Solution of Chapter 13. Motion and Time (NCERT Science Book)

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.

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 kilometers.

iv. The time period of a given pendulum is not constant.

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

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

The distance between two stations is 240 km. A train takes 4 hours to cover this distance. Calculate the speed of the train.

The odometer of a car reads 57,321.0 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 57,336.0 km? Calculate the speed of the car in km/min during this time. Express the speed in km/h also.

Salma takes 15 minutes from her house to reach her school on a bicycle. If the bicycle has speed of 2m/s, calculate the distance between her house and the school.

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

Which of the following relations is correct?

i. Speed = Distance × Time

ii. Speed=

iii. Speed=

iv. Speed=

The basic unit of speed is:

i. km/min ii. m/min

iii. km/h iv. m/s

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

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 blue car.

Fig. 13.15 shows the distance-time graph for the motion of two vehicles A and B. Which one of them is moving faster?

Distance – Time graph for a motion of two cars

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

(i)

(ii)

(iii)

(iv)

You can make your own sundial and use it to mark the time of the day at your place. First of all find the latitude of your city with the help of an atlas. Cut out a triangular piece of a cardboard such that its one angle is equal to the latitude of your place and the angle opposite to it is a right angle. Fix this piece, called gnomon, vertically along a diameter of a circular board a shown in Fig. 13.16. One way to fix the gnomon could be to make a groove along a diameter on the circular board.

Next, select an open space, which receives sunlight for most of the day. Mark a line on the ground along the North-South direction. Place the sundial in the sun as shown in Fig. 13.16. Mark the position of the tip of the shadow of the gnomon on the circular board as early in the day as possible, say 8:00 AM. Mark the position of the tip of the shadow every hour throughout the day. Draw lines to connect each point marked by you with the centre of the base of the gnomon as shown in Fig. 13.16. Extend the lines on the circular board up to its periphery. You can use this sundial to read the time of the day at your place. Remember that the gnomon should always be placed in the North-South direction as shown in Fig. 13.16.

Collect information about time measuring devices that were used in the ancient times in different parts of the world. Prepare a brief write up on each one of them. The write up may include the name of the device, the place of its origin, the period when it was used, the unit in which the time was measured by it and a drawing or a photograph of the device, if available.

Make a model of a sand clock which can measure a time interval of 2 minutes (Fig. 13.17).

You can perform an interesting activity when you visit a park to ride a swing. You will require a watch. Make the swing oscillate without anyone sitting on it. Find its time period in the same way as you did for the pendulum. Make sure that there are no jerks in the motion of the swing. Ask one of your friends to sit on the swing. Push it once and let it swing naturally. Again, measure its time period. Repeat the activity with different persons sitting on the swing. Compare the time period of the swing measured in different cases. What conclusions do you draw from this activity?