NCERT Solutions for Class 7 Science Chapter 13 Motion And Time

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 on a see-saw.
(v) Motion of the hammer of an electric bell.
(vi) Motion of a train on a straight bridge.

Answer :

(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

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 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.

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

Answer :

Number of oscillations = 20

Total time taken to complete 20 oscillations = 32 s

Time period = \(\frac{total time taken}{number of oscillation} = \frac {32}{20} \) = 1.6 seconds

Therefore, the time period of the pendulum is 1.6 seconds.

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.

Answer :

Distance = 240 km

Time taken = 4 hours

Speed =Distance covered/time taken = 240km/4 hours =60 km/h

Therefore , the speed of train is 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 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

Final reading of the odometer = 57336.0

Distance covered by the car = Final reading of the odometer - Initial reading of the odometer = 57336.0 km – 57321 km =15 km

Speed =\(\frac{15km}{20 min} =\frac{3}{4} km/min \)

Speed in km/hr =\(\frac{3}{4} \) × 60 =(15 x 3) km/hr =45 km/hr.

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/min, calculate the distance between her house and the school.

Answer :

Time taken by Salma to reach her school by bicycle = 15 mins= 15 x 60 = 900 s

Speed = 2 m/s

Distance = speed x time = 2 x 900= 1800 m

Also 1 km= 1000 m

Therefore 1800x (1/1000) = 1.8 km

Therefore, the distance between her house and the school is 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

Answer :

Q.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

Answer :

(ii) Speed = Distance/Time

Q.9 The basic unit of speed is:
(i) km/min
(ii) m/min
(iii) km/h
(iv) m/s

Answer :

(iii) km/h

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

Answer :

(ii) 25 km

Solution :

When the speed of the car is 40 km/h

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

Speed formula : Distance covered d1 = speed x time taken = 40 x 0.25 = 10 kms

When the speed of the car is 60 km/ h

Speed formula : Distance covered d2 = speed x time taken = 60 x 0.25= 15 kms

Total distance covered by the car = d1 + d2 = 10 + 15 = 25 kms

Q.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 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 taken to cover this distance = Time interval between the two photographs = 10 s

Speed = distance/time = 120/10 = 12 m/sec

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?

Answer :

Vehicle A is moving faster.

Solution: The graph is between distance and time .

Distance = speed x time

Slope is telling about speed.

The slope for Vehcile A is more than that of Vehicle B.

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

Answer :

(iii)