# WBBSE Class 7 Science Solutions Chapter 2 Time and Motion

Detailed explanations in West Bengal Board Class 7 Science Book Solutions Chapter 2 Time and Motion offer valuable context and analysis.

## WBBSE Class 7 Science Chapter 2 Question Answer – Time and Motion

(Page no. 70)

Select the correct option from below and fill in the blanks:
1. Rectilinear motion.
2. Motion in a circular path.
3. Rotational motion.
4. Combination of Rotational and Rectilinear motion.
5. Curvilinear motion

 Examples of different types motion Name of the motion 1. The motion of the pencil tip while drawing a straight line with a ruler. Rectilinear motion 2. Motion of the tips of clock-hand Motion in a circular path. 3. Motion of a stone which is thrown away from the roof. Curvilinear motion. 4. Motion of merry-go-round. Motion in a circular path. 5. Motion of a spinning top at a fixed point. Combination of rotational motion and rectilinear motion. 6. Motion of a clock’s pendulum. Rectilinear motion. 7. Motion of a car along a straight road. Rectilinear motion. 8. Motion of a cycle along a straight line. Rectilinear motion. 9. Motion of a screw driver. Curvilinear motion. 10. Motion of an electric fan’s blade. Rotational motion.

### Class 7 Science Chapter 2 Question Answer West Bengal Board – Time and Motion

Multiple Choice Questions & Answers : (1 mark for each question)

Question 1.
The rate of change of displacement with time is called-
i) speed
ii) retardation
iii) velocity
iii) velocity Question 2.
Distance travelled by a moving object-
i) can be zero
ii) cannot be zero
ii) cannot be zero

Question 3
If the velocity of the body decreases with time, the motion is said to be
i) acceleration
ii) retardation
iii) displacement
ii) retardation

Question 4.
In SI system unit of retardation is-
i) MS2
ii) MS-1
iii) MS-2
iii) MS-2

Question 5.
The state of motion and rest of an object is-
i) absulute
ii) relative
ii) relative

Question 6.
The average velocity of an object-
i) could be zero
ii) could not be zero
ii) could not be zero Question 7.
A rotating fan take sometime to stop when it is suitched off due to
i) interia of motion
ii) momentum
iii) air current
i) interia of motion

Question 8.
The distance travelled by the body in a unit time interval is called
i) velocity
ii) speed
iii) acceleration
ii) speed

Question 9.
If force is applied from outside on a moving body then its momentum
i) decrease
ii) increase
iii) does not change
ii) increase

Question 10.
If a rubber ball is thrown on a wall it comes back, this can be explained by Newton’s-
i) First law
ii) Second law
iii) Third law
iii) Third law Question 11.
The action and the reaction both act on-
i) two different objects
ii) same object
i) two different objects

Question 12.
We come to know about the rate of change of momentum when force is applied, from the Newton’s-
i) first law
ii) second law
iii) third law
ii) second law

Question 13.
We know about definition of force from Newion’s-
i) first law
ii) second law
iii) third law
i) first law

Question 14.
In CGS unit of force is-
i) Newton
ii) gram-weight
iii) dyne
iii) dyne

Question 15.
The property of a body to remain at rest forever is called-
i) inertia of motion
ii) inertia of rest
iii) momentum
ii) inertia of rest Question 16.
Th property of inertia of a body is-
i) fundaniental
ii) not fundamental
i) fundamental

Fill in the blanks : (1 mark for each question)

1. The tendency of a body to keep itself at rest for ever is known as _______ of rest.
inertia.

2. Speed is a _______ quantity.
scalar.

3. Two vector quantities with same units are _____ and ________.
acceleration, retardation.

4. ____ is the rate of change of velocity with time.
Acceleration. 5. Dimensional formula of retardation ______.
LT-2

6. To change the state of inertia we apply ______.
force.

7. Dimensional formula of velocity _____.
LT-1

8. A body which changes is position with time is called a ____ object.
moving.

9. Force = mass × ________.
acceleration.

10. 1 Kgf = ______.
9.8 N (Newton). 11. One newton is the force which when acts on a body of mass 1 kg produces an acceleration of _____.
1 m/ sec-2

12. Dimensional formula of momentum _______.
MLT-1

13. In SI system the gravitational unit of force is ______.
Newton.

14. 1 Newton = 1 kg × _________.
1 m/sec2

15. In SI system the unit of momentum is _______.
Kilogram-meter/second.

16. The action and the reaction both act on ______ bodies.
two different. 17. The rate of change of momentum is proportional to _______.
force.

