Q.1 Paheli wondered how time was measured when pendulum clocks were not available. (page 148)
Ans. Many times, measuring devices were used in different parts of the world before the pendulum clock water clocks, sand clocks and sundials are some example of such devices.
Q.2 Boojo wants to know whether there is any device that measures the speed. (page 150)
Ans. Yes, speedometer.
NCERT TEXTBOOK EXERCISE (Page 156-158)
Q.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 electrical bell.
(vi) motion of a train on a straight Bridge.
Ans. (i) oscillatory (ii)straight line (iii) circular (iv) oscillatory (v) oscillatory (vi) straight line.
Q.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 not constant.
(v) The speed of a train is expressed in m/ h.
Ans. (ii) and (v).
Q.3 A simple pendulum takes 32s to complete 20 oscillations. What is the time period of the pendulum?
Ans. Given, time taken to complete 20 oscillations = 32s
time taken to complete 1 oscillation = 32/20s
Thus, the time period of pendulum is 1.6s.
Q.4 The distance between two stations is 240 km. A train takes 4 hours to cover this distance. Calculate the speed of the train.
Ans. Given, distance between two stations =240 km
and time taken to cover this distance = 4 hours
We know that, Speed = Distance covered / Time taken
Thus, the speed of the train is 60 km/hour.
Q.5 The odometer of a car reads 57321.0 km when the clock shows the time 8:30 AM. What is the distance moved by the car, if at 8 : 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.
Ans. Given, reading of odometer at time 8:30 AM = 57321.0 km
and reading of Odometer at time 8:50 AM = 57336.0km
Distance covered by car = (57336.0-57321.0) km
Time taken = 8.50-8.30
= 20 minutes
Speed = 15/20 km/minutes
= Distance covered/ Time taken
= 0.75/1/60 km/hour
(1 minute=1 / 60 hour)
= 0.75 X 60 km/hour
= 45 km/hour
Q.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.
Ans. Given, time taken by Salma from her house to her school = 15 minutes
= 15 X 60 seconds
= 900 seconds
speed of bicycle = 2 m/s
We know that,
speed = Distance covered / Time taken
or 2 = Distance covered / Time taken
or Distance covered = 2 X 900m = 1800 m
= 1800/ 1000 km
Thus, the distance between Salma’s school from her house is 1800 m or 1.8 km.
Q.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.
Ans. (i) Graph – Distance-time graph
(ii) Graph – Distance- time graph
Q.8 Which of the following relations is correct ?
() Speed = Distance X Time;
(ii) Speed = Distance / Time;
(iii) speed = Distance/Time;
(iv) Speed =1/Distance X Time.
Ans. Speed = Distance / Time.
Q.9 The basic unit of speed is:
(i) km/min ; (ii) m/min; (iii) km/h; (iv) m/s.
Ans. (iv) m/s.
Q.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.
Ans. (ii) 25 km.
Distance travelled in first 15 minutes = speed X Time
= 40 km/h X 15 minutes
= 40 km/h X 15/60 hour
= 10 km
Distance travelled in last 15 minutes = speed X Time
= 60 km/h X 15 minutes
= 60 Km/h X 15/60 hour
= 15 km
Total distance = (10 + 15) km
= 25 km.
Q. 11 Suppose the two photographs, shown in Fig. (a) and Fig. (b) had been taken at an interval of 10 seconds. If a distance of 100 meters is shown by 1 cm in these photographs, calculate the speed of blue car.
Fig. (a) Vehicles moving in the same direction on a road.
Fig. (b) Position of vehicles shown in (a) after some time.
Ans. Speed = 100 km/ 10h
= 10 km/h.
Q.12 Fig. below the distance- time graph for the motion of two vehicle A and B. Which one of them is moving faster?
Fig. Distance- time graph for the motion of two cars.
Ans. Car A is moving faster.
Q.13 Which of the following distance- time graphs shows a truck moving with speed which is not constant?
Fig. Graph (page 157)