Motion

Objective Questions

Question 1

Write true or false for each statement:

(a) Two trains going in opposite directions with the same speed are at rest relative to each other.

(b) A ball is thrown vertically upwards. Its motion is uniform throughout.

(c) The motion of a train starting from one station and reaching at another station is non-uniform.

(d) A motion which repeats itself after a fixed interval of time is called periodic motion.

(e) A ball thrown by a boy from a roof-top has oscillatory motion.

(f) Mass has both magnitude and direction.

(g) Weight always acts vertically downwards.

(h) Mass varies from place to place but weight does not.

Answer

(a) False
Correct Statement — Two trains going in opposite directions with the same speed are at motion relative to each other.

(b) False
Correct Statement — A ball is thrown vertically upwards. Its motion is non-uniform throughout.

(c) True

(d) True

(e) False
Correct Statement — A ball thrown by a boy from a roof-top has curvilinear motion.

(f) False
Correct Statement — Mass has only magnitude.

(g) True

(h) False
Correct Statement — Weight varies from place to place but mass does not.

Question 2

Fill in the blanks:

(a) Two boys cycling on the road with the same speed are ................... relative to each other.

(b) The motion in a .................. is rectilinear motion.

(c) One to and fro motion of a clock pendulum takes time = .................. .

(d) 36 km h^-1 = ....................... m s^-1.

(e) Total distance travelled = .................... x total time taken.

(f) The weight of a girl is 36 kgf. Her mass will be .................. .

(g) The weight of a body is measured using ..................... .

Answer

(a) at rest

(b) straight line

(c) 2 s

(d) 10

(e) average speed

(f) 36 kg

(g) spring balance

Question 3

Match the following:

Answer

Question 4a

Select the correct alternative:

A book lying on a table is an example of:

1. a body at rest
2. a body in motion
3. a body neither at rest nor in motion
4. none of these

Answer

a body at rest

Reason — A book lying on a table will not change its position if it is not disturbed, so it is said to be at rest.

Question 4b

Select the correct alternative:

The motion of a pendulum is:

1. rotatory
2. oscillatory
3. curvilinear
4. rectilinear

Answer

oscillatory

Reason — The motion of pendulum is to and fro motion which is called oscillatory motion.

Question 4c

Select the correct alternative:

A car moving on a straight road is an example of:

1. rotatory motion
2. rectilinear motion
3. oscillatory motion
4. periodic motion

Answer

rectilinear motion

Reason — As the car is moving on a straight road so it is in rectilinear motion.

Question 4d

Select the correct alternative:

A ball falls down vertically. Its motion is:

1. periodic
2. linear
3. circular
4. vibratory

Answer

linear

Reason — A ball falls down vertically i.e. in a straight line, so its motion is linear.

Question 4e

Select the correct alternative:

If a body covers equal distances in equal intervals of time, the motion is said to be:

1. uniform
2. non-uniform
3. oscillatory
4. rotatory

Answer

uniform

Reason — If a moving body travels equal distances in equal intervals of time, then the motion is said to be uniform.

Question 4f

Select the correct alternative:

A boy goes from his house to school by bus at a speed of 20 km h^-1 and returns back through the same route at a speed of 30 km h^-1. The average speed of his journey is:

1. 24 km h^-1
2. 25 km h^-1
3. 30 km h^-1
4. 20 km h^-1

Answer

24 km h^-1

Reason — Suppose the distance between school and house is D km.

In first part the boy goes to school by bus from home at speed (V₁) = 20 km h^-1.

time taken (t₁) = $\dfrac{\text{distance}}{\text{speed}}$ =$\dfrac{\text{D}}{\text{20}}$

In second part the boy returns from school to home through same route at speed(V₂) = 30 km h^-1.

time taken (t₂) = $\dfrac{\text{distance}}{\text{speed}}$ =$\dfrac{\text{D}}{\text{30}}$

Total distance = D + D = 2D km

Total time taken = $\dfrac{\text{D}}{\text{20}}$ + $\dfrac{\text{D}}{\text{30}}$ = $\dfrac{\text{5D}}{\text{60}}$

