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Chapter 1: Nutrition in Plants

Chapter 2: Nutrition in Animals

Chapter 3: Fibre to Fabric

Chapter 4:Heat

Chapter 5: Acids, Bases and Salts

Chapter 6: Physical and Chemical Changes

Chapter 7: Weather, Climate and Adaptations of Animals to Climate

Chapter 8: Winds, Storms and Cyclones

Chapter 9:Soil

Chapter 10: Respiration in Organisms

Chapter 11:Transportation in Animals and Plants

Chapter 12:Reproduction in Plants

Chapter 13:Motion and Timers

Chapter 14: Electric Current and Its Effects

Chapter 15:Light

Chapter 16:Water: A Precious Resource

Chapter 17: Forests: Our Lifeline

Chapter 18: Wastewater Story

NCERT Solutions for class 7 science chapter 13

Motion and Timers

NCERT Solutions for Class 7 Science chapter 13 Exercise Question

Question 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 on a see-saw.

(v) Motion of the hammer of an electric bell.

(vi) Motion of a train on a straight bridge.

Answer

i) oscillatory

ii) Motion along a straight line

iii) Circular motion

iv) Oscillatory motion

v) Oscillatory motion

vi) Motion along a straight line.

Question 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 kilometres.

(iv) The time period of a given pendulum is constant.

(v) The speed of a train is expressed in m/h.

Answer

Incorrect statements are:

(ii) Every object moves with a constant speed.

(iv) The time period of a given pendulum is constant.

(v) The speed of a train is expressed in m/h.

Question 3
A simple pendulum takes 32 s to complete 20 oscillations. What is the time period of the pendulum?

Answer

Number of oscillations = 20

time = 32 s

Time period =
Total time / Number of oscillations
=
32 / 20
= 1.6s

Question 4
The distance between two stations is 240 km. A train takes 4 hours to cover this distance. Calculate the speed of the train.

Answer

Distance s = 240 km

Total time = 4 hr

Speed =
Distance / total time
=
240 / 4
= 60 km/h

Question 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 08:50 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.

Answer

Initial reading = 57321.0

Final reading = 57336.0

time taken by car = 20 min

Distance covered by the car = Final reading – Initial reading

= 57336.0 – 57321.0 = 15 km

Speed =
Distance / total time
=
15 / 20
= 0.75km/min Now in km/hr
1 min =
1 / 60
h
so 0.75km/min = 0.75 x 60 = 45 km/h

Question 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.

Answer

Time taken = 15 mins= 15 x 60 = 900 s

Speed of bicycle= 2m/s

Speed =
Distance / total time

Distance = speed x time taken

= 2 x 900 = 1800 m

1m =
1 / 1000
km = 1.8 km

Question 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.

Answer

Question 8
Which of the following relations is correct?

(i) Speed = Distance × Time

(ii) Speed = Distance/Time

(iii) Speed = Time/Distance

(iv) Speed = 1/Distance x Time

Answer

(ii) Speed = Distance/Time

Question 9
. The basic unit of speed is:

(i) km/min

(ii) m/min

(iii) km/h

(iv) m/s

Answer

(iv) m/s

Question 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

Answer

Speed = 40 km/h

Time taken = 15 min = 15/60 = 0.25 h

Speed =
Distance / total time

Distance covered (when speed is 40km/h) = speed x time

= 40 x 0.25 = 10 km

When the speed of the Car is 60 km/ h

Speed =
Distance / total time

Distance covered (when speed is 60km/h) = speed x time

= 60 x 0.25= 15 km

Total distance covered by the car =Distance covered (when speed is 40km/h) + Distance covered (when speed is 60km/h)

= 10 + 15 = 25 km

So (ii) 25 km is correct

Question 11
Suppose the two photographs, shown in Fig. 13.1 and Fig. 13.2, had been taken at an interval of 10 seconds. If a distance of 100 metres is shown by 1 cm in these photographs, calculate the speed of the fastest car.

Answer
Given 1cm = 100m
Distance travelled by the blue car in the picture= 2 cm
So 2 x 100 = 200 m

Time taken = 10sec
Speed of the blue car =
Distance / total time

=
200 / 10
=20m/s
So Speed of the blue car is 20 m/s

Question 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?

Answer

Vehicle A is moving faster than vehicle B because large distance coverd by A in less time than B .

Question 13
Which of the following distance-time graphs shows a truck moving with speed which is not constant?



Answer

( iii)