Force & Laws Of Motion Class 9 Science Notes

Please refer to Force & Laws Of Motion Class 9 Science Notes and important questions below. The Class 9 Science Chapter wise notes have been prepared based on the latest syllabus issued for the current academic year by CBSE. Students should revise these notes and go through important Class 9 Science examination questions given below to obtain better marks in exams

Force & Laws Of Motion Class 9 Science Notes and Questions

The below Class 9 Force & Laws Of Motion notes have been designed by expert Science teachers. These will help you a lot to understand all the important topics given in your NCERT Class 9 Science textbook.

Refer to Chapter 9 Force & Laws Of Motion Notes below which have been designed as per the latest syllabus issued by CBSE and will be very useful for upcoming examinations to help clear your concepts and get better marks in examinations.

1 Balanced and Unbalanced Forces

Balanced Forces: The net force is when two or more forces are applied on the same object and at the same time. The applied forces combined are called the net force.

Balanced Forces The force I apply in one direction plus the force you apply in the opposite direction are added together. 2 Because the forces are equal and balanced

Unbalanced Forces: A force is applied in one direction and either another smaller or larger force is applied in the opposite direction or no force is applied at all in the opposite direction.

Unbalanced Forces If I have a chair and I push on one side of it with a force of 50 N and you push on the other side, with a force of 25 N, will the chair move? Which way will it move? The direction in which the most force is applied. What is the net force? 50 N 25 N.

2. Laws of Motion

Newton’s First Law
1st Law – An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.
An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.

Newton’s Second Law
“If the net force on an object is not zero, the object will accelerate. The direction of the acceleration is the same as the direction of the net force. The magnitude of the acceleration is directly proportional to the net force applied, and inversely proportional to the mass of the object.”

Mathematical symbols provide a convenient shorthand for all of this:
a=FnetmorFnet=maa=FnetmorFnet=ma

Important Questions Class 9 Science Chapter 9 Force & Laws Of Motion

The Effect of Mass
A force applied to an automobile will not have the same effect as the same force applied to a pencil. An automobile resists accelerating much more than a pencil does, because it has more inertia, or mass.

The acceleration of an object depends not only on how hard you push on it, but also on how much the object resists being pushed.

What is the effect of mass on acceleration? This, too, turns out to be quite simple (I wonder why…). For the same force, an object with twice the mass will have half the acceleration. If it had three times the mass, the same force will produce one-third the acceleration. Four times the mass gives one-fourth of the acceleration, and so on.

This type of relationship between quantities (double one, get half the other) is called an inverse proportion or inverse variation. In other words, then:

Newton’s Second Law of Motion The acceleration of an object is dependent upon both force and mass. Thus, if the colliding objects have unequal mass, they will have unequal accelerations as a result of the contact force which results during the collision.

Newton’s Third Law
Newton’s Third Law is stated as: For every action there is an equal and opposite reaction.

“Action…Reaction” means that forces always occur in pairs. (Forces are interactions between objects, like conversations are interactions between people.) Single, isolated forces never happen. The two forces involved are called the “action force” and the “reaction force.”
These names are unfortunate for a couple of reasons :
Either force in an interaction can be the “action” force or the “reaction” force
The action and reaction forces exist at the same time.

“Equal” Means
Both forces are exactly the same size. They are equal in magnitude. Both forces exist at exactly the same time. They both start at exactly the same instant, and they both stop at exactly the same instant. They are equal in time.

“Opposite” means that the two forces always act in opposite directions – exactly 18001800 apart.

Newton’s third law of motion In every interaction, there is a pair of forces acting on the two interacting objects. The size of the force on the first object equals the size of the force on the second object. The direction of the force on the first object is opposite to the direction of the force on the second object. Forces always come in pairs – equal and opposite action-reaction force pairs.

Newton’s third law of motion applied to collisions between two objects. In a collision between two objects, both objects experience forces which are equal in magnitude and opposite in direction. Such forces cause one object to speed up (gain momentum) and the other object to slow down (lose momentum). According to Newton’s third law, the forces on the two objects are equal in magnitude.

3. Inertia and Mass Inertia is the tendency of an object to resist any change in its motion. An object will continue to move at the same speed in the same direction unless acted upon by an unbalanced force. Inertia & Mass Inertia & Mass of a bowling ball rolled down the road would eventually come to a stop. Friction is an unbalanced force that causes the ball to stop or slow down. Without friction, the ball would keep going.

