NCERT Class 7 Science Chapter 13 Motion and Time

Chapter 13 - Motion and Time

Question and Answers
Question 1: Classify the following as motion along a straight line, circular or oscillatory motion:
(i) Motion of your hands while running.
(Oscillatory Motion)
(ii) Motion of a horse pulling a cart on a straight road.
(Motion along a straight line)
(iii) Motion of a child in a merry-go-round.
(Circular motion)
(iv) Motion of a child on a see-saw.
(Oscillatory motion)
(v) Motion of the hammer of an electric bell.
(Oscillatory motion)
(vi) Motion of a train on a straight bridge.
(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:
(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 taken to complete 20 oscillations = 32 seconds
Time period = Number of oscillations/Time taken
= 32/20
= 1.6 seconds
Therefore, the time period of the simple pendulum is 1.6 seconds.

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 between two stations = 240 kilometre
Time taken to reach from station 1 to station 2 = 4 hours
Speed = Distance/Time
= 240/4
= 60 km/hr
Therefore, the speed of the train is 60 km/hr.

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 of the odometer = 57321.0 km
Final reading of the odometer = 57336.0 km
Distance covered by the car = Final reading of the odometer - Initial reading of the odometer
= 57336.0 - 57321.0 = 15 km
Starting time = 08:30 AM
Ending time = 08:50 AM
Time taken to complete 15 km = Starting time - Ending time
= 08:50 - 08:30 = 20 minutes
(Speed of car in km/min)
Speed = Distance/Time
= 15/20
= 3/4 km/min
(Speed of car in km/hr)
1 hour = 60 minutes
20 minutes = 1/6 x 20
= 1/3 hr
Speed = Distance/Time
= 15/1/3 km/hr
Therefore, the speed of car in km/min is 3/4 km/min and the speed of the car in km/hr is 15/1/3 km/hr.

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 by Salma to reach school from home = 15 minutes
Speed of bicycle = 2 m/s
Distance between her school and her home = ?
15 minutes = _____ seconds
= 15 x 60
= 900 seconds
Distance = Speed x Time
= 2 x 900
= 1800 m
1800 m = ________ km
= 1800/1000
= 1.8 km
Therefore, the distance between her house and her home is 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:

(i)

(ii)

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

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:
(ii) 25 km
(When the speed of the car is 40 km/hr)
Time taken = 15 min (15/60 = 1/4 hr)
Speed = Distance/Time
Distance covered = speed x time taken
= 40 x 1/4 = 10 km
(When the speed of the Car is 60 km/ h)
Speed = Distance/Time
Distance covered = speed x time taken
= 60 x 1/4= 15 km
Total distance covered by the car = 10 + 15 = 25 km

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:
The distance covered by the blue car (as evident from the photograph) from one horizontal white strip to another, which is measured by scale is 1.2 cm.
Given that 1 cm is equivalent to 100 m. Therefore, 1.2 cm is equivalent to 120 m.
Distance travelled by the car = 120 m
Time taken to cover this distance = Time interval between the two photographs = 10 s
Speed = Distance/Time
= 120/10
= 12 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.

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

Answer: (iii)

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