18. Momentum is a _____ quantity.
Vector.

State whether True or False : (1 mark for each question)

1. If a body has zero speed it can have acceleration.
2. Acceleration is a scalar quantity.
3. Velocity is a vector quantity but speed is not.
4. In speed magnitude and direction both are to be mentioned.
5. A body may have uniform speed but it may not have uniform velocity.
6. The rate of increasing velocity is called uniform velocity.
7. Motion of the tips of clock-hand is called rotation.
8. When a moving object change its place it is called displacement.
9. If an object have zero displacement, it can have distance.
10. To express velocity we have to mention both its derection and magnitude.
11. We got defination of force from Newton’s second law.
12. In SI system unit of force is Newton.
13. Measure of force is known from Newton’s third law.
14. Action and reaction cannot suppress each other.
15. If a moving bus suddenly stops the passengers moves in front ward direction.
16. If 1 Newton force works on 1 gm mass then 1cm / sce2 acceleration is produced the body.
17. Reaction remains as long as action persists.
18. If a body has less mass then it has less accleration.
19. If a vehicle suddenly starts to move it’s pas: engers lean backward direction.
20. Force applied = mass × retardation.
1. true
2. false
3. true
4. false
5. true
6. false
7. true
8. false
9. true
10. true
11. false
12. true
13. false
14. true
15. true
16. false
17. true
18. false
19. true
20. false Match the column A with column B.

 Column A Column B a) Capacity of doing work i) more work is done b) Increase in the rate of respiration ii) was the president of Royal Society c) Increase in the rate of heartbeat iii) to do work d) When we spend more energy iv) is called energy e) Energy’ is the ability v) is Newton f) To every action there is an vi) we apply force g) Sir Issac Newton vii) generates more heat h) In SI system the unit of force viii) increase in the rate of respiration i) To change the state of inertia ix) is Dyne j) In CGS system the unit of force x) equal and opposite reaction

a) Capacity of doing work is called energy.
b) Increase in the rate of respiration generates more heat.
c) Increase in the rate of heartbeat increase in the rate of respiration.
d) When we spend more energy more work is done.
e) Energy is the ability to do work.
f) To every action there is an equal and opposite reaction.
g) Sir Issac Newton was the president of Royal Society.
h) In SI system the unit of force is Newton.
i) To change the state of inertia we apply force.
j) In CGS system the unit of force is Dyne. Short Questions & Answers : (1,2 marks for each question)

Question 1.
How many types of motion are there?
There are two types of motion.
i) Rotation and ii) Revolution.

Question 2.
What is Revolution?
When a moving object travels in an orbit while rotating on its own axis then the motion of that object is called revolution.

Question 3.
What type of motion is seen in a moving car?
Moving car has mixed motion.

Question 4.
Why displacement is a vector quantiny?
Displacement has both magnitude and direction. So it is a vector quantity.

Question 5.
What type of motion has a moving electric fan?
It has rotational motion. Question 6.
What is the unit of speed in SI system?
In SI system the unit of speed in meter/second.

Question 7.
Is speed vector or scalar quantity?
Speed is vector quantity.

Question 8.
Which physical quantity has unit as meter/second2 ?
In SI system acceleration has unit as meter/second2.

Question 9.
What is the unit of velocity? Is it a scalar or vector quantity?
In CGS system unit of velocity in centimeter/sec. and in SI system unit of velocity is meter/second. Velocity is a vector quantity.

Question 10.
A body after moving in a circular part comes back to its starting point, what is its displacement?
If a body after travelling in a circular part comes back to its starting point, the displacement is zero. Question 11.
What we come to know from Newton’s Fast law?
From Newton’s fast law, we come to know about- (i) Inertia and (ii) force.

