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

Question 1.

Define force.

Answer:

A physical quantity that changes the state of rest or of velocity of an object is called force.

Question 2.

Which are the various kinds of forces found in nature?

Answer:

The forces found in nature are gravitational force, magnetic force, electric force and nuclear force.

Question 3.

What are the various effects that can happen on an object when force is applied on it?

Answer:

A force acting on an object can:

- Change the velocity of the object
- Change the shape of the object
- Change its direction of motion
- Cause acceleration in it.

Question 4.

What are balanced forces?

Answer:

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 5.

What are unbalanced forces? Explain.

Answer:

A set of forces acting on an object are called unbalanced forces if they change its speed or direction of motion or both.

Question 6.

Distinguish between balanced and unbalanced forces.

Answer:

Balanced forces | Unbalanced forces |

a. They keep the object in equilibrium position. | They change the speed or direction of motion or both of the object. |

b. Resultant of the forces is zero. | Resultant of the forces is not zero. |

Question 7.

In a tug of war game, if the two teams involved exert equal force, then none of the teams can win. Why.

Answer:

In a tug of war game, if the two teams exert equal force, then they constitute balanced forces. The net effect of the force is zero. Therefore, no team can win. To win the game, one team should exert a higher force than the other.

Question 8.

Which were the three situations studied by Galileo to understand motion of objects?

Answer:

Galileo studied the following situations to understand the motion of objects:

- The motion of a ball rolling down an inclined plane,
- A ball rolling on the inner surface of a shallow hemisphere, and
- The motion of a pendulum.

Question 9.

Is a force necessary to maintain the motion of an object?

Answer:

A force is not necessary to maintain the motion of an object. In the absence of any external forces, an object continues to remain in a state of motion in a straight line, and at constant speed continues indefinitely.

Question 10.

State Newton’s first law of motion.

Answer:

An object continues to remain in its state of rest or uniform motion unless it is acted upon by an external force.

Question 11.

Which are the two important implications of Newton’s first law of motion?

Answer:

Newton’s first law gives a working definition of force and it also talks about an important property of all matter called inertia.

Question 12.

Define force according to Newton’s first law of motion.

Answer:

According to Newton’s first law, force is that which can change the state of rest or of uniform motion of an object.

Question 13.

What is inertia?

Answer:

The tendency of the objects to remain at rest or in state of uniform motion when no forces act on them is called ‘inertia’.

Question 14.

Which are the three types of inertia shown by objects? Explain.

Answer:

Objects show three kinds of inertia namely

- Inertia of rest,
- Inertia of motion
- Inertia of direction.

Objects oppose the change of their state of rest. This is inertia of rest. They oppose the change of their constant speed. This is inertia of motion. They also oppose the change of the direction of their motion. This is inertia of direction.

Question 15.

Why do passengers standing inside a moving bus get pushed forward when brakes are applied suddenly?

Answer:

Standing passengers are pushed forward when a bus stops suddenly. This is due to the inertia of motion of the upper part of the body. When a bus comes to a sudden halt, the portion of the body in contact with the bus will come to a halt while the upper portion of the body tends to remain in motion. This causes the pushing forward of passengers.

Question 16.

In the absence of any resistance, what can a force do to an object?

Answer:

In the absence of external resistance, a force will accelerate the object.

Question 17.

What is the factor on which inertia of an object depends?

Answer:

The inertia of objects depends on their mass. Heavier objects have greater inertia than those objects which have less mass. This means that an object which is heavier shows higher resistance to the change of state of rest or of uniform motion than an object which is having lower mass.

Question 18.

How is the inertia of an object measured? Explain.

Answer:

The inertia of an object is measured by the magnitude of force required to change its state of rest or of uniform motion. The force required to change the state of a heavier object is more than the force required to change the state of a lighter object. This is because, the mass of the heavier object is more than the mass of the lighter object. This means that the mass of an object is a measure of its inertia.

Question 19.

Explain the card and coin experiment to demonstrate the principle of inertia.