Mass is the amount of matter in an object. A bowling ball has more mass than a tennis ball. The greater the mass of an object the greater its inertia.

Mass is the measurement of inertia.

4. Conservation of Momentum

Law of Conservation of Momentum
In a closed system, the vector sum of the momenta before and after an impact must be equal.
Before After
m₁v₁ +m₂v₂m₁v₁ +m₂v₂ = m₁v′₁m₁v₁′ + m₂v′₂m₂v₂′
Internal and External Forces

Questions for Class 9 Science Chapter 9 Force & Laws Of Motion

Question. Calculate the force required to impart to a car a velocity of 30 m/s in 10 s. The mass of the car is 1500 kg.
Ans :
u = 0, v = 30 m/s, t = 10 s
v = u + at
30 = 0 + a × 10
a = 3 ms^–2
Now m = 1500 kg, a = 3 ms^–2
Required Force = F = ma
1500 × 3 = 4500 N

Question. Calculate the force required to impact to a car, a velocity of 30 ms^–1 in 10 seconds. The mass of the car is 1,500 kg.
Ans :
Here u = 0 ms–1; v = 30 ms–1; t = 10 s; a = ?
Using v = u + at, we have
30 = 0 + a (10)
a = 3 ms^–2
Now F = ma = 1,500 × 3
or F = 4,500 N

Question. An object undergoes an acceleration of 8 ms^–2 starting from rest. Find the distance travelled in 1 second.
Ans :
Given,
Acceleration, a = 8 ms^–2
Initial velocity, u = 0
Time interval, t = 1 s
Distance travelled, s = ?
Using the equation of motion, s = ut + 2
1 at2, one gets
s = 0 × 1 + 2
1 × 8 × 12 = 4 m
The object travels a distance of 4 m.

Question. Name three non-SI units of force. Define them.
Ans : Some commonly used non-SI units force are :
Dyne : The dyne is the CGS unit of force. One dyne is the force which produces an acceleration of 1 cm/s²in a body mass 1 g, i.e.,
dyne = 1 G * 1 cm/s²
Gram-weight : It is the gravitational unit of force in CGS system. This is denoted as g-wt.
One gram weight is the force which produces an acceleration of 981 cm/s2 in a body of mass 1 g. Thus,
g-wt = 1 g * acceleration due to gravity (g)
Or, 1 g-wt = 1 g * 981 cm/s² = 981 dynes
[a 1 g * 1 cm/s2 = 1 dynes]
Kilogram-weight : It is the gravitational unit of force in SI system. This unit is denoted as kg-wt and it is the force which produces an acceleration of 9.81 m/s²
in a body of mass 1 kg. Thus,
1 kg-wt = 1 kg * 9.81 m/s²

Question. Explain inertia and momentum.
Ans : Inertia : The natural tendency of an object to resist a change in their state of rest or of uniform motion is called inertia. For example : A book lying on a table will remain there until an external force is applied on it to remove or displace it from that position. Momentum : Momentum of body is the quantity of motion possessed by the body. It is equal to the
product of the mass and velocity of the body and is denoted by p. p = mv
Momentum is a vector quantity and its direction is same as the direction of velocity of the object. Its SI unit is kilogram metre per second (kg ms_–1).

Question. Give 3 examples in daily life which shows inertia.
Ans : Three examples of inertia in daily life are :
(i) When we are travelling in a vehicle and sudden brakes are applied we tend to fall forward.
(ii) When we shake the branch of a tree vigorously, leaves fall down.
(iii) If we want to remove the dust from carpet we beat the carpet so that dust fall down.

Question. Name the various types of forces.
Ans : Various types of forces are :
(i) Muscular force,
(ii) Tension,
(iii) Gravitational,
(iv) Reaction,
(v) Frictional forces,
(vi) Electrical forces,
(vii) Magnetic forces.

Question. Use law of conservation to explain why a gun recoils.
Ans : Gun and bullet both are at rest before firing. Hence, their initial velocities are zero. So, the initial momentum of the gun and the bullet is zero before fire. When a bullet is fired from the gun, it moves forward with a large velocity. The bullet imparts an equal and opposite momentum to the gun due to which the gun recoils backwards. Thus, the final momentum of the system is zero.

