# Motion and Time

## Exercise

1). Classify the following as motion along a straight line, circular orÂ 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

Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â = 32/20

Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â = 16/10

Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â = 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}

Â Â Â Â Â Â Â Â Â Â Â Â Â = ^{240}/_{4}

Â Â Â Â Â Â Â Â Â Â Â Â Â = 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 â€“ 57321. 0

Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â = 15 km

Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â = 15000m

Time = 08: 50 â€“ 08:30 = 20 min

Speed = ^{distance}/_{time}

Â Â Â Â Â Â Â Â Â Â Â Â Â = ^{15000} / _{20}

Â Â Â Â Â Â Â Â Â Â Â Â Â = ^{1500}/_{2}

Â Â Â Â Â Â Â Â Â Â Â Â Â = 750 m/min

Â Â Â Â Â Â Â Â Â Â Â Â Â = ^{750 X 60}/_{1000}

Â Â Â Â Â Â Â Â Â Â Â Â Â = ^{45000}/_{1000}

Â Â Â Â Â Â Â Â Â Â Â Â Â = 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 aÂ bicycle. If the bicycle has a speed of 2 m/s, calculate the distanceÂ 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

Â Â Â Â Â Â Â Â Â Â Â Â Â = 2 X 900

Â Â Â Â Â Â Â Â Â Â Â Â Â = 1800 m

Ans: The distance between Salmaâ€™s house and the school = 1800 m.

7). Show the shape of the distance-time graph for the motion in theÂ 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 Ã— Time Â Â Â Â Â Â (ii) Speed = Distance/Time

(iii) Speed = Time /DistanceÂ Â Â Â Â Â (iv) Speed = Distance Ã— Time

Ans: Distance/time

9). The basic unit of speed is:

(i) km/min Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â (ii) m/min

(iii) km/h Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â (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 Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â (ii) 25 km

(iii) 15 km Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â (iv) 10 km

Ans: speed = 40 km/h

Time = 15 min = 15/60 hr = Â¼ hr

Distance = speed X time

Â Â Â Â Â Â Â Â Â Â Â Â Â = 40 X Â¼

Â Â Â Â Â Â Â Â Â Â Â Â Â = 10 km

speed = 60 km/h

Time = 15 min = 15/60 hr = Â¼ hr

Distance = speed X time

Â Â Â Â Â Â Â Â Â Â Â Â Â = 60 X Â¼

Â Â Â Â Â Â Â Â Â Â Â Â Â = 15 km

Total distance = 10 + 15

Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â = 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). Â 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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