Class 9 Science Chapter 8 Important Questions of Force and Laws of Motion. Class 9 Science Chapter 8 Extra Question answers contains all the intext and exercises important questions with answers. These are helpful for the preparation of class tests and terminal exams.

## Class 9 Science Chapter 8 Important Questions for 2024-25

### Class 9 Science Chapter 8 Important Extra Questions Set – 1

### Name the scientist who proved for the first time that objects move with constant speed when no force acts on them.

Galileo.

### Why do bicycles begin to slow down when we stop pedaling?

This is because of the frictional forces acting opposite to the direction of motion.

### Which law of motion gives the measure of force?

Newton’s second law of motion.

### Write the C.G.S unit of force.

Dyne.

### Can every force produce motion in every object?

No.

#### Why mass is sometimes called coefficient of linear inertia?

It is easier to pull a lighter body than a heavier body. Therefore, more the mass more will be the inertia. That is why mass is sometimes termed as coefficient of linear inertia.

#### Why does a boat tend to leave the shore, when passengers are alighting from it?

When the passengers alight from the boat, they push the boat in backward direction. As a result, the boat has a tendency to slip back into water. This difficulty is usually overcome by the boatman by tying the boat to some rigid support.

### Class 9 Science Chapter 8 Important Extra Questions Set – 2

### When a force is applied to a body, what are the two essential effects it can produce?

(a) It can bring the change in the state of motion of a body or

(b) It can deform a body, i.e. can change its shape.

### Define 1 newton force.

1 newton is the magnitude of force which produces an acceleration of 1m/s2 in a body of mass 1kg.

### What do you mean by an impact force?

The force produces by the impact of a fast moving object on another is called impact force.

### Define force of friction.

The force acting between any two surfaces in contact and tending to oppose motion is called force of friction.

### Define electrostatic force.

The force exerted by an electrically charged body is called electrostatic force.

### Class 9 Science Chapter 8 Important Extra Questions Set – 3

### If the body is found to be accelerated, is the force acting on it balanced or unbalanced?

Unbalanced.

### What do balanced forces usually do to a body?

Balanced force usually produce a change in the shape of the body.

### When a body move on flat surface, will its speed change?

No.

### What did Galileo conclude on the basic of his experiments on the motion of objects?

A body continuous to move with the same velocity if no unbalanced force acts on it.

### What do you mean by a resultant force?

When two or more forces act on a body simultaneously, then the single force which produces the

#### Why does a cricket player move his hand backward while catching the ball?

A fast moving cricket ball has a large momentum. In shopping or catching this ball, its momentum has reduced to be zero. Now, when a cricket player moves back hands on catching the fast ball, then the time taken to reduce the momentum of ball to zero is increased. Due to more time taken to stop the ball, the rate of change of momentum of ball is decreased and hence a small force is exerted on the hands of player. So, the hands of player do not get hurt.

#### When small boy is trying to push a heavy stone, mention various forces acting on the stone?

The various forces acting on the stone are:

- The gravitational force exerted by the earth which pulls the stone downwards.
- The force of reaction exerted by the ground on the stone vertically upwards.
- The force of pushing exerted by the boy.
- The force of friction exerted by the stone.
- When a small boy tries to push a heavy stone, then all these forces are balanced, and therefore the stone does not move.

### Class 9 Science Chapter 8 Important Extra Questions Set – 4

### Do action and reaction act on the same body?

No, action and reaction act on different bodies.

### Write the SI unit of impulse.

Ns.

### What is the total momentum of a bullet a and a gun a before firing?

Zero.

### Name the principle on which a rocket works.

Newton’s third law of motion.

### Body A is heavier than body Q. Which has more inertia?

A has more inertia.

### Class 9 Science Chapter 8 Important Extra Questions Set – 5

### A body is moving with uniform acceleration. Is its momentum constant?

No.

### Name the physical quantity that corresponds to the rate of change of momentum.

Force.

### Which principle is involved in the working of a jet plane?

Newton’s third law of motion.

### When a force acting on a body has equal and opposite reaction, then why should the body move at all?

The action and reaction act on different bodies. Therefore, the body moves under the action of the acting forces.

