Page No 157: - Chapter 13 Motion & Time class 7 ncert solutions Science - SaraNextGen [2024-2025]

Updated By SaraNextGen

On April 24, 2024, 11:35 AM

Question 9:

The basic unit of speed is:

(i) km/min

(ii) m/min

(iii) km/h

(iv) m/s

Answer:

(iv) m/s

The basic unit of distance is metre (m).

The basic unit of time is second (s).

Therefore, the basic unit of speed is 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:

(ii) 25 km

Case I

Speed of the car = 40 km/h

Time taken = 15 min = 

Distance covered, d₁ = Speed × Time taken = 40 × 0.25 = 10 km

Case II

Speed of the car = 60 km/h

Time taken = 15 min = 

Distance covered, d₂ = Speed × Time taken = 60 × 0.25 = 15 km

Total distance covered by the car, d = d₁ + d₂ = 10 + 15 = 25 km

Therefore, the total distance covered by the car is 25 km.

Question 11:

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

Answer:

The distance covered by the blue car (as evident from the photograph) from one white strip to another, which is measured by scale is 1.4 cm.

It is given that 1 cm is equivalent to 100 m.

Therefore, 1.4 cm is equivalent to 140 m.

Distance travelled by the car = 140 m

Time interval between the two photographs = 10 s

Question 12:

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

Answer:

Vehicle A is moving faster than vehicle B.

Speed is given by the relation

This relation shows that speed of a vehicle is greater if it covers maximum distance in a given interval of time. To compare the distance, draw a line perpendicular to the time-axis, as shown in the following distance-time graph.

From the graph, it is evident that for a given time t, the distance covered by vehicle A is more than vehicle B. Hence, vehicle A is moving faster than vehicle B.

Question 13:

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

Answer:

Graph (iii)

In a distance-time graph, the constant speed of a truck will be represented by a straight line.

In a distance-time graph, a straight line parallel to the time axis indicates that the truck is not moving.

A curved line on the distance-time graph indicates that the truck is moving with a speed which is not constant.