# KSEEB Solutions for Class 8 Science Chapter 9 Force and Newton’s Laws of Motion

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

### KSEEB Class 8 Science Chapter 9 Textual Questions & Answers

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

Question 1.
Which one of the following is not correct – Action and reaction
a) act on the same body
b) are equal
c) are opposite
d) act on different bodies
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 have zero acceleration,
d) the object loses its shape
c) the object will have zero acceleration

Question 3.
The correct way to write S.I. unit of force is
a) Newtons
b) newton
c) newtons
d) newton
b) newton

II. Fill in the blanks

Question 1.
Inertia of an object is proportional to its ……………
mass

Question 2.
The SI unit of momentum is ………..
kg ms-1

Question 3.
To cause acceleration in an object, the necessary factor is unbalanced ……….
force

Question 4.
Newton’s first law of motion is also called law of …………..
inertia

Question 5.
The suitcases kept in a moving bus move forward when the brakes are applied suddenly this is due to ……….
inertia

Question 1.
In a tug of war game if the two teams exert equal forces none of the team wons. Why?
In a tug of war game, if the two team exert equal forces none of the team wons since it maintain the tug in the equilibrium position or in the same state as the forces are balanced forces.

Question 2.
Which of the following has greater momentum?
a) An object of mass 2 kg moving with uniform velocity of 2 ms-1
b) An object of 1 kg mass moving with uniform velocity of 3 ms-1
a) Mass = 2 kg
Velocity = 2 ms-1
Momentum M = mass × velocity
= 2 x 2 = 4 kg ms-1

b) Mass m = 1 kg
Velocity v = 3 ms-1
Momentum M = mass × 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 paisa coin
b) one rupee coin
c) five rupee coin
c) five rupee coin has greater inertia.

Question 4.
State any two illustrations for Newton’s third law of motion.

1. A swimmer pushes the water backward to move forward
2. A boatman Standing in a boat which 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.

Question 5.
Distinguish between balanced and unbalanced forces.

1. Balanced forces :
• The effect of the force cannot be observed.
• No change in the speed or direction of the motion or both.
• Equilibrium position is maintained,
2. Unbalanced force :
• The effect of the force can be observed.
• Change in the speed or direction of the motion or both.
• Equilibrium position is not maintained.

Question 6.
State newton’s second law of motion in the terms of momentum.
The acceleration given to a body by a force applied to it is directly proportional to the force and in the same direction of the force and inversely proportional to the mass of the body. F = ma

IV. Give a reason for the following.

Question 1.
It is dangerous to get down from a moving bus even if the bus is moving slowly.
The person getting down from a moving bus runs for a certain distance in the direction of the bus due to inertia which may lead to an accident. Therefore it is dangerous to get down from a moving bus.

Question 2.
An athlete in a long-jump runs a distance before taking the leap.
It is possible to jump a longer distance if the athlete runs a distance before taking the leap. 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 wilt continue to move with some velocity when no force acts on it. According to this principle, if an athlete runs from a distance, he/she can jump a longer distance with the same 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.
Usually, a fielder in a cricket match takes a catch by after taking the moving his hands in the direction of the motion Of cricket ball. Due to inertia, the ball continues in its state of uniform motion in a straight line in the same direction. So the impact on the hands will be less.

V. Solve the problems.

Question 1.
Calculate the force required to change the velocity, of an object of mass 8kg from 4 ms-1 to 6 ms-1 in 2 seconds.
Data :
m = 8 kg
u = 4 ms-1
v = 6 ms-1
t = 2 sec
a = ?
F = ?
Solution:
a = $$\frac { v-u }{ t }$$
= $$\frac { 6-4 }{ 2 }$$
= $$\frac { 2 }{ 2 }$$ = 1 ms-1
F = ma
=8 × 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. Calculate the time interval.
Data :
F = 25 N
m = 10 kg
u = 5 ms-1
v = 10 ms-1
a = ?
t = ?
Solution :
F = ma
ma = F
10 a = 25
a = $$\frac { 25 }{ 10 }$$ = 2.5 ms-2
v = u + at
10 = 5 + 2.5 t
2.5 t = 10 – 5
2.5 t = 5
t = $$\frac {5 }{ 2.5 }$$ = 2 sec

Question 1.
Define force? Mention the effect of force.
Force is a physical quantity that changes the state of the rest of a body or that changes the velocity of a body. Effects of force applied on an object can

1. Change the velocity of an object
2. Change the shape of an object.

Question 2.
What are unbalanced forces?
The effect of force will be observed when an unbalanced force acts on an object. Unbalanced forced acting on an object, changes its speed or direction of motion.

Question 3.
What are balanced forces?
If two or more forces acting on the same object keep the object in equilibrium position or in the same state, then the forces are balanced forces.

Question 4.
What is meant by Frictional force?
Friction is the opposing force which comes, whenever there is relative motion between two bodies.

Question 5.
What is meant by inertia?
The tendency of the objects to remain at rest or state of motion when no forces act on them is called inertia.

Question 6.
State Newton’s first law of motion.
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 7.
What are the SI units of mass and acceleration?
SI unit of mass is kg SI unit of acceleration is ms-2

Question 8.
Calculate the force required to change the velocity of an object of mass 4 kg from 2 ms-2 to 4 ms-2 in one second.
Solution
F = ma
m = 4 kg
a = 2 ms-2
F = ?
F = 4 kg x acceleration
F = 4 x 2
= 8 kg ms-1 or 8 newton or 8 N.

Question 9.
An object of mass 10 kg is moving with an initial velocity of 2 ms-1. To make it come to rest in one second, what should be the unbalanced force?
Solution,
initial velocity = 2 ms-1
velocity at rest = 0 ms-1
time interval = 1 second
mass m = 10 kg
change in velocity in one second or acceleration
= $$\frac { 0-2 }{ 1 }$$ = 2 ms-1
F = ma
F = 10 × 2
= 20 kg ms-2 or -20 newton or-20 N.
The negative sign tells us that the force exerted is opposite to the direction of motion.

Question 10.
An object of mass 20 kg is moving with an initial velocity of 2 ms-1. If its velocity changes to 4 ms-1 in one second. What should be the force acting on it? What will be the direction of the motion?
Solution
F = ma
m = 20 kg
a = $$\frac { 4-2 }{ 1 }$$ = 2 ms-1
F = 20 x 2 = 40 newton or 40 N.
The object continues to move in the same direction as the force applied is in the same direction.

Question 11.
State the conservation of momentum
Conservation of momentum states that the total momentum of a system conserved when the net external force acting on the system is zero.
mv + MV = 0 or mv = – MV

Question 12.
State Newton’s third law of motion.
“To every action there is always an opposed and equal reaction or The mutual actions of two bodies upon each other are always equal and directed in opposite direction.

Question 13.
A bullet of mass 25 g is fired from a rifle of mass 2 kg. Imagine that no other forces are acting on the system. If the bullet travels with a velocity of 2 ms-1, what will be the velocity of the recoil of the rifle?
2 × velocity of recoil = $$\frac { 25 }{ 1000 }$$ × 2
2 × velocity of recoil = $$\frac { 1\times 2 }{ 40 }$$ = $$\frac { 1 }{ 2 }$$
velocity of recoil = $$\frac { 1 }{ 2 }$$ ×  $$\frac { 1 }{ 20 }$$ =  $$\frac { 1 }{ 40 }$$ =0.0025 ms-1