### Does Newton’s third law apply to a system where bodies do not actually touch each other?

Yes, whenever the bodies are in actual contact or even if there is an interaction between the bodies (e.g., attraction or repulsion between two magnets charges etc.) Newton’s third law is applicable.

#### Why do the driver and the person seated in front seat need a seat belt?

In a car accident, a fast running car stops suddenly. Due to this the car’s large momentum is reduced to zero in a very short time. The stretchable seat belts tightened by the passengers of the car increases time taken by the passengers to fall forward. Due to longer time, the rate change of momentum of passengers is reduced and hence less stopping force acts on them. So, the passengers may either not get injured at all or may get less injuries. It is obvious that seat belts reduce the passenger’s momentum more gently and hence prevent injuries.

#### Newton’s First Law of Motion

According to Newton’s first law of motion, a body at rest or in uniform motion will remain at rest or in uniform motion unless an unbalanced force acts upon it. This law consists of three parts:

- The first part says that a body at rest continuous in its state of rest. For instance, a boy standing in a train falls backwards when the train suddenly starts moving forwards. This is because when the bus moves, the lower part of his body begins to move along with the train while the upper part of his body continuous to remain at rest due to inertia.
- The second part says that a body in uniform motion continues to move in straight line path with a uniform speed. E.g. when a moving train stops suddenly a person sitting in it falls forward. This is because as train stops, the lower parts of the person’s body comes to rest along with the bus while upper part of his body continues to remain in motion due to inertia of motion and thus he falls forwards.
- Third part says that a body moving with a uniform speed in a straight line cannot change its direction of motion by itself. For example, when a bus takes a sharp turn, a person sitting in a bus get force acting away from the centre of the curved path due to his tendency to move in the original direction.

### Class 9 Science Chapter 8 Important Extra Questions Set – 6

### Suppose a ball of mass m is thrown vertically upwards with an initial speed v, its speed decreases continuously till it becomes zero. Therefore, the ball begins to fall downward and attains the speed v again before striking the ground. It implies that the magnitude of initial and final moment of the ball are same. Yet, it is not an example of conservation of momentum. Explain why.

Law of conservation of momentum is applicable to isolated system (no external force is applied). In this case, the change in velocity is due to the gravitational force of earth.

### What is the ratio of SI units to CGS units of momentum? How do you measure the effect of an impulsive force on the body?

A ratio of SI units to CGS units of momentum is (kg m/s)/(g cm/s) i.e., 105.

The effect of an impulse force on the body is measured only in terms of impulse.

### Why can a small mass such as a bullet kill a person when fired from a gun?

It is so because even if the mass of the bullet is small, it moves out of the gun with a very high velocity, due to which the momentum produced is high (p=mv). This high momentum of the bullet kills a person.

### Describe our walking in terms of Newton’s third law of motion.

When we walk on the ground or road, our foot pushes the ground backward (action) and the ground pushes our foot forward (reaction). Thus, the forward reaction exerted by the ground on our foot makes us walk forward.

### Class 9 Science Chapter 8 Important Extra Questions Set – 7

### There are three solids made up of Aluminium, steel and wood, of the same shape and same volume. Which of them would have highest inertia?

Steel has the highest inertia. As the mass is a measure of inertia, the ball of same shape and size, having more mass than other balls will have highest inertia. Since steel has greatest density and greatest mass, therefore it has highest inertia.

### Two identical bullets are fired one by a light rifle and other by a heavy rifle with the same force. Which rifle will hurt the shoulder more and Why?

According to conservation of momentum, the rifle recoils with same momentum as that of bullet. As momentum = mass × velocity; so light rifle will recoil with larger velocity and hence, will hurt the shoulder more.

### Water sprinkler used for grass lawns begins to rotate as soon as the water is supplied. Explain the principle on which it works.

The working of the rotation of sprinkler is based on third law of motion. As the water comes out of the nozzle of the sprinkler, an equal and opposite reaction force comes into play. So the sprinkler starts rotating.

### Deduce Newton’s first law from the second law.

According to second law, F = ma

If F = 0, a = 0 since m ≠ 0,

But a = (v – u )/t

Or v – u = 0 so, v = u for whatever time t is taken.

This means that the object will continue moving with the uniform velocity, u throughout the time, t. If u is zero, then v will be zero. That is the object will remain at rest.