Question 12.
From which law of Newton we come to know about force?
We come to know about force from Newton’s fast law.

Question 13.
What is the unit of force in CGS system?
In CGS system the unit of force is Dyne.

Question 14.
What is the unit of force in SI system?
In SI system the unit of force is Newton.

Question 15.
One Newton = ? Dyne.
One Newton = 105 Dyne. Question 16.
Name the types of Inertia.
Inertia are of two types – i) Inertia of rest and ii) Inertia of motion.

Question 17.
Is force vector quantity or scalar quantity?
Force is vector quantity.

Question 18.
What is’ the unit of momentum in SI system?
In SI system the unit of momentum is Kilogram-meter/second.

Question 19.
1 gm. force = how much Dyne?
lgf = 980 dyne.

Question 20.
What is the unit of work in SI system?
A unit of work in SI system is Joule.

Question 21.
What is energy? How we get energy?
The energy of a body is its capacity to do work.
We get energy from food. We get something from food which gives us ability for doing work. Question 22.
How can we feel that energy is being spent inside our body?
Fatigue, increase in the rate of heart beat, increase in the rate of respiration, increase in the temperature of the body etc help us to understand that energy is being spent inside our body.

Question 23.
Sometimes you may have noticed that birds glide some disatance without moving its wings. Why does the birdmove in this manner?
A flying bird is a moving object. It tries to remain in uniform motion when there is no force on it. So, without moving its wings it goes farther distance because of inertia of motion.

Question 24.
What is inertia?
The tendency of a body of remaining at rest or in uniform motion is known as ‘inertia’ of the body.

Question 25.
What is ‘inertia of rest’ and ‘inertia of motion’?
The tendency of a body to keep itself at rest forever is known as ‘inertia of rest’.
The tendency of remaining in uniform motion of the body is called as ‘inertia of motion’.

Question 26.
What is force?
What we apply to change the state of inertia is called as force. Question 27.
Explain Newton’s Third law of motion?
Newton’s third law of motion is – To every action, there is an equal and opposite reaction.

Question 28.
When we can say a work is done?
Work is said to be done only when body move (i.e. there is a displacement of the body).

Long Questions & Answers : (3 marks for each question)

Question 1.
Explain the concept of Newtons first law of motion.
If there is no external force acting on a body, then-
i) The body at rest will remain at rest forever,
ii) The moving body will go with constant speed in the same direction of its motion. Question 2.
Explain Newton’s second law of motion.
Newton’s second law of motion states that-
i) If we apply more force on a body of fixed mass, its change of velocity will be more in one second. If we double the force, the acceleration of the body also doubles. That is to say there is a linear relation between force and acceleration.
ii) The direction to which the froce acts, the direction of acceleration is also the same. That is the velocity of the body increases along the direction of applied force.

Question 3.
Write down the equation for the magnitude of applied force, according to Newton’s second law of motion.
Applied force = mas of the body × change in velocity in 1 second. = mass of the body × acceleration produced (change in velocity in one second = acceleration)

F = m × a[F = force, m = mass, a = acceleration]
The unit of force in SI system is 1 Newton.
1 Newton =1 kg × 1 m / sec2

Question 4.
Who was Sir Issac Newton?
Sir Issac Newton was an English physicist, mathematician and astronomer. He proposed the Three Laws of Motion, Spectrum of Sunlight. Universal Law of Gravitation, Binomial Theo rem. Differential calculus etc. On December, 1642. Newton was born in a peasant family of Willsthrope in England. He got BA degree from Trinity college of Cambridge University in 1669 at the age of 27 . He died in 1727.

Question 5.
Why was Sir Issac Newton famous?
Sir Issac Newton invented laws of Motion, Spectrum of sunlight, Binomial theorem, Differential Calculus etc expanded a new era of science and mathematics. He was the president of Royal society from 1672 for next 25 years. He wrote the famous book “Principia Mathematica”. Question 6.
When a bus suddenly starts moving from rest, the passengers in the bus leans in backward direction, explain why?