Motion and Time
Exercise
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 electric bell.
(vi) Motion of a train on a straight bridge.
Answer:
(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
(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.
2). Which of the following are not correct?
(i) The basic unit of time is second.
Ans: Correct.
(ii) Every object moves with a constant speed.
Ans: Incorrect.
(iii) Distances between two cities are measured in kilometres.
Ans: Correct.
(iv) The time period of a given pendulum is constant.
Ans: Incorrect.
(v) The speed of a train is expressed in m/h.
Ans: Incorrect.
3). A simple pendulum takes 32 s to complete 20 oscillations. What is the time period of the pendulum?
Given: No. of oscillations = 20
Time taken = 32 sec
To find: Time Period= ?
Time period = time taken/no. of oscillations
= 32/20
= 16/10
= 1.6
Ans: Time period of the pendulum = 1.6 sec
4). The distance between two stations is 240 km. A train takes 4 hours to cover this distance. Calculate the speed of the train.
Given: Distance = 240 km
Time = 4 hours
To find: Speed of the train = ?
Speed = distance / time
= 240/4
= 60 km/hr.
Ans: Speed of the train = 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.
Distance travelled by the car = 57336.0 – 57321. 0
= 15 km
= 15000m
Time = 08: 50 – 08:30 = 20 min
Speed = distance/time
= 15000 / 20
= 1500/2
= 750 m/min
= 750 X 60/1000
= 45000/1000
= 45 km/hr
Ans: The distance moved by the car = 15 km
Speed of the car = 45 km/hr
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.
Given: Time = 15 min = 15 X 60 = 900 sec
Speed = 2 m/s
To find: Distance = ?
Distance = speed X time
= 2 X 900
= 1800 m
Ans: The distance between Salma’s house and the school = 1800 m.
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.
8). Which of the following relations is correct?
(i) Speed = Distance × Time (ii) Speed = Distance/Time
(iii) Speed = Time /Distance (iv) Speed = Distance × Time
Ans: Distance/time
9). The basic unit of speed is:
(i) km/min (ii) m/min
(iii) km/h (iv) m/s
Ans: 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
Ans: speed = 40 km/h
Time = 15 min = 15/60 hr = ¼ hr
Distance = speed X time
= 40 X ¼
= 10 km
speed = 60 km/h
Time = 15 min = 15/60 hr = ¼ hr
Distance = speed X time
= 60 X ¼
= 15 km
Total distance = 10 + 15
= 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 fastest car.
Do yourself.
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?
Ans: A is moving faster.
13). Which of the following distance-time graphs shows a truck moving with speed which is not constant?
Ans: (ii) and (iii).
Additional Questions:
Fill in the blanks.
(i). The metallic ball is called the bob of the pendulum.
(ii). The to and fro motion of a simple pendulum is an example of a periodic or an oscillatory motion.
(iii). The basic unit of time is second.
(iv). The basic unit of speed is m/s.
(v). The distance-time graph for the motion of an object moving with a constant speed is a straight line.
True or false.
(i). The distance moved by objects in a given interval of time can help us to decide which one is faster or slower.
Ans: True.
(ii). Car seldom moves with a constant speed.
Ans: True.
(iii). In case of uniform motion, the average speed is same as the actual speed.
Ans: True.
(iv). The basic unit of time is minutes.
Ans: False
(v) A pendulum of a given length takes always the same time to complete one oscillations.
Ans: True.
(vi). The symbols of all units are written in singular.
Ans: True.
Define.
(i). Speed
Ans: Distance covered by an object in a unit time is called as speed of that object.
(ii). Uniform motion
Ans: If an object is moving along a straight line with a constant speed it is said to be in uniform motion.
(iii). Non-uniform motion
Ans: If the speed of an object moving along a straight line keeps changing, its motion is said to be non-uniform.
(iv). Time period
Ans: The time taken by the pendulum to complete one oscillations is called its time period.
Answer the following questions.
1). What is a simple pendulum?
Ans: A simple pendulum consitsts of a small metallic ball or a piece of stone suspended from a rigid stand by a thread.
2) What are quartz clocks?
Ans: Clocks or watches that have an electric circuit with one or more cells are called as quartz clocks.
3). What is the advantage of using quartz clocks?
Ans: The time measured by quartz clocks is much more accurate than that by the earlier clocks.
4). What are x-axis and y-axis in a graph?
Ans: The horizontal line in a graph is called as x-axis and vertical line is called as y-axis.
5). What is origin in a graph?
Ans: The intersection of x-axis and y-axis is called origin.
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