### Describe in brief an activity to illustrate the property of inertia of rest.

(a) Make a pile of similar carom coins on a table.

(b) Attempt a sharp horizontal hit at the bottom of the pile using another carom coin or the striker. If the hit is strong enough, the bottom coin moves out quickly. Once the lowest coin is removed, the inertia of the other coins make them fall vertically on the table.

##### What is the relationship between mass and inertia? Explain with the help of examples.

The mass of a body is a measure of its inertia. It means larger the mass of a body, larger will be the inertia offered by the body to change its state of motion. The following examples will clarify the concept:

- When we kick a football, it flies a long way but at the same time if the we kick a stone of the same size, it hardly moves, as the stone resists a change in its motion better than the football because of its more mass.
- We can increase the velocity of our bicycle by pedaling harder i.e. on applying more force. But the same force will produce a negligible change in the motion of a bus, because in comparison to the bicycle, a bus has more tendency to oppose any change in its state of motion because of its larger mass. i.e. the bus has more inertia than the bicycle.

### Class 9 Science Chapter 8 Important Extra Questions Set – 8

### Give few examples of Newton’s third law of motion.

(a) Jet aeroplanes and rockets work on the principle of third law of motion.

In this case, the hot gases come out of a nozzle with the great force, i.e. action and the rocket moves with high speed upwards as a reaction.

(b) If we fill a balloon with air and hold it with its mouth downwards then we released the balloon the air rushes out vertically downwards (action). The balloon moves vertically upwards (reaction).

### Why does an athlete puts some sands or cushion on the ground while high jumping?

Separation between them will increase. Initially the momentum of both of them are zero as they at rest. In order to conserve the momentum, the one who throws the ball would move backward. The second will experience a net force after catching the ball and therefore will move backwards that is in the direction of the force.

### Two friends on roller skates are standing 5m apart facing each other. One of them throws a ball of 2 kg towards the other, who catches it. How will this activity affect the position of the two? Explain your answer.

When a higher jumper falls on a soft landing site (such as cushion or a heap of sand) then the jumper takes a longer time to come to stop. The rate of change of momentum of athlete is less due to which a smaller stopping force acts on the athlete. And the athlete does not get hurt. Thus the cushion or sand, being soft, reduces the athlete momentum more gently. If however, a high jumping athlete falls from a height on to hard ground, then his momentum will be reduced to zero in a very short time. The rate of change of momentum will be large due to which a large opposing force will acting on the athlete. This can cause series injuries to the athlete.

### Describe balanced forces with the help of two examples.

If the resultant of various forces acting on a body is zero, the forces are said to be balanced forces. These forces do not change the speed but usually change the shape of an object.

Examples:

(a) Consider a wooden block lying on a table, the strings tied to its two opposite faces.

If we pull a point P, it being to move towards left. If we pull at point Q it begins to move towards right. But if we pull from both the sides with equal force, the blood does not move. The two forces have now balanced each other.

(b) In a tug of water the two teams pull the rope with equal effort; the rope is not moved in any direction. This is clearly because the forces exerted by the two teams are equal and opposite and thus get balanced.

### Derive the mathematically relation of Newton’s second law of motion.

Consider an object of mass m moving along a straight line with an initial velocity u (say). It is uniformly accelerated to velocity u in time t by the application of a constant force F in time t.

Then, initial momentum of the object. = mu

P₁ = mu

Final momentum of the object = mv

P₂ = mv

Change in momentum = mv – mu = m(v – u)

The rate of change in momentum = (m ×(v – u))/t

According to Newton’s second law of motion, we have

F ∝ (m ×(v – u))/t

F = km ((v – u))/t

F = kma …. (1)

Here a = ((v – u))/t = the rate of change of velocity

= acceleration

k = a constant of proportionality

Putting m = 1 kg, a = 1 ms⁻²

F becomes 1N.

So, 1N = k × 1 kg × 1 ms⁻²

k = 1

From equation (1) we have

F = ma

This represent the second law of motion.

Thus the second law of motion gives a method to measure the force acting on an object as a product of its mass and direction.

