# NCERT Solutions for Class 7 Science Chapter 13

## Motion and Time Class 7

### Exercise : Solutions of Questions on Page Number : 156

Q1 :

Classify the following as motion along a straight line, circular oroscillatory 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.

(i) Oscillatory motion

While running, the hands move to and fro and repeat their motion after a given interval of time. Hence, it is an oscillatory motion.

(ii)Straight line

The horse is pulling a cart on a straight road. Therefore, it has a motion along a straight line.

(iii)Circular motion

Merry-go-round has a circular motion. Therefore, a child sitting inside it will also have a circular motion.

(iv) Oscillatory motion

The child on a see-saw goes up and down continuously. It oscillates up-down. Therefore, it is an oscillatory motion.

(v)Oscillatory motion

The hammer hits the electric bell and vibrates rapidly. Therefore, it is an oscillatory motion.

(vi) Straight line

The train is moving on a straight bridge. Therefore, it has a motion along a straight line.

Q2 :

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 not constant.

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

(i) Correct

Second is the SI unit of time.

(ii) Not correct

An object can move with constant or variable speed.

(iii) Correct

The distance between two cities can be very large. Since kilometre is a bigger unit of distance, the distance between two cities is measured in kilometres.

(iv) Not correct

Time period of a pendulum depends on the length of the thread. Hence, it is constant for a particular pendulum.

(v) Not correct

The speed of a train is measured either in km/h or in m/s.

Q3 :

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

Q4 :

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

Q5 :

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.

Q6 :

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.

Q7 :

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.

Q8 :

Which of the following relations is correct?

(i) Speed = Distance × Time

(ii) (iii) (iv) Q9 :

The basic unit of speed is:

(i) km/min

(ii) m/min

(iii) km/h

(iv) m/s

Q10 :

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

Q11 :

Suppose the two photographs, shown in Figure 1 and Figure 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. Figure 1 Vehicles moving in the same direction of on a road Figure 2 Position of vehicles shown in Figure 1 after some time

Q12 :

Figure 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 the motion of two cars

Q13 :

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