Answer:

Place the coin on a card. Place the card over the open end of a glass tumbler. When the card is flipped quickly, the cardboard piece flies away and the coin falls into the tumbler due to the inertia of rest of the coin. Similar effect is seen when the card is pushed suddenly.

Question 20.

Which of the following has highest inertia: a 50 paise coin, a one rupee coin or a five rupee coin?

Answer:

The inertia of a coin does not depend on its value. It just depends on its mass. The coin with highest mass will have the highest inertia. The mass of five rupee coin, one rupee coin and 50 paise coin are 9 g,4.8 g and 3.8 g respectively. Therefore, a five rupee coin has the highest inertia.

Question 21.

Why are car passengers advised to wear seat belts?

Answer:

During an accident or when the brakes are applied suddenly, the car stops suddenly. As a result the passengers are pushed forward due to inertia and may get injured severely. Wearing seat belts will slow down and prevent the passengers from falling forward suddenly. Therefore, the impact becomes less. Therefore, passengers travelling in a car must wear seat belts.

Question 22.

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

Answer:

When we are in a moving bus, our entire body is in motion. When we get down from a moving bus, our feet (lower part of the body) will suddenly come to rest on coming in contact with the ground. However, the upper part of the body will remain in motion due to inertia. Thus, there is a chance that we may fall off. Therefore, it is dangerous to get down from a moving bus.

Question 23.

State Newton’s second law of motion. Express it in the form of an equation.

Answer:

Newton’s second law of motion states that “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”.

Consider an object of mass ‘m’ under the action of an unbalanced force ‘F’. If the acceleration produced in the object is ‘a’, then, according to Newton’s second law,

a ∝ \(\frac{\mathrm{F}}{\mathrm{m}}\) a = K \(\frac{\mathrm{F}}{\mathrm{m}}\)

where K is the constant of proportionality whose value is equal to 1 in S.I. system. Thus, F = ma.

Question 24.

What is the S.I. unit of force? Define it.

Answer:

The S.I. unit of force is called ‘newton’. It is represented by the symbol ‘N’.

One newton (1 N) of force is defined as the force required to give an object of mass 1 kg an acceleration of 1 m s^{-2}.

1 N = I kg m s^{-2}

Question 25.

Calculate the force required to give an object of mass 5 kg an acceleration of 0.8 m s^{-2}.

Answer:

Mass of the object m = 5 kg .

Acceleration produced a = 0.8 m s^{-2}

Force required F = ?

According to Newton’s second law, F = ma

= 5 kg × 0.8 m s^{-2}

= 4 kg m s^{-2}

= 4 N.

Question 26.

Calculate the force required to change the velocity of an object of mass 4 kg from 2 s^{-2} to 4 s^{-2} in one second.

Answer:

F = ma Acceleration, a = \(\frac{v-u}{t}=\frac{v-u}{t}\) = 2 ms^{-2}

m = 4 kg, a = 2 ms^{-2}, F = ?

F = 4 kg × 2 ms^{-2}

F = 4 × 2 = 8 kg ms^{-2} or 8 newton or 8 N.

Question 27.

Find the acceleration produced in an object of mass 25 kg by an unbalanced force of 40 N that acts on it.

Answer:

Mass of the object m = 25 kg; Force acting F = 40 N; Acceleration produced a = ?

Acceleration a = \(\frac{\mathrm{F}}{\mathrm{m}}\)

= \(\frac{40 \mathrm{N}}{25 \mathrm{kg}}\)

= 1.6 m s^{-2}.

Question 28.

An object of mass 10 kg is moving with a uniform velocity of 2 ms^{-1}. What is the unbalanced force required to make it come to rest in 1 second?

Answer:

Mass of the object m = 10 kg ; Initial velocity u = 2 m s^{-1}; Final velocity v = 0

Time taken t = 1 s; Force required F = ?

Change in velocity in one second or acceleration = \(\frac{\text { Change in velocity }}{\text { Time taken }}\)

a = \(\frac{v-u}{t}=\frac{0-2}{1}\) = -2 ms^{-2}.