Average speed = $\dfrac{\text{total distance travelled}}{\text{total time taken}}$

$\text{Speed} = \dfrac{\text{distance}}{\text{time}} \\[1em] = \dfrac{2D}{\dfrac{5D}{60}} \\[1em] = 2 \times 12 \\[1em] = 24 \text{km h}^{-1}$

Question 4g

Select the correct alternative:

The earth attracts a body of mass 1 kg with a force of 10 N. The mass of a boy is 50 kg. His weight will be:

1. 50 kg
2. 500 N
3. 50 N
4. 5 N

Answer

500 N

Reason — Weight = 50 x 10 = 500 N

Short/Long Answer Questions

Question 1

Explain the meaning of the terms rest and motion.

Answer

A body is said to be at rest if it does not change its position with respect to its immediate surroundings.

A body is said to be in motion if it changes its position with respect to its immediate surroundings.

Question 2

Comment on the statement 'rest and motion are relative terms'. Give an example.

Answer

An object can be in motion relative to one set of objects while at rest relative to some other set of objects. Thus, rest and motion are relative terms.
Example — If a boy is sitting on a bench in a bus stop and he sees a moving bus, for him the bus is in motion but the trees near by will appear to him at rest.
But a boy who is sitting inside a bus, to him the trees and the boy outside sitting on the bench will appear to move in opposite direction. But the roof of the bus and driver will appear to him at rest.
From the above example it can be concluded that rest and motion are relative terms.

Question 3

Fill in the blanks using one of the words: at rest, in motion.

(a) A person walking in a compartment of a stationary train is ................... relative to the compartment and is ................... relative to the platform.

(b) A person sitting in a compartment of a moving train is .................... relative to the other person sitting by his side and is ....................... relative to the platform.

Answer

(a) in motion, in motion

(b) at rest, in motion

Question 4

Name five different types of motion you know.

Answer

The five different types of motion are:

1. Translatory motion
2. Circular motion
3. Oscillatory motion
4. Vibratory motion
5. Rotatory motion

Question 5

What do you mean by translatory motion? Give one example.

Answer

A body is said to have translatory motion if every point on the moving body moves through the same distance in the same interval of time.
Example — Motion of a box when pushed from one corner of a room to the other.

Question 6

Explain the meanings of:

(i) rectilinear motion

(ii) curvilinear motion

Give one example of each.

Answer

(i) The motion of a body in a straight line is called rectilinear or linear motion.
Example — A car moving on a straight road.

(ii) If the motion of a body is along a curved path, it is said to be a curvilinear motion.
Example — The motion of a cycle while taking a turn on the road.

Question 7

What is rotatory motion? Give two examples.

Answer

A body is said to be in a rotatory motion if it moves about a fixed axis.
Examples — a spinning top, a merry-go-round.

Question 8

What is meant by circular motion? Give one example.

Answer

The motion of a body along a circular path is called circular motion. Example — Motion of a satellite around the earth.

Question 9

How does rotatory motion differ from circular motion?

Answer

Question 10

Explain oscillatory motion by giving one example.

Answer

The to and fro motion of a body from its rest position is called the oscillatory motion.
Example — Motion of a pendulum of a wall clock. The bob of pendulum moves to side A from its rest position O, again comes back to position O and then moves to side B and then comes back to rest position O. This process is continuously repeated and this to and fro motion is called oscillatory motion.

Question 11

What is vibratory motion? Give one example.

Answer

The to and fro motion of a part of the body keeping its rest part fixed is called vibratory motion.
Example — Vibration of the membrane of tabla.

Question 12

Differentiate between periodic and non-periodic motions by giving an example of each.

Answer

Question 13

What is random motion? Give an example.

Answer

The motion of an object which takes path in no specific direction is called random motion.
Example — Motion of molecules of a gas.

Question 14

Name the type/types of motion being performed by each of the following:

(a) Vehicle on a straight road.

(b) Blades of an electric fan in motion.

(c) Pendulum of a wall clock.

(d) Smoke particles from chimney.