### Class 9 Science Chapter 8 Important Extra Questions Set – 9

### Derive the mathematical formula of conservation of momentum.

To explain conservation of momentum, let us take the following examples. Considered two balls A and B having masses m1 and m2 respectively. Let the initial velocity of ball A be u₁ and that of ball B be u₂ (u₁ > u₂). Their collection takes place for a very short interval of time t and after that A and B start moving with velocities v₁ and v₂ (now v₁ < v₂) respectively. The momentum of ball of ball A and after the collision is m1u1 and m₁v₁ respectively. If there are no external forces acting on the body, then the rate of change of momentum of ball A, during the collision will be = (m₁(v₁ - u₁))/t and similarly the rate of change in momentum of ball B = (m₂(v₂ - u₂))/t Let F₁₂ be the force exerted by ball A on B and F₂₁ be the force exerted by ball B on A. Then, according to Newton’s second law of motion F₁₂ = (m₁(v₁ - u₁))/t and F₂₁ = (m₂(v₂ - u₂))/t According to Newton’s third law of motion, we have F₁₂ = F₂₁ Or (m₁(v₁ - u₁))/t = -(m₂(v₂-u₂))/t Or m₁v₁ – m₁u₁ = -m₂v₂ or m₂u₂ or m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂ i.e. Total momentum before collision = Total momentum after collision Thus we find that in a collision between the two balls the total momentum before and after the collision remains unchanged or conserve provided no net force acts on the system. This result is law of conservation of momentum.

### A bullet of 10g strikes a sand bag at a speed of 103ms-1 and gets embedded after travelling 5cm. Calculate: (i) the resistive force exerted by the sand on the bullet. (ii) the time taken by the bullet to come to rest.

(i) m= 10g = 10/100 kg, u= 103 m/s, v = 0, s = 5/100 m

v^2 – u² = 2as

0 – (103)² = 2a x 5/100

a = (-1000 ×1000)/(2 ×5) × 100

= 10⁷ ms⁻²

F = m, a = -10⁵ N

v = u + at

0 = 10³ – 10⁷ t

10⁷ t = 10³

t = 10³/ 10⁷ = 10⁻⁴ s

### A body mass 300g kept at rest breaks into two parts due to internal forces. One part of mass 200g is found to move at a speed of 12 m/s towards the east. What will be the velocity of the other part?

Initially the body was at the rest. The linear momentum of the body is thus p = mu = 0. The body breaks due to internal forces. As the external force acting in it is zero, its linear momentum will remain constant that is zero.

p₁ = m₁v₁ = (200g) × (12 m/s) towards the east.

The linear momentum of the other part must have the same magnitude and should be opposite in direction. It therefore moves towards the west. If its speed is v₂, its linear momentum is

p₂ = m₂v₂ = (100g) × v₂.

Now, m₁v₁ = m₂v₂.

Thus (200g) × (12m/s) = (100g) × v₂ or, v₂ = 24m/s

### A force of 5N produces an acceleration of 8ms-2 on a mass m1 and an acceleration of 24ms-2 on a mass m2. What acceleration would the same force provide if both the masses are tied together?

We know F= ma = 5N or 5kg ms⁻²

m₁ = F/a₁ = 5/8kg

m₂ = F/a₂ = 5/24kg

M = (5/8+ 5/24)kg = (5/6)kg

Acceleration produced in M,

a = F/M = 5/(5/6) = 6 ms⁻²

### The velocity time graph of a ball moving on the surface of floor. Calculate the forces acting on the ball, if mass of the ball is 100g.

The velocity time graph shows that the velocity of the ball at t= 0 is zero. Initial velocity of ball, u = 0

Velocity of ball at t = 4 s is 20m/s

That is final velocity v = 20 m/s

Time, t = 4s

Acceleration of the ball, a = (v – u)/(t )

Or, a = (20 ms⁻¹ – 0)/4s

a = 5 ms⁻²

Also mass of ball,

m = 100g = 100/1000 kg = 1/10 kg

Force acting on the ball, F = ma

Or F = 1/(10 ) kg ×5 ms⁻²

= 0.5 kg ms⁻² = 0.5 N [1 kg ms⁻² = 1N]

##### Factors for Friction

The force of friction is directly proportional to the weight of the weight of the body sliding over the surface. The force of friction also depends on the nature of the surfaces in contact.