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Class seven science Motion and Time NCERT Solution
Chapter 13: Motion and Time
N C E R T Questions
1. Classify the following as motion along a straight line, circular or oscillatory motion:
(i) The motion of your hands while running.
(ii) The motion of a horse pulling a cart on a straight road.
(iii) The motion of a child in a merry-go-round.
(iv) The motion of a child on a see-saw.
(v) The motion of the hammer of an electric bell.
(vi) The motion of a train on a straight bridge.
Answers
(i) oscillatory
(ii) Motion along a straight line
(iii) Circular motion
(iv) Oscillatory motion
(v) Oscillatory motion
(vi) Motion along a straight line.
2. Which of the following is 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.
Answers
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.
3. A simple pendulum takes 32 s to complete 20 oscillations. What is the time period of the pendulum?
Answers
Number of oscillations = 20
Total time is taken to complete 20 oscillations = 32 s
4. The distance between two stations is 240 km. A train takes 4 hours to cover this distance. Calculate the speed of the train.
Answers
Distance between two stations = 240 km
Total time take = 4 hrs/240 minutes
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 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.
Answers
An initial reading of the odometer = 57321.0
The final reading of the odometer = 57336.0
Distance covered by the car = Final reading of the odometer – Initial reading of the odometer
= 57336.0 – 57321.0 = 15 km
Starting time of the car is 8:30 and it stops at 8:50
Hence, time is taken by car = 20 mins
Hence, time is taken by car = 20 mins
Distance/Time
= 15/20
= 0.75 km/min
20 min = 1/60 x 20 h
=1/3h
15 km/ 1/3h
= 45 km/h
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 is taken by Salma to reach her school by bicycle
= 15 mins = 15 x 60 = 900 s
Speed of Salma’s bicycle = 2m/s
Distance covered = speed × time taken
= 2 × 900 = 1800 m
1000 m = 1 km
= 1.8 km
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.
Answers
8. Which of the following relations is correct?
(i) Speed = Distance × Time
(ii) Speed = Distance/Time
(iii) Speed = Time/Distance
(iv) Speed = 1/Distance × Time
Answers (ii) Speed = Distance/Time
9. The basic unit of speed is:
(i) km/min
(ii) m/min
(iii) km/h
(iv) m/s
Answers (iv) m/s
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
Answers (ii) 25 km
When the speed of the car is 40 km/h Time taken = 15 min = 15/60 = 0.25 h
Distance covered d1 = speed x time taken
= 40 × 0.25 = 10 km
When the speed of the car is 60 km/ h
Distance covered d2 = speed × time taken
= 60 × 0.25 = 15 km
Total distance covered by the car = d1 + d2
= 10 + 15
= 25 km
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.
Answers
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. It is given that 1 cm is equivalent to 100 m.
Therefore, 1.2 cm is equivalent to 120 m.
Distance travelled by car = 120 m
Time is taken to cover this distance = Time interval between the two photographs = 10 s
Answers Vehicle A is moving faster than vehicle B.
13. Which of the following distance-time graphs shows a truck moving with a speed which is not constant?
Answers (iii)