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## Karnataka State Syllabus Class 8 Science Chapter 9 Force and Newton’s Laws of Motion

### KSEEB Class 8 Science Force and Newton’s Laws of Motion Textbook Exercise Questions and Answers

I. Four alternatives are given to each of the following incomplete statements/questions. Choose the right answer.

Question 1.

Which one of the following ¡s not correct Action and reaction _______.

(a) act on the same body

(b) are equal

(C) are opposite

(d) act on different bodies

Answer:

(d) act on different bodies

Question 2.

If the forces acting on an object are balanced, then _______.

(a) the object will be accelerated.

(b) the object will be in motion.

(c) the object will be at rest.

(d) the object loses its shape.

Answer:

(c) the object will be at rest.

Question 3.

The correct way to write the SI unit of free is _______.

(a) Newtons

(b) newton

(c) newtons

(d) Newton

Answer:

(b) newton

II. Fill in the blanks with suitable words:

1. Inertia of an object is proportional to its _______.

2. The S.l unit of momentum is _______.

3. To cause acceleration in an object, the necessary factor is unbalanced _______.

4. Newton’s first law of motion is also called the law of _______.

5. The suitcases kept in a moving bus, move forward when the brakes are applied suddenly. This is due to _______.

Answer:

1. mass,

2. Kg ms^{-1}

3. force,

4. inertia,

5. inertia

III. Answer the following:

Question 1.

In a tug of war game, if the two teams exert equal forces, none of the team wons. Why?

Answer:

In a tug. of war game, if the two teams exert equal forces none of the team wins because the rope is maintained in the equilibrium position or in the same state, as the forces are balanced forces.

Question 2.

Which of the following has greater momentum?

Answer:

(a) An object of mass 2 Kg moving with a uniform velocity of 2 ms^{-1}

(b) An object of 1 kg mass moving with a uniform velocity of 3 ms^{-1}

(a) mass = 2 kg

Velocity = 2 ms^{-1}

Momentum = mass x velocity

= 2 x 2 = 4 kg ms^{-1}

(b) mass = 1 kg

Velocity = 3 ms^{-1}

Momentum = mass x velocity

= 1 x 3

= 3 kg ms^{-1}

therefore object of mass 2 kg, moving with uniform velocity of 2 ms^{-1} has greater momentum.

Question 3.

Which of the following has greater inertia?

(a) 50 paise coin

(b) One rupee coin

(c) Five rupee coin

Answer:

Five rupee coin has greater insertions.

Question 4.

State any two illustrations for Newton’s third law of motion.

Answer:

a) A boatman standing in a boat that is in the river pushes the river bank with the help of a long pole. So the boat – moves forward, equal to the force applied in the opposite direction.

b) A swimmer pushes the water backward to move forward.

Question 5.

Distinguish between balanced and unbalanced forces.

Answer:

A. Balanced Force:

- The effect of the force cannot be observed.
- No change in the speed or direction of the motion or both.
- The equilibrium position is maintained.

B. Unbalanced Force:

- The effect of the force can be observed.
- Change in the speed or direction of the motion or both.
- The equilibrium position is not maintained.

Question 6.

State Newton’s second law of motion in terms of momentum.

Answer:

Newton’s second law of motion in terms of momentum can be stated as the Rate of change of momentum is directly proportional to the impressed force and takes place in the direction of the application of the force.

IV. Give reasons for the following:

Question 1.

It is dangerous to get down from a moving bus even if the bus is moving slowly.

Answer:

When a person tries to get down from a moving bus, his/her feet come to rest, but the upper body due to moment of Intertia will be moving forward and the chances of tumbling forward are there.

Question 2.

An athlete in a long jump, runs distance, before taking the leap.

Answer:

This is because the acceleration, given to a body by a force applied to it is directly proportional to the force when a body moves, it will continue to move with the same velocity when no force act on it. According to this principle, If an athlete runs from a distance he can jump a long distance with greater force.

Question 3.