According to Newton’s second law, F = ma.

F = 10 × -2

= -20 kg ms^{-2} or -20 N.

Question 29.

An object of mass 20 kg is moving with a uniform velocity of 2 m s^{-1}. If its velocity changes to 4 m s^{-1} in one second, what should be the force acting on it? What is the direction of motion of the object?

Answer:

Mass of the object m = 20 kg; Initial velocity u = 2 m s^{-1};

Final velocity v = 4 m s^{-1};

Time taken t = 1 s; Force applied F = ?

a = \(\frac{v-u}{t}\)

= \(\frac{4-2}{1}\) = 2 ms^{-2}

According to Newton’s second law, F = ma.

∴ F = 20 × 2 = 40 N.

Question 30.

What is meant by momentum of an object? Express it in the form of an equation. What is the S.I. unit of momentum?

Answer:

The product of mass and velocity of an object is called its momentum.

Consider an object of mass m moving with a velocity v. Let its momentum be P.

Now, Momentum = Mass × Velocity

P = mv

The S.I unit of momentum is kg m s^{-1}.

Question 31.

What are the factors on which the momentum of an object depends?

Answer:

The momentum of an object is directly proportional to the mass and velocity of the object.

Question 32.

A body of mass 20 kg has a momentum of 150 kg m^{-1}. Calculate the velocity of the body.

Answer:

Mass of the object m = 20 kg; Momentum P = 150 kg m s^{-1};

Velocity of the object v = ?

Momentum P = mv

v = \(\frac{P}{m}\)

= \(\frac{150 \mathrm{kg} \mathrm{ms}^{-1}}{20 \mathrm{kg}}\)

= 7.5 m s^{-1}.

Question 33.

An iron ball and a cricket ball of the same volume are moving with the same velocity. Which of them has higher momentum? Why?

Answer:

The iron ball has higher momentum than the cricket ball. This is because the iron ball has higher mass. When two objects are moving with the same velocity, the object with higher mass will have higher momentum.

Question 34.

Which of the following has greater momentum: an object of mass 2 kg moving with uniform velocity of 2ms^{-1} or an object of 1 kg moving with uniform velocity of3m s^{-1}?

Answer:

We know that momentum P = m × v

Momentum of object of mass 1 kg = 1 kg × 3 m s^{-1} = 3 kg m s^{-1} Momentum of object of mass 2 kg = 2 kg × 2 m s^{-1} = 4 kg m s^{-1}

Hence, the object of mass 2 kg has higher momentum.

Question 35.

State the law of conservation of momentum. Explain with a suitable illustration.

Answer:

Whenever two or more objects collide, their total momentum before collision is equal to their total momentum after collision. In other words, the total momentum of a system is always conserved. It is neither created nor destroyed.

Consider two objects of mass m_{1} and m_{2} having initial velocities u_{1} and u_{2}. Let the two masses collide. Let their velocity after collision be v_{1} and v_{1}.

Now, their total momentum before collision = m_{1}u_{1} + m_{2}u_{2}

Their total momentum after collision…………. = m_{1}v_{1} + m_{2}v_{2}

According to the law of conservation of momentum,

Total momentum before collision = Total momentum after collision

m_{1}u_{1} + m_{2}u_{2} = m_{1}v_{1} + m_{2}v_{2}.

Question 36.

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

Answer:

The rate of change of momentum of an object is directly proportional to the unbalanced force acting on it.

Question 37.

Why does an athlete in long jump run a distance before taking the leap?

Answer:

An athlete always runs before jumping to gain momentum. This helps in jumping higher and longer as the rate of change of momentum becomes less.

Question 38.

Why does a fielder in cricket usually move his hands back while trying to catch a cricket ball?

OR

Give reason: Usually a fielder in a cricket match moves his hands in the direction of motion of the cricket ball after taking the catch.

Answer:

In a cricket match a fielder moves his arms back while trying to catch a cricket ball in order to reduce the rate of change of momentum of the ball. If the fielder tries to stop the fast moving ball suddenly, then the speed decreases to zero in a very short time. This could hurt the fielder as pressure on the hands is high. When the fielder moves his arms back, the force exerted by the ball on the hand decreases and the fielder does not get hurt.