Question. Why is the movement of a rocket in the upward direction?
Ans : (i) The movement of a rocket in the upward direction can also be explained with the help of the law of conservation of momentum.
(ii) The momentum of a rocket before it is fired is zero. When the rocket is fired, gases are produced n the combustion chamber of the rocket due to the burning of fuel. These gases come out of the rear of the rocket with high speed. The direction of the Momentum of the gases coming out of the rocket is in the downward direction. To conserve the momentum of the system (rocket gases), the rocket moves upward with a momentum equal to the momentum of the gases. The rocket continues to move upward as long as the gases are ejected out of the rocket.

Question. What is momentum?
Ans : The momentum of an object is the product of its mass and velocity and has the same direction as that of the velocity. The SI unit is kg m/s. (p = mv)

Question. State Newton’s third law of motion.
Ans : To every action, there is an equal and opposite reaction and they act on two different bodies.

Question. Which type of force does not change the state of rest or of motion of an object?
Ans : Balanced force.

Question. Why do athletes have a special posture with their right foot resting on a solid supporter for athletic races?
Ans : Athletes have to run the heats and they rest their foot on a solid supports before start so that duringthe start of the race the athlete pushes the support with lot of force and this support gives him equal and opposite push to start the race.

Question. Give any three examples in daily life which are based on Newton’s third law of motion.
Ans : Three examples based on Newton’s third law are : Swimming : We push the water backward to move forward.
(i) Action – water is pushed behind
(ii) Reaction – water pushes the swimmer ahead Firing gun : A bullet fired from a gun and the gun recoils.
(i) Action – gun exerts force on the bullet
(ii) Reaction – bullet exerts an equal and opposite force on the gun Launching of rocket :
(i) Action – hot gases from the rocket are released
(ii) Reaction – the gases exert upward push to the rocket

Question. How are safety belts helpful in preventing any accidents?
or
Why do you think it is necessary to fasten your seat belts while travelling in your vehicle?
Ans : While travelling in a moving car, our body remains in the state of rest with respect to the seat. But when driver applies sudden breaks in the car, our body tendsto continue in the same state of motion because of its inertia. Therefore, this sudden break may cause injury to us by impact or collision. Hence, safety belt exerts a force on our body to make the forward motion slower.

Question. If a ball is thrown up in a moving train, it comes back to the person’s hands. Why?
Ans :
This is because no horizontal force acts on it. It moves with the same horizontal speed.

Question. (a) What is friction?
(b) What are the advantages of friction?
or
Why is friction necessary?
Ans :
(a) The force which opposes the motion of one body over the surface of another is called friction or the force of friction.
(b) Friction is very desirable and an important force in our daily life.
Some advantages of friction are :
(i) The nails and screws hold the wooden boards together due to friction.
(ii) It is due to the friction between the ground/road and the soles of our shoes that we are able to walk.
(iii) The friction between the road and the surface of the tyres permits safe driving.
(iv) We are able to write on paper because of the friction between the pen/pencil and the paper.
That is why, it is very difficult to write on a glazed/waxed paper.
(v) The application of brakes to stop a cycle, scooter or car, etc., is possible due to friction between the brake-lining and the rim of the wheel.

Question. The fruits fall off the branches when a strong wind blows. Give reason.
Ans : Fruits tend to continue in the state of rest on account of inertia while branches suddenly come into motion.

Question. Name the physical quantity which is determined by the rate of change of linear momentum.
Ans : Force.

Question. Two similar trucks are moving with the same velocity on a road. One of them is loaded while the other one is empty. Which of the two will require a larger force to stop it?
Ans : The loaded truck will require a larger force to stop because it has greater momentum.

Question. Name the scientist who introduced the property of momentum.
Ans : Newton.

Question. (i) What are the causes of friction?
(ii) How can friction between any two surfaces can be reduced?
or
Describe some method for reducing friction.
Ans :
The friction rises due to the following factors :
1. Due to the force of attraction between the molecules of the two surfaces in contact. This iscalled the force of adhesion.
2. Due to the interlocking of the surface irregularities.
3. Thus, friction is due to the roughness of the two surfaces in contact.
4. The friction between two surfaces can be reduced by following methods :
(i) By polishing surfaces : Rough surfaces can be made smooth by polishing. Therefore, polishing reduces friction.
(ii) By applying oil or grease on the surfaces : Oil/ grease forms a thin layer between the two surfaces and reduces friction.