(e) Hands of a clock.

(f) Earth around the Sun.

(g) A spinning top.

Answer

(a) Rectilinear motion

(b) Rotatory motion

(c) Oscillatory motion and Periodic motion

(d) Non-periodic motion

(e) Circular motion

(f) Periodic motion, circular motion

(g) Rotatory motion

Question 15

Give two examples to illustrate that a body can have two or more types of motion simultaneously.

Answer

The two examples that show a body can have two or more types of motion (mixed motion) simultaneously are:

1. The Earth rotates about its axis (rotatory motion) and at the same time, it revolves around the sun in a curved path (curvilinear or circular motion) in a fixed time interval (periodic motion).

2. A ball rolling on the ground has rotatory and translatory motion as it moves on the ground.

Question 16

State the types of motion of the following:

(a) The needle of a sewing machine

(b) The wheel of a bicycle

(c) The drill machine

(d) The carpenter's saw

Answer

(a) Periodic motion

(b) Translatory and rotatory motion

(c) Translatory and rotatory motion

(d) Translatory and oscillatory motion

Question 17

Distinguish between uniform and non-uniform motions, giving an example of each.

Answer

Question 18

How do you determine the average speed of a body in non-uniform motion?

Answer

The average speed is calculated by finding the ratio of the total distance travelled by the body to the total time taken in the journey.

Average speed = $\dfrac{\text{total distance travelled}}{\text{total time taken}}$

Question 19

Define the term weight and state its S.I. unit.

Answer

The weight of the body on earth is the force with which earth attracts the body. The S.I. unit of weight is newton (N).

Question 20

How are the units of weight, kgf and newton related?

Answer

1 kgf = 9.8 (nearly 10) N

Question 21

State three differences between mass and weight.

Answer

Question 22

Which quantity: mass or weight, does not change by change of place?

Answer

Mass does not change by change of place.

Question 23

State which of the quantities, mass or weight is always directed vertically downwards.

Answer

The direction of weight is always directed vertically downwards.

Numericals

Question 1

A car covers a distance of 160 km between two cities in 4 h. What is the average speed of the car?

Answer

Distance = 160 km;
time = 4 h

$\text{Average Speed} = \dfrac{\text{Total distance travelled}}{\text{total time taken}} \\[1em] \text{Average Speed} = \dfrac{\text{160}}{\text{4}} = 40 \text{km h}^{-1}$

So, average speed of the car = 40 km h^-1.

Question 2

A train travels a distance of 300 km with an average speed of 60 km h^-1. How much time does it take to cover the distance?

Answer

Distance = 300 km;
Average speed = 60 km h^-1
time = ?

$\text{Average speed} = \dfrac{\text{Distance travelled}}{\text{time}} \\[1em] \text{60} = \dfrac{\text{300}}{\text{time}} \\[1em] \text{time} = \dfrac{\text{300}}{\text{60}} = 5 \text{hr}$

So, time taken = 5 hour.

Question 3

A boy travels with an average speed of 10 m s^-1 for 20 min. How much distance does he travel?

Answer

Average speed = 10 m s^-1;
time = 20 min
1 min = 60 sec
20 min = 60 x 20 = 1200 sec
distance = ?

$\text{Average speed} = \dfrac{\text{distance}}{\text{time}} \\[1em] \text{10} = \dfrac{\text{distance}}{\text{1200}} \\[1em] \text{distance} = 1200 \times 10 = 12000 \text{m}$

So, distance covered = 12000 m or 12 km.

Question 4

A boy walks a distance of 30 m in 1 minute and another 30 m in 1.5 minute. Describe the type of motion of the boy and find his average speed in m s^-1.

Answer

The motion of the boy is non-uniform.
total distance travelled = 30 m + 30 m = 60 m
total time taken = 1 min + 1.5 min = 2.5 min
1 min = 60 sec
2.5 min = 2.5 x 60 = 150 sec
Average speed = ?