Question 39.

Calculate the force required to change the velocity of an object of 8 kg from 4 m s^{-1} to 6 m s^{-1} in 2 second.

Answer:

Mass of the object m = 8 kg ; Initial velocity u = 4 m s^{-1}; Final velocity v = 6 m s^{-1};

Time taken t = 2s; Force required F = ?

Force required F = Rate of change of momentum

Question 40.

A force of25 N is applied on an object of 10 kg moving with a velocity of 5 ms^{-1}. If the object gains a velocity to 10 m s^{-1}, calculate the time taken.

Answer:

Force applied F = 25 N ; Mass of the object m = 10 kg ; Initial velocity u = 5 m s^{-1};

Final velocity v = 10 m s^{-1}; Time taken t = ?

Question 41.

State Newton’s third law of motion.

Answer:

Newton’s third law states that “For every action, there is an equal and opposite reaction.”

Question 42.

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

Answer:

- When a swimmer swims, he pushes the water backwards (action). The water pushes the swimmer forward (reaction) with the same force.
- In rocket motion, the rocket exerts force (action) on the gases to expel them through the nozzle backwards. The gases exert an equal and opposite force (reaction) on the rocket due to which it moves in the forward direction.

Question 43.

What is the principle on which a rocket works?

Answer:

A rocket works on the principle of conservation of momentum and Newton’s third law of motion.

Question 44.

Describe a simple activity to demonstrate Newton’s third law of motion.

Answer:

Fill a balloon with air. Make its mouth narrower by tying the narrow tube of a ball point pen. Now release the balloon. The air moves downwards and the balloon moves upwards. Here, there are two forces. One is the force with which air moves out of the balloon and the other is the force with which the balloon moves up. These forces are equal and opposite. The air moving downwards is the action and the balloon moving up is the reaction.

Question 45.

Explain briefly how a rocket works on the basis of Newton’s third law of motion.

Answer:

In a rocket, burning gas is expelled from the rocket and the rocket moves upwards in accordance with Newton’s third law of motion.

The hot gas rushes out from the nozzle at the bottom of the rocket at high speed. There is a downward force that expels the hot gas out of the rocket. Another force of the same magnitude acts on the body of the rocket in the opposite direction causing it to move. The force exerted on the hot gas and the force exerted on the rocket form an action-reaction pair.

Question 46.

How does Newton’s third law explain the motion of a person while swimming?

Answer:

A swimmer pushes the water in the backward direction with his hands and legs. The water exerts a force on the swimmer which pushes him forward. Here, one force is acting on water and the other on the swimmer’s body. These two forces are equal and opposite. Pushing of the water backwards is the action and the swimmer moving forward is the reaction.

Question 47.

How is Newton’s third law applicable to rowing of a traditional boat?

Answer:

A boat, when rowed, accelerates by action-reaction principle. If we push the water in one direction with an oar, the boat moves in the opposite direction. Here, the momentum exerted on water is equal and opposite to the momentum acquired by the boat.

Question 48.

A bullet of mass 25 g is fired from a rifle of mass 2 kg. Imagine that no other force acts on the system. If the bullet travels with a velocity of 2 m s^{-1}, what will be the velocity of the recoil of the rifle?

Answer:

Mass of the bullet m_{1} = 25 g = 0.025 kg; Initial velocity of the bullet u_{1} = 0;

Final velocity of the bullet v_{1} = 2 m s^{-1}; Mass of the rifle m_{2} = 2 kg;

Initial velocity of the rifle u_{2} = 0; Final velocity of the rifle v_{2} = ?