Question. What is the relation between Newton’s three laws of motion?
Ans : (i) Newton’s first law explains about the unbalanced force required to bring change in the position of the body.
(ii) Second law explains about the amount of force required to produce a given acceleration.
(iii) While Newton’s third law explains how these forces acting on a body are interrelated.

Question. Why does a ball rebound after striking against a floor?
Ans : When a ball strikes against a floor, it exerts a force on the floor. According to Newton’s third law of motion,
the floor exerts an equal and opposite force on the ball. Due to this reaction, the ball rebounds.

Question. How do we swim?
Ans : While swimming, a swimmer pushes the water backward with his hands. The reaction offered by the water to the swimmer pushes him forward.

Question. An iron sphere of mass 10 kg is dropped from a height of 80 cm. If the downward acceleration of the ball is 10 ms^–2, calculate the momentum transferred to the ground by the ball.
Ans :
Here, initial velocity of sphere, u = 0
Distance travelled, s = 80 cm = 0.8 m
Acceleration of sphere, a = 10 ms–2
Step 1. Final velocity of sphere when it just reaches the ground can be calculated using
v² – u² = 2as
u² – 0 = 2 × 10 ms^–2 × 0.8 m
= 16 m2s–2
or v = √16 m²s^-2 = 4 ms^–1.
Momentum of the sphere just before it touches the ground = mv = 10 kg × 4 ms^–1 = 40 kg ms^–1
Step 2. On reaching the ground, the iron sphere comes to rest, so its final momentum = 0
According to the law of conservation of momentum,
Momentum transferred to the ground = momentum of the sphere just before it comes to rest = 40 kg ms^–1.

Question. Explain why- An inflated balloon lying on the surface of a floor moves forward when pierced with a pin.
Ans : The momentum of the inflated balloon is zero before it is pierced with a pin. Air comes out with a speed in the backward direction from balloon after it is pierced with a pin. The balloon moves in the forward direction to conserve the momentum.

Question. Explain our walking in terms of Newton’s third law of motion.
Ans : When we walk on ground, our foot pushes the ground backward and in return the ground pushes our foot forward. The forward reaction exerted by the ground on our foot makes us walk forward.

Question. There are three solid balls, made up of aluminium,steel and wood of the same shape and volume. Which of them would have highest inertia? Why?
Ans : Steel ball because the density of steel is maximum. For the same shape and volume, steel ball will have maximum mass.

Question. State Newton’s three laws of motion.
Ans : Sir Isssac Newton further studied the idea of Galileo’s on force and motion and presented three laws of motion. These laws are as follows :
(i) First Law : A body remains in resting position unless it is not introduced with an unbalanced external force.
(ii) Second Law : The rate of change of momentum of a body is directly proportional to the applied unbalanced force and change takes place in the direction of the force.
(iii) Third law : Action and reaction are equal and opposite and they act on different bodies.

Question. What is frictional force?
Ans : The force that always opposes the motion of object is called force of friction.

Question. Give example to show the effects of force.
Ans : (i) Place a ball in the ground. Kick it with your foot. The ball starts moving. The ball moves because of the force applied to it.
(ii) If ball is coming towards you, you can kick it in any direction. The direction of motion of the ball changes because of the force applied to it.
(iii) Place a rubber on the ground. Press it with your foot. It is found that the ball is no longer round but takes the shape of an egg, i.e. it is oblong. The shape of the ball has changed because of the force applied on the ball.

Question. Why we tend to get thrown to one side when a motorcar makes a sharp turn at a high speed?
Ans : We tend to get thrown to one side when a motorcar makes a sharp turn at a high speed due to law of inertia. When we are sitting in moving car on a straight road, we tend to continue in our straight-line motion. But when an unbalanced force is applied on car to change the direction of motion, we slip to one side of the seat due to the inertia of our body.

Question. Why do fielders pull their hand gradually with the moving ball while holding a catch?
Ans : While catching a fast moving cricket ball, a fielder on the ground pulls his hands backwards with the moving ball. This is done so that the fielder increases the time during which the high velocity of the moving ball decreases to zero. Thus, the acceleration of the ball is decreased and therefore, the impact of catching the fast moving ball is reduced.

Question. What is inertia?
Ans : The natural tendency of an object to resist a change in their state of rest or of uniform motion is called inertia.

