 15 km
(iv) 10 km
Ans
Total time =15 min + 15 minutes =30 minutes
or ½ hr.
40 + 60
km/h
Average
Speed= --------------------- = 50 km/h
2
Total distance= 50km/h X 1/2h = 25 km.
Q.11
Suppose the two photographs, shown in fig. 13.1 and 13.2, had been taken at an
interval of 10 seconds. If a distance of 100 meters is shown by 1 cm in these
photographs, calculate the speed of the faster car.
Fig 13.1 Fig 13.2
(Photographs Courtesy, NCERT)
Ans- Here green car
and blue car are moving faster than white car. The distance covered by the blue
car from one white strip to the next is 1.3 cm approximately.
Given that, 1 cm = 100 m So, 1.3 cm = 1.3 x 100 m = 130 m
Also,
the time interval between two photographs= 10 s
So,
Speed = total distance covered/ Total time taken
= 130 m/ 10 s
= 13 m/ s
Q.12
Fig.13.15 shows the distance- time graph for the motion of two vehicles A and
B. Which one of them is moving faster?
Ans- On comparing the distance covered by
car A with car B for given points of time, it can be concluded that car A is
moving faster.
Q.13
Which of the following distance-time graphs shows a truck moving with speed
which is not constant?
Ans- (iii) is not a constant motion. It is
increasing (or ascending).
Useful links for reference-
1. https://www.youtube.com/watch?v=NM6Q958lpJc
5.
www.askiitians.com
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