## NCERT CLASS 7 SCIENCE CHAPTER 13 MOTION AND TIME

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## NCERT CLASS 7 SCIENCE CHAPTER 13 MOTION AND TIME

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## NCERT CLASS 7 SCIENCE CHAPTER 13 MOTION AND TIME

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**NCERT IN – TEXT QUESTIONS OF BOOJHO AND PAHELI**

**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

= 1.6s

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

= 240/4

=60 km/hour

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

= 15km

Time taken = 8.50-8.30

= 20 minutes

Speed = 15/20 km/minutes

= Distance covered/ Time taken

= 0.75km/minutes

= 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

= 1.8km

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)**

**Ans.**(iii)

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Chapter 12. Reproduction in Plants

Chapter 13. Motion and Time

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Chapter 18. Wastewater Story