Question. It is required to increase the velocity of a scooter of mass 80 kg from 5 to 25 ms–2 in 2 seconds. Calculate the force required.
Ans :
Given : m = 80 kg,
u = 5 ms^–2
v = 25 ms^–2
and t = 2 s
Now acceleration a = change in velocity time
= v – u/t
= 25 – 5/2 = 10 ms^–2
Force = mass × acceleration of F
= ma
Therefore, F = 80 × 10 = 800 N

Question. State Newton’s first law of motion.
Ans : An object remains in a state of rest or of uniform motion in a straight line unless acted upon by an external unbalanced force.

Question. How is force represented graphically?
Ans : Force is a vector quantity. So, force is represented by a line with an arrow head.
(i) The head of arrow represents the direction of the force.
(ii) The length of the line is proportional to the magnitude of the force.
(iii) This is done by choosing a convenient scale.
(iv) For example : A force of 10 N can be represented by a line with an arrow head.
(v) The length of the line depends upon the scale chosen. So, if a scale of 1 cm = 2 N is chosen, the force of 10 N is represented by a line of 5 cm in length.

Question. What are the disadvantages of friction?
or
Why friction is considered wasteful?
Ans : Friction is considered wasteful because :
(1) Friction leads to a loss of energy. Therefore, it reduces the efficiency of machines.
(2) Friction cause wear and tear of machine’s parts.

Question. Why are tyres corrugated and made rough?
Ans : Tyres are corrugated and made rough to increase friction.
(1) This provides better grip on the road.
(2) It also prevents the vehicles from slipping.

Question. Why you get hurt by hitting a stone while when you kick a football it flies away?
Ans :
This is because stone is heavier than football and heavier objects offer larger inertia. When we kick a football its mass is less and inertia is also less so force applied by our kick acts on it and hence it shows larger displacement but in case of stone, it has more mass and offers larger inertia. When we kick (action) the stone it exerts an equal and opposite force (reaction) and hence it hurts the foot.

Question. State the law of conservation of momentum.
Ans : (i) Momentum of two bodies before collision is equal to the momentum after collision.
(ii) In an isolated system, the total momentum remains conserved.

Question. Why is it dangerous to jump out from a moving bus?
Ans : While moving in a bus our body is in motion. On jumping out of a moving bus our feet touches the ground and come to rest. While the upper part of our body fall in motion and moves forward due to inertia of motion and hence we can fall in forward direction.
Thus, to avoid this we need to run forward in the direction of bus.

Question. When we hold a bag full of books in our hand steady at some height above the ground, what kind of forces are acting on it?
Ans : The bag full of books is steady. This means its position is fixed. So, it must be under balanced forces. The forces acting on it are :
(i) Weight (or the gravitational pull) downwards,
(ii) Upward force applied by the person holding the bag.

Question. Why all cars are provided with seat belts?
Ans : Sudden movement of the vehicle results in the sudden change in the state of motion of the vehicle when our feet are in contact with it. But the rest of our body opposes this change due to its inertia and tends to remain where it was. Seat belts are provided to protect the passengers from falling backward or forward during such situation.

Question. Why are athletes made to fall either on a cushioned bed or on a sand bed in a high jump athletic event?
Ans : In a high jump athletic event, athletes are made to fall either on a cushioned bed or on a sand bed so as to increase the time of the athlete’s fall to stop after making the jump. This decreases the rate of change of momentum and hence the force.

Question. Which concept is behind the phenomenon- “A boatman pushes the river bank with a bamboo pole to take his boat into the river”.
Ans : When the boatman pushes the river bank with a bamboo pole, the river bank offers an equal and opposite reaction. This reaction helps the boat to move into the river.

Question. Why does a fireman struggle to hold a hose-pipe?
Ans : A fireman has to make a great effort to hold a hosepipe to throw a stream of water on fire to extinguish it. This is because the stream of water rushing through the hose-pipe in the forward direction with a large speed exerts a large force on the hose-pipe in the backward direction.

Question. What happens when a quick jerk is given to a smooth thick cardboard placed on a tumbler with a small coin placed on the cardboard? The coin will fall in the tumbler. Why?
Ans : The coin was initially at rest. When the cardboard moves because of the jerk, the coin tends to remain at rest due to inertia of rest. When the cardboard leaves contact with the coin, the coin falls in the tumbler on account of gravity.

Question. Why is it necessary to bend knees while jumping from greater height?
Ans :
(i) Our feet come to rest at once during the jump.
(ii) Due to the less time a large force acts on the feet.
(iii) If we bend out knees, the time of impact increases. Hence, less force acts on the feet.