Usually, a fielder in a cricket match takes a catch by moving his hands in the direction of the motion of a cricket ball.

Answer:

Usually, a fielder in a cricket match takes a catch by moving his hands in the direction of the motion of cricket ball due to inertia. Always the ball continues in its state of uniform motion in a straight line in the same direction.

V. Solve the problems.

Question 1.

Calculate the force required to change the velocity of an object of mass 8 Kg from 4 ms^{-1} to 6 ms^{-1} in 2 seconds,

mass = 8 kg

u = 4 ms^{-1}

v = 6 ms^{-1}

t = 2 sec.

a = ?

F = ?

\(a=\frac{v-u}{t}=\frac{6-4}{2}=\frac{2}{2}=1 \mathrm{ms}^{-2}\)

F = ma = 8 x 1 = 8 kg ms^{-2} = 8 N

Question 2.

A force of 25 newtons is applied on an object of mass 10 kg moving with a velocity of 5 ms^{-1}. If the object gains a velocity of 10 ms^{-1} calculates the time taken.

Answer:

Force = 25 N,

mass =10 kg

u = 5 ms^{-1}

v = 10 ms^{-1}

a = ?

t = ?

F = ma

ma = F 10a = 25

a= \(\frac{25}{10}\) = 2.5 ms^{-2}

V = u + at

10 = 5 + 2.5t

2.5t = 10 – 5

2.51 = 5

t = \(\frac{5}{2.5}\) = 2 sec

### KSEEB Class 8 Science Force and Newton’s Laws of Motion Additional Questions and Answers

I. Answer the following:

Question 1.

What is Force?

Answer:

Force is a physical quantity that changes the state of the rest of a body or that changes the velocity of a body or tends to change the shape of the body.

Question 2.

Mention the effects of force?.

Answer:

Force applied on an object can

- Change the velocity of an object
- Change the shape of an object

Question 3.

What are balanced forces?

Answer:

If two or more forces acting on the same object keep the object in on equilibrium position or the same state, then the forces are balanced forces.

Question 4.

What is meant his frictional force?

Answer:

Friction is the opposing force which . comes, into play whenever there is relative motion between two bodies.

Question 5.

Give some examples of motion studied by Galileo Galilei.

Answer:

The examples of motion studied by Galileo Galilei are

- ball moving down an inclined plane.
- ball moving in a hollow hemisphere
- motion of a pendulum.

Question 6.

What is meant by inertia?

Answer:

The tendency of the objects to remain at rest or in a state of motion, when do external forces act on them is called inertias.

Question 7.

State Newton’s first law of motion.

Answer:

Everybody continues in its state of rest or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed on it.

Question 8.

State Newton’s Second law of motion.

Answer:

The acceleration given to a body by a force applied to it is directly proportional to the force and is in the same direction of the force and inversely proportional to the mass of the body.

Question 9.

What is momentum?

Answer:

Momentum is the product of mass and velocity.

Question 10.

What is the conservation of momentum?

Answer:

Conservation of momentum states that the total momentum of a system conserved (remain constant) when the net external force acting on the system is zero.

mv + MV = 0 or mv = -MV

Question 11.

What is unit force?

Answer:

Unit force is that force that produces an acceleration of 1 ms^{-2} in a mass of 1 kg.

Question 12.

State Newton’s third law of motion.

Answer:

To every action, there is always an opposite and equal reaction or The mutual actions of two bodies upon each other are always equal and directed in opposite directions.

II. Fill in the blanks:

- The laws of motion were proposed by __________.
- The famous Mathematics book written by Isaac Newton was __________.
- Newton’s first law of motion is also called as __________.
- To accelerate an object, an external __________ is necessary.
- The resistance to change the state of motion depends upon the __________ of the objects.
- Massive objects have __________ inertia than lighter ones.
- S.l unit of force is __________.
- Action and reaction act on __________ bodies.
- Action and reaction are __________ and
- Momentum = mass x __________.

