1. Classify the following as motion along a straight line, circular or oscillatory motion:
(i) Motion of your hands while running.
Ans: oscillatory motion.
(ii) Motion of a horse pulling a cart on a straight road.
Ans: straight line motion
(iii) Motion of a child in a merry-go-round.
Ans: circular motion
(iv) Motion of a child on a see-saw.
Ans: oscillatory motion
(v) Motion of the hammer of an electric bell.
Ans: oscillatory motion
(vi) Motion of a train on a straight bridge.
Ans: straight line motion
2. Which of the following are not correct?
(i) The basic unit of time is second.
Ans: This is correct.
(ii) Every object moves with a constant speed.
Ans: This is incorrect.
(iii) Distances between two cities are measured in kilometres.
Ans: This is correct.
(iv) The time period of a given pendulum is constant.
Ans: This statement is generally incorrect.
(v) The speed of a train is expressed in m/h.
Ans: This is correct.
3. A simple pendulum takes 32 s to complete 20 oscillations. What is the
time period of the pendulum?
Ans: Time period of a pendulum is time taken by it to complete 1 oscillation. Hence, the time period of the pendulum is 1.6 seconds.
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: So the train takes 4 hours to travel a distance of 240 km between the two stations. So the train is supposed to travel with a constant velocity along a straight track. So the velocity of the train is 60 Km/hr.
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.
Ans: Distance covered by car = (57336 - 57321) km = 15 km Time taken between 08:30 AM to 08:50 AM = 20 minutes = 20/60 hour = 1/3 hour So Speed in km/min Speed = (Distance travelled)/ (Time) = (15km)/ (20min) = 0.75km/min Speed in km/h Speed = (Distance travelled)/ (Time) = (15km)/ (1/3h) = (15 x 3) km/ (1h) = 45km/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.
Ans: If the bicycle has speed of 2m/s, calculate the distance between her house and the school. \ Converting into seconds, t= 15 x 60 = 900 seconds. Distance between her house and school = 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.
Ans:
8. Which of the following relations is correct?
(i) Speed = Distance × Time (ii) Speed = Distance Time iii) Speed = Time Distance (iv) Speed = 1 Distance Time
Ans: (i) Speed = Distance / Time
9. The basic unit of speed is:
(i) km/min (ii) m/min
(iii) km/h (iv) m/s
Ans: (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:
(iii) 15 km (v) 10 km
Ans: (ii) 25 km.
11. Suppose the two photographs, shown in Fig. 9.1 and Fig. 9.2, 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 fastest car.
12. Fig. 9.15 shows the distance-time graph for the motion of two vehicles A and B. Which one of them is moving faster?
Fig. 9.15 Distance-time graph for the motion of two cars
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. 9.16. One way to fix the gnomon could
be to make a groove along a diameter on the circular board.
Ans:
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. 9.16. 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. 9.16.
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. 9.16.
Ans:
2. Collect information about time-measuring devices that were used in
the 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.
Ans:
Colour By: Himashree Bora.