According to the law of conservation of momentum,

m_{1}u_{1} + m_{2}u_{2} = m_{1}v_{1} + m_{2}v_{2}

m_{1}v_{1} + m_{2}v_{2} = 0

0.025 kg × 2 m s^{-1} = 2 kg × v_{2}

v_{2} = \(\frac{-0.025 \mathrm{kg} \times 2 \mathrm{ms}^{-1}}{2 \mathrm{kg}}\) = -0.25 m s^{-1}

The negative sign indicates that the rifle moves in the direction opposite to that of the bullet.

Multiple Choice Questions

Question 1.

Which of the following is not correct? Action and reaction

(a) act on the same object

(b) are equal and opposite

(c) are equal but act in the same direction

(d) unequal but act on the same object

Answer:

(b) are equal and opposite

Question 2.

When a force of l N acts on a mass of l kg which is able to move freely, the object moves with a

(a) speed of 1 m s^{-1}

(b) speed of 1 km s^{-1}

(c) acceleration of 10 m s^{-2}

(d) acceleration of 1 m s^{-2}

Answer:

(d) acceleration of 1 m s^{-2}

Question 3.

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

(a) will be accelerated

(b) will be in motion

(c) will be at rest

(d) loses its shape

Answer:

(c) will be at rest

Question 4.

The inertia of an object tends to cause an object to

(a) resist any change in its state of motion

(b) increase its speed

(c) decrease its speed

(d) decelerate due to friction

Answer:

(a) resist any change in its state of motion

Question 5.

If a set of forces acting on an object are balanced, then, the object must

(a) be at rest

(b) be accelerating

(c) not be accelerating

(d) be slowing down.

Answer:

(c) not be accelerating

Question 6.

Action and reaction act on

(a) same body in opposite directions

(b) different bodies in opposite directions

(c) different bodies but in same direction

(d) same body in same direction.

Answer:

(b) different bodies in opposite directions

Question 7.

Which one of the following terms is used to indicate the natural tendency of an object to remain at rest or in motion at a constant speed along a straight line?

(a) velocity

(b) acceleration

(c) inertia

(d) force

Answer:

(c) inertia

Question 8.

The correct way to write S.I. unit of force is

(a) Newtons

(b) newton

(c) newtons

(d) Newton

Answer:

(b) newton

Question 9.

Which of the following statements is not true about momentum?

(a) Momentum has both quantity and direction.

(b) An object with mass will have momentum.

(c) Momentum of an object is the product of its mass and velocity.

(d) The momentum of an object varies inversely with its velocity.

Answer:

(b) An object with mass will have momentum.

Question 10.

1 newton =

(a) 1 kg

(b) 1 kg m s^{-1}

(c) 1 kg m s^{-2}

(d) 1 m s^{-2} kg^{-1}

Answer:

(c) 1 kg m s^{-2}

Question 11.

A force of 10 N is acting on an object of mass 10 kg. What is the acceleration produced in it?

(a) 20 m s^{-2}

(b) 1 m s^{-2}

(c) 100 m s^{-2}

(d) 10 m s^{-2}

Answer:

(b) 1 m s^{-2}

Question 12.

The product of mass and velocity of an object is known as

(a) acceleration

(b) force

(c) momentum

(d) velocity

Answer:

(c) momentum

Question 13.

The working of a rocket is based on

(a) Newton’s first law of motion

(b) Newton’s second law of motion

(c) Newton’s third law of motion

(d) Newton’s first and second law

Answer:

(c) Newton’s third law of motion

Question 14.

A and B are two objects with masses 8 kg and 32 kg respectively. If this is so, then,

(a) A has more inertia than B

(b) B has more inertia than A

(c) A and B have the same inertia

(d) A has twice the inertia of B

Answer:

(b) B has more inertia than A

Fill In The Blanks

1. Inertia of an object is proportional to its mass

2. The S.I. unit of momentum is kg m s^{-1}

3. Newton’s first law of motion is also called the law of inertia

4. causes motion in a body. Force

5. Action and reaction are always equal and opposite

6. On a frictionless horizontal surface, the velocity of a ball does not change

7. The product of mass and velocity of an object is its momentum

8. Newton’s second law can be written in the form of an equation as F = ma

9. To cause acceleration in an object, the necessary factor is unbalanced force

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