NCERT Solutions for Class 7 Science Chapter 13 Motion and Time

1. Classify the following as motion along a straight line, circular or oscillatory motion:

(i) Motion of your hands while running.

Answer: Motion of your hands while running is an oscillatory motion. This is because, while running our hands move in forward and backward continuously.

(ii) Motion of a horse pulling a cart on a straight road.

Answer: It is a straight line motion

(iii) Motion of a child in a merry-go-round.

Answer: It is a circular motion. This is because, merry go round has circular motion, therefore, child in merry go round will have circular motion.

(iv) Motion of a child on a see-saw.

Answer: It is an oscillatory motion. This is because, child playing in the see saw moves up and down continuously, thereby, showing an oscillatory motion.

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

Answer: It is an oscillatory motion, in which hammer moves forward and backward.

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

Answer: It is a straight line motion

2. Which of the following are not correct?

(i) The basic unit of time is second.

Answer: Correct. SI unit of time is second

(ii) Every object moves with a constant speed.

Answer: Incorrect. Object can move with constant speed or changing speed.

(iii) Distances between two cities are measured in kilometres.

Answer: Correct. Large distances are usually measured in kilometer.

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

Answer: Incorrect. Time period depends on length of thread attached to pendulum.

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

Answer: Incorrect. Speed of train can be measured in m / s or in km / h.

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

Answer: Given: 1) Time required for 20 oscillation is 32 seconds. 2) Number of oscillation is 20.

We know that,

time period = total time taken / number of oscillations = 32 / 20 = 1.6 s

Therefore, time period of simple pendulum is 1.6 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.

Answer: Given: 1) Distance between two stations = 240 km. 2) Time taken by train = 4 hours

Speed = distance / time = 240 / 4 = 60 km / h

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: Given: 1) Initial reading in car’s odometer = 57321.0 km. 2) Final reading in car’s odometer 57336.0.

Distance covered = Final reading – initial reading = 57336.0 – 57321.0 = 15 km

Time taken by car to travel 08:50 – 08:30 = 20 min

Also, 60 min = 1 hr. Therefore, 20 min = (1/60) x 20 = 1/3 hr

Time taken by car in hours is 1/3 hr

We know that,

Speed = distance/ time = 15 / (1/3)= 15 x 3 = 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: Given: 1) time required by Salma to reach school = 15 min. 2) Speed of bicycle = 2 m/s.

Lets convert time from minutes to seconds. Therefore 15 min= 15 x 60 s= 900s

We know that, speed = distance / time.

Therefore, distance = speed x time = 2 x 900 = 1800 m.

We know that,

1000 m = 1 km, lets convert distance from m to km.

Therefore, 1800 m = (1/ 1000) x 1800 = 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.

Answer:

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

9. The basic unit of speed is:

(i) km/min (ii) m/min (iii) km/h (iv) m/s

Answer: (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

Answer: In first case, speed of car is 40 km/hr in 15 min. Therefore, time taken in hours will be 15 / 60 = 0.25 hr.

As we know, speed = distance / time ,

∴ Distance D = speed x time = 40 x 0.25 = 10km

In second case, speed of car is 60 km / h in 15 min. Therefore, time taken in hours will be 15/ 60 = 0.25 hr.

We know that, speed = distance / time,

∴ Distance d = speed x time = 60 x 0.25 = 15 km

Therefore, total distance = D + d= 10 + 15 = 25km

11. Suppose the two photographs, shown in Fig., 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:

12. Fig. shows the distance-time graph for the motion of two vehicles A and B. Which one of them is moving faster?

Answer: We know that, speed = distance / time …… (1)

From (1) it can be understood that, speed increases with increase in distance at a given time. Thus, to compare distance from the figure, draw a line parallel to distance axis, as show in figure below. Now, it can be observed that, for any time t, distance covered by vehicle A is greater than distance covered by vehicle B. Thus, it can be concluded that, vehicle is moving faster than vehicle B.

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Download free NCERT textbook class 7 Science – Chapter 13 Motion and Time