**Class 7 | Science | Chapter 13 | Motion and Time | NCERT Solutions**

**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 1**

(i) Motion of your hands while running- **Oscillatory motion**

(ii) Motion of a horse pulling a cart on a straight road- **Straight line motion**

(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-**Straight line motion**

**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 2 **

(i) The basic unit of time is second-**Correct**

(ii) Every object moves with a constant speed- **Not correct**

(iii) Distances between two cities are measured in kilometres- **Correct**

(iv) The time period of a given pendulum is constant- **Not correct**

(v) The speed of a train is expressed in m/h- **Not correct**

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

**Answer 3** The time period to complete one oscillation is known as time period of the pendulum.

Time period= Total time taken / Number of oscillations

Time period= 32 / 20 = 1.6 sec

**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 4** Speed= Distance travelled / Time taken

Speed= 240/4 = 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 5** Distance covered by car = 57336 – 57321 = 15 km

Time taken between 08:30 AM to 08:50 AM = 20 minutes= 20/60 = 1/3 hrs

Speed= Distance travelled / Time taken

Speed= 15 Km/ (1/3) hr = 45 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 6** Speed= 2 m/s

Time taken= 15 min= 15 × 60 = 900 sec

Distance= speed × time= 2 × 900 = 1800 m = 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 7** (i) A car moving with a constant speed.

(ii) A car parked on a side road.

**Question 8.** Which of the following relations is correct?

**Answer 8 **

**Question 9.** The basic unit of speed is:

(i) km/min (ii) m/min

(iii) km/h (iv) m/s

**Answer 9** (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 10** (ii) 25 km

**Case 1 **

speed = 40 Km/h

Time= 15 min= 15/60 hr

Distance= Speed × Time = 40 × (15/60)

Distance= 10 Km

**Case 2**

speed = 60 Km/h

Time= 15 min= 15/60 hr

Distance= Speed × Time = 60 × (15/60)

Distance= 15 Km

Total distance= 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 11** From the figures we conclude that distance covered by blue car is 2 cm.

The distance covered = 2 × 100 =200 m

Time taken= 10 seconds

Speed = Distance/ Time

Speed= 200/ 10 = 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?

Fig. 13.15 Distance-time graph for the motion of two cars

**Answer 12** Vehicle A is travelling longer distance in lesser time as compared to vehicle B.So, vehicle A is moving faster.

**Question 13.** Which of the following distance-time graphs shows a truck moving with

speed which is not constant?

**Answer 13** (iii) Graph is not a straight line , so it shows a truck moving with speed which is not constant.