Answers:

- Isaac Newton,
- Principia Mathematica,
- law of inertia,
- unbalanced force,
- mass,
- more,
- kg ms
^{-2}or newton or N, - different bodies,
- equal and opposite,
- velocity.

III. Four alternatives are given to each of the following incomplete statements/questions, choose the right answer.

Question 1.

The unit of momentum is

(a) Kg sec^{-1}

(b) Kg ms^{-1}

(c) Kg ms^{-2}

(d) Kg N.

Answer:

(b) Kg ms-1

Question 2.

The relationships between force, mass and acceleration

(a) F = ma

(b) F ± m/a

(c) F = a/m

(d) F = m a^{2}

Answer:

(a) F = ma

Question 3.

Which of the following weight has the greatest inertia

(a) 500 gm

(b) 1 Kg

(c) 2 Kg

(d) 5 Kg

Answer:

(d) 5 Kg

Question 4.

A coin pushed on the table stops because

(a) of its weight

(b) of the attraction of the earth

(c) of frictional force

(d) no force acts on it.

Answer:

(c) of frictional force

Question 5.

A force of 10 N acts on a body of mass 5 kg. The acceleration produced is

(a) 2m/sec^{2}

(b) 0.5 m/sec^{2}

(c) 50 m/sec^{2}

(d) 2 cm/sec^{2}

Answer:

(a) 2m/sec^{2}

Question 6.

1 N is the force, which produces

(a) An acceleration of 1 cm /sec^{2} on a body of mass 1g

(b) An acceleration of 1 m/sec^{2} on a body – of mass 1kg

(c) An acceleration of 1 m/sec^{2} on a body of mass Igm

(d) An acceleration of 1 cm/sec^{2} on a body of mass 1 kg.

Answer:

(b) An acceleration of 1 m/sec^{2} on a body – of mass 1kg

Question 7.

According to Newton’s 3rd law, action and reaction act on

(a) the same body in opposite directions.

(b) different bodies in opposite directions.

(c) different bodies but in the same direction.

(d) same body in the same direction.

Answer:

(b) different bodies in opposite directions.

Question 8.

The momentum of a body of a given mass is proportional to

(a) its volume

(b) its shape

(c) its speed

(d) it’s color

Answer:

(c) its speed

Question 9.

When a bus suddenly starts, the passengers who are standing lean backward. This is an example of

(a) Newton’s 1^{st} law

(b) Newton’s 2^{nd} law

(c) Newton’s 3^{rd} law

(d) gravitational force.

Answer:

(a) Newton’s 1st law

Question 10

If Naveen and Vikram are of weights 20 kg and 30 kg respectively then

(a) Naveen has more inertia than Vikram

(b) Vikram has more inertia than Naveen

(c) Naveen a Vikram have the same inertia

(d) Neither Naveen nor Vikram has inertia

Answer:

(b) Vikram has more inertia than Naveen

Problems:

Question 1.

Find out the force required to produce an acceleration of 2 m/sec2 on a body of mass 0.5 kg.

Answer:

m = 0.5 kg

a = 2m

F = ma = 0.5 x 2 = 1 N

Question 2.

A ball of mass 2 kg is initially moving with a velocity of 50 m/sec. On applying a constant force on the ball for 2.0 sec, it acquires a velocity of 70 m/sec. Calculate,

a) the initial momentum of the ball

b) the final momentum of the ball

c) acceleration of the ball

d) the magnitude of the force applied, m = 2kg, u = 50 m/sec, v = 70 m/sec, t = 2 sec.

Answer:

A initial momentum + m x initial velocity 2 x 50 = 100 kg ms^{-1}

final momentum = m x final velocity

= 2 x 70 = 140 kg ms^{-1}

Rateofchangemomentum = m(v-u)/t(F)

\(\frac{(70-50)}{2}\)

Force = 20 N

acceleration x mass = F

F = ma

2a = 20

a = \(\frac{20}{2}\) = 10 m sec^{-2}