$\text{Average Speed} = \dfrac{\text{Total distance travelled}}{\text{total time taken}} \\[1em] \text{Average Speed} = \dfrac{\text{60}}{\text{150}} = 0.4 \text{m s}^{-1}$

So, average speed of the boy = 0.4 m s^-1.

Question 5

A cyclist travels a distance of 1 km in the first hour, 0.5 km in the second hour and 0.3 km in the third hour. Find the average speed of the cyclist in:

(i) km h^-1

(ii) m s^-1.

Answer

(i) total distance travelled = 1 km + 0.5 km + 0.3 km = 1.8 km
total time taken = 1 h + 1 h + 1 h = 3 h

$\text{Average Speed} = \dfrac{\text{Total distance travelled}}{\text{total time taken}} \\[1em] \text{Average Speed} = \dfrac{\text{1.8}}{\text{3}} = 0.6 \text{km h}^{-1}$

So, average speed of the cyclist = 0.6 km h^-1.

(ii) total distance travelled = 1 km + 0.5 km + 0.3 km = 1.8 km
1 km = 1000 m
1.8 km = 1.8 x 1000 = 1800 m

total time taken = 1 h + 1 h + 1 h = 3 h
1 h = 3600 sec
3 h = 3 x 3600 = 10800 sec

$\text{Average Speed} = \dfrac{\text{Total distance travelled}}{\text{total time taken}} \\[1em] \text{Average Speed} = \dfrac{\text{1800}}{\text{10800}} = 0.167 \text{m s}^{-1}$

So, average speed of the cyclist = 0.167 m s^-1.

Question 6

A car travels with speed 30 km h^-1 for 30 minutes and then with speed 40 km h^-1 for 1 hour. Find:

(a) the total distance travelled by the car.

(b) the total time of travel, and

(c) the average speed of the car.

Answer

(a) In first part of journey
speed = 30 km h^-1

time = 30 min
60 min = 1 h
30 min = $\dfrac{\text{1}}{\text{2}}$ h

$\text{Speed} = \dfrac{\text{distance}}{\text{time}} \\[1em] \text{30} = \dfrac{\text{distance}}{\dfrac{1}{2}} \\[1em] \text{distance} = 30 \times {\dfrac{1}{2}} = 15 \text{km}$

In second part of journey
speed = 40 km h^-1

time = 1 h

$\text{Speed} = \dfrac{\text{distance}}{\text{time}} \\[1em] \text{40} = \dfrac{\text{distance}}{\text{1}} \\[1em] \text{distance} = 40 \times 1 = 40 \text{km}$

So, total distance travelled = 15 km + 40 km = 55 km.

(b) time taken in first part of journey = 30 min

60 min = 1 h
30 min = $\dfrac{\text{1}}{\text{2}}$ h

time taken in second part of journey = 1 h

total time taken = 1 h + $\dfrac{\text{1}}{\text{2}}$ h = 1.5 h

So, total time taken = 1.5 h.

(c) $\text{Average Speed} = \dfrac{\text{Total distance travelled}}{\text{total time taken}} \\[1em] \text{Average Speed} = \dfrac{\text{55}}{\text{1.5}} = 36.67 \text{km h}^{-1}$

So, average speed of the car = 36.67 km h^-1.

Question 7

On earth the weight of a body of mass 1.0 kg is 10 N. What will be the weight of a boy of mass 37 kg in

(a) kgf

(b) N

Answer

(a) Mass of boy = 37 kg

So, Weight of boy in kgf = 37 kgf.

(b) 1 kgf = 10 N
37 kgf = 37 x 10 = 370 N

So, weight of boy in N = 370 N.

Question 8

The weight of a body of mass 6.0 kg on moon is 10 N. If a boy of mass 30 kg goes from earth to the moon surface, what will be his

(a) mass,

(b) weight?

Answer

(a) Mass of boy = 30 kg on moon surface as mass remains constant. Mass of a body does not change with place.

(b) Weight of body of mass 6.0 kg on moon surface = 10 N

Weight of body of mass 30 kg on moon surface = $\dfrac{\text{10}}{\text{6.0}}$ x 30 = 50 N

So, weight of boy of 30 kg on moon surface = 50 N.