## Tuesday, 5 July 2011

### Class VII | Science | Lesson 13. Motion and Time

Exercises Solution
1. Classify the following as motion along a straight line, circular or oscillatory motion:
(i) Motion of your hands while running. :  - oscillatory motion
(ii) Motion of a horse pulling a cart on a straight road. :-  straight line motion
(iii) Motion of a child in a merry-go-round. : - circular motion
(iv) Motion of a child on a see-saw. :  -  oscillatory motion
(v) Motion of the hammer of an electric bell.- oscillatory motion
(vi) Motion of a train on a straight bridge : - straight line motion

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.

Answer : The following statements are incorrect :
(ii) Every object moves with a constant speed.
(iv) The time period of a given pendulum is not constant.
(v) The speed of a train is expressed in m/h.

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

Solution :
Time taken to complete 20 oscillations = 32 s
Time period of the pendulum              = Nos of oscillations/Total Time taken
= 32/20 s =  1.6s

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

Solution :
The distance between two stations                       =  240 km
Time taken by the train to cover that distance      = 4 hours
Speed of the train                                               = 240/4 Km/h =60 km/h

5. 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.

Solution :     Initial reading of car odometer        = 57321.0 km
Final reading of     car odometer        = 57336.0 km
Time at the time of Initial reading        = 08:30 AM
Time at the time of Initial reading        = 08:50 AM

Distance traveled by the car                 =  57336.0 km - 57321.0 km
= 15 Km
Time taken by the car to cover distance                  =  08:50 AM -08:30 AM
= 20 m
Speed of the car  in Km/m                            = 15 km/20m
= .75 Km/m
Speed of the car  in Km/h                             = 15 km/20m X 60
= 45 Km/h

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.

Solution :
Speed of bicycle                                        = 2 m/s
Time taken by Salma to reach school          = 15 minutes = 15 × 60 Seconds
Distance between her house and the school =  Speed of bicycle × Time taken
= 2 m/s × 15 × 60 s
= 1800 m or 1.8 Km

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. (i) A car moving with a constant speed. (ii) A car parked on a side road.

8. Which of the following relations is correct?
(i) Speed = Distance Time (ii) Speed = Distance / Time
(iii) Speed = Time / Distance
(iv) Speed = 1/ Distance Times

Answer : (ii) Speed = Distance / Time

9. The basic unit of speed is:
(i) km/min (ii) m/min (iii) km/h (iv) m/s

Answer : (iv) m/s

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

Solution :   (ii) 25 km
Distance covered by the car with a speed
of 40 km/h in 15 minutes                             = Speed  × Time
= (40 Km/h /60 minute)  × 15 minute
= 10 Km
Distance covered by the car with a speed
of 60 km/h in 15 minutes                            = Speed  × Time
= (60 Km/h /60 minute)  × 15 minute
= 15 Km

Total distance covered by the car              =(10 + 15 )km = 25 Km

11. 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. 13.1 Fig. 13.2

Solution :
1 cm of Distance  in photographs             = 100 m
Distance moved by the blue car             =100 m
Time taken to cover this distance            = 10 s
Speed of the car                                    = 100 m /10 s  = 10 m /s
= 10 m/s × 60 × 60 s /1000 m = 36 km/h

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

Answer : The vehicle A is moving faster as distance represented by vertical axis of A is more as compare to B for a given point of time.

Solution : The distance-time graphs number 3 shows a truck moving with speed which is not constant as distance being covered with respect to time is not the same and graph is a curve.

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Extend Learning — Activities and Projects
1. 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.
2. Collect information about time-measuring devices that were used in 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.
3. Make a model of a sand clock which can measure a time interval of 2
minutes (Fig. 13.17).
4. 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?
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Did you know?
The time-keeping services in India are provided by the National Physical Laboratory, New Delhi. The clock they use can measure time intervals with an accuracy of one-millionth of a second. The most accurate clock in the world has been developed by the National Institute of Standards and Technology in the U.S.A. This clock will lose or gain one second after running for 20 million years.
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Chapter 13. Motion and Time | Science | Class VII   CBSE NCERT

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