Question. Why is it difficult to achieve a zero unbalanced force in practical situations? In practice what happens to a rolling marble? How can we reduce the effect of friction on a marble?
Ans :
(i) It is difficult to achieve a zero unbalanced force because of the presence of the frictional force acting opposite to the direction of motion.
(ii) In practice, the rolling marble stops after travelling some distance.
(iii) The effect of frictional force may be reduced by using a smoother marble and a smoother plane and providing a lubricant.

Question. A bullet of mass 100 g is fired from a gun of mass 20 kg with a velocity of 100 msl. Calculate the velocity of recoil of the gun.
Ans :
Mass of bullet, m = 100 g – 1000 kg
Velocity of bullet, u = 100 ms–1
Mass of gun, M = 20 kg
Let recoil velocity of gun = V
Step 1. Before firing, the system (gun + bullet) is at rest, therefore, initial momentum of the system = 0
Final momentum of the system
= momentum of bullet + momentum of gun
= mu + MV = 10
1 × 100 + 20 V
V = 10 + 20 V
Step 2. Apply law of conservation of momentum Final momentum = Initial momentum
i.e. 10 + 20 V = 0
20 V = 10
or V = – 0.5 ms–1
Negative sign shows that the direction of recoil velocity of the gun is opposite to the direction of the velocity of the bullet.

Question. Define force. What are different types forces?
Ans : Force : It is a push or pull on an object that produces acceleration in the body on which it acts. The S.I. unit of force is Newton.
Types of forces :
Balanced force : When the forces acting on a body from the opposite direction do not change the state of rest or of motion of an object, such forces are called balanced forces.
Unbalanced force : When two opposite forces acting on a body move a body in the direction of the greater force or change the state of rest, such forces are called as unbalanced forces.
Frictional force : Force of friction is the force that always opposes the motion of object.

Question. Why are road accidents at high speeds very much worse than accidents at low speeds?
Ans : The time of impact of vehicles is very small at high speed. So, they exert very large forces on each other.
Hence, road accidents at high speeds are highly fatal.

Question. Name two factors which determine the momentum of a body.
Ans : Two factors on which momentum of a body depend is mass and velocity. Momentum is directly proportional to the mass and velocity of the body.

Question. What decides the rate of change of momentum of an object?
Ans : The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of force.

Question. Why do athletes run some distance before jumping?
Ans : Athlete has the inertia of motion and thus continues to move past the line.

Question. Is force a scalar quantity or a vector quantity?
Ans : Force is a vector quantity. It has both magnitude and direction.

Question. Why action and reaction do not cancel each other?
Ans : Action and reaction act simultaneous but on different objects. Hence, they do not cancel each other.

Question. What is inertia? Explain different types of inertia.
Ans : Inertia : The natural tendency of an object to resist change in their state of rest or of motion is called inertia. The mass of an object is a measure of itsinertia. Its S.I. unit is kg.
Types of inertia :
Inertia of rest : The object remain in rest unless acted upon by an external unbalanced force.
Inertia of motion : The object in the state of uniform motion will continue to remain in motion with same speed and direction unless external force is not appliedon it.

Question. What is the acceleration produced by a force of 5 N exerted on an object of mass 10 kg?
Ans :
Here F = 5 N; m = 10 kg; a = ?
Now F = ma or a = F/m

a = 0.5 ms–2

Question. Which would require greater force : accelerating a 10 g mass at 5 ms–2 or 20 g mass at 2 ms–2?
Ans :
In first case m1 = 10 g = kg = 0.010 kg;
Now al = 5 ms^–2 ; F1 = ?
F1 = m1a1 = 0.010 × 5
F1 = 0.050 Newton
In second case, m2 = 20 g = 0.020 kg
a2 = 2 ms^–2; F2 = ?
Now F₂ = m2a2 = 0.020 × 2
or F₂ = 0.04 Newton
We find that F1 > F2, hence more force is required to accelerate 10 g at 5 ms^–2 than accelerating 20 g at 2 ms^–2.

Question. State Newton’s second law of motion.
Ans : The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force.

Question. Mention the factors on which the force of friction depends?
Ans : (a) The force of friction is directly proportional to the weight of the body sliding over the surface.
(b) The force of friction depends on the surfaces in contact.

Question. What does first law of motion indicates when an unbalanced external force acts on an object?
Ans : First law of motion indicates that when an unbalanced external force acts on an object, its velocity changes i.e. the object gets acceleration.

Question. What happens to the velocity of an object when the force acting on the object are balanced?
Ans : An object can move with uniform velocity only when the forces (pushing force and friction force) acting on the object are balanced.

Question. What is resultant force?
Ans : The resultant force of several forces acting simultaneously on a body is that single force which produces the same effect on a body as all these forces produce together.

Question. How is inertia measured quantitatively?
Ans : Quantitatively the inertia of an object is measured by its mass.

Question. Why are roads on mountains inclined inwards at turns?
Ans : A vehicle moving on mountains is in the inertia of motion. At a sudden turn there is a tendency of vehicle to fall off the road due to sudden change in the line of motion hence the roads are inclined inwards so that the vehicle does not fall down the mountain.

Question. If a person jumps from a height on a concrete surface he gets hurt. Explain.
Ans :
When a person jumps from a height he is in state of inertia of motion. When he suddenly touches the ground he comes to rest in a very short time and hence the force exerted by the hard concrete surface on his body is very high, and the person gets hurt.

Question. When a force of 40 N is applied on a body it moves with an acceleration of 5 ms2. Calculate the mass of the body.
Ans :
Let m be the mass of the body.
Given : F = 40 N, a = 5 ms²
From the relation F = m a, we have
40 = m × 5
m= 40/5 = 8 kg

Question. Which will have more inertia a body of mass 10 kg or a body of mass 20 kg?
Ans : A body of mass 20 kg will have more inertia, as inertia is measure of mass.

Question. Name the factor on which the inertia of the body depends.
Ans : Inertia of a body depends upon the mass of the body.

Question. If a man jumps out from a boat, the boat moves backwards. Why?
Ans : When a man jumps out of a boat to the bank of the river, he supplies a force in the forward direction. Due to the reaction of this, the boat moves backwards.

Question. It is easier to push an empty box than to push the box full of books. Give reason.
Ans : This is because the empty box has less inertia of rest than the box full of books.

Question. Define SI unit of force.
or
What is one Newton force?
Ans : A Newton is that force which act on a body of mass of 1 kg to produce an acceleration of 1 ms–2 in it.

Question. What is the direction of momentum?
Ans : The direction of momentum is the same as that of velocity.

Question. Which type of force brings an object in motion?
Ans : Unbalanced force.

Question. Define force.
Ans : It is a push or pull on an object that produces acceleration in the body on which it acts.

Question. What is balanced force?
Ans : When forces acting on a body from the opposite direction do not change the state of rest or of motion of an object, such forces are called balanced forces.

Question. No force is required to move an object with a constant velocity. Why?
Ans : For an object moving with a constant velocity, a = 0,
so F = ma = 0

Question. Why do the passengers fall backward when a bus accelerates from rest?
Ans : The passengers fall backward when a bus accelerates from rest due to their inertia. The passengers tend to remain in their state of rest even when the bus starts moving.

Question. What are the changes possible on an object at rest if we apply on it?
(a) A balanced force?
(b) An unbalanced force?
Ans :
(a) Object may changes its shape or size.
(b) Objects changes its speed, velocity or accelerates.

Question. What happens when you shake a wet piece of cloth? Explain your observation.
Ans : When a wet piece of cloth is shaken, small droplets of water fall down. This is because in the beginning both water and the piece of cloth were at rest. When he cloth is moved, the water in it tends to remain in the state of rest due to inertia of rest. As a result, the droplets of water fall down and the clothes dry quickly.

Question. State all 3 Newton’s law of motion.
Ans : Newton’s I law of motion : An object remains in a state of rest or of uniform motion in a straight line unless acted upon by an external unbalanced force. Newton’s II law of motion : The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force. Newton’s III law of
motion : To every action, there is
an equal and opposite reaction and they act on two different bodies.

Question. Write the effect of force.
or
What change will be in a body by force?
Ans : Force can produce three effects :
(a) It can change the magnitude of velocity of an object (i.e. to make the object move faster or slower).
(b) It can change the direction of motion of an object.
(c) It can change the shape and size of an object.

Question. What do you understand by the gravitational unit of force?
Ans : A gravitational unit of force is defined as that force which produces an acceleration equal to the acceleration due to gravity in a body of one unit mass. Gram-weight and kilogram-weight are the gravitational unit of force.