NCERT CLASS 9TH Science

Class 9 Force and Laws of Motion Notes

In this article, we will learn about force and laws of motion. This is the 2nd lecture of physics of class 9th. The notes given by us are very important for you because we have not kept any loophole in it. After reading this article all your doubts will be cleared. Also, you will not need any other reference book. You will get a very beautiful mark in your exam. If you need notes for any other topic, then you can tell us in the comments.

Chapter Name	FORCE AND LAWS OF MOTION
Class	9th
Language	English
Website	Allexamsolutions.in

What is Force?

A push or pull on a body is called force. The direction in which a body is pushed or pulled is called the direction of force. We open or close a door by applying force. Now, when we push the door to open it, we apply a force on the door in a direction away from us. And when we pull the door to close it, then we exert a force on the door in a direction towards us.

Forces are used in our everyday actions like pushing, pulling, lifting, stretching, twisting, and pressing.

Effects of Force

A force cannot be seen. A force can be judged only by the effects which it can produce in various bodies (or objects) around us. A force can produce the following effects:

1. A force can move a stationary body.
2. A force can stop a moving body.
3. A force can change the speed of a moving body.
4. A force can change the direction of a moving body.
5. A force can change the shape (and size) of a body.

Force can make a stationary body move and a force can make a moving body stop.

The speed of a falling ball (or any other falling body) increases because the earth applies a pulling force on it which is called the force of gravity.

Balanced and Unbalanced Force

Forces are of two types: Balanced forces and Unbalanced forces. We will now discuss balanced and unbalanced forces in detail, one by one.

Balanced force

If the resultant of all the forces acting on a body is zero, the forces are called balanced forces. A body under the action of balanced forces does not change its position of rest (or of uniform motion) and it appears as if no force is acting on it.

if a number of balanced forces act on a stationary body, the body continues to remain in its stationary position. Similarly, if a number of balanced forces act on a body in uniform motion, the body continues to be in its state of uniform motion.

Though balanced forces cannot produce motion in a stationary body or stop a moving body, they can, however, change the shape of the body.

Unbalanced Force

If the resultant of all the forces acting on a body is not zero, the forces are called unbalanced forces. When unbalanced forces act on a body, they produce a change in its state of rest or of uniform motion. That is, unbalanced forces can move a stationary body, or they can stop a moving body. In other words, unbalanced forces acting on a body can change its speed or direction of motion.

An unbalanced force acts on a body, it produces motion in the body. Another point to be noted is that an unbalanced force can also stop a moving body. For example, when a ball is rolling on the ground, an unbalanced force of friction acts on it which brings the ball to a stop after some time.

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Newton’s laws of Motion

Newton has given three laws to describe the motion of bodies. These laws are known as Newton’s laws of motion.

Newton’s 1^st law of Motion

According to Newton’s first law of motion: A body at rest will remain at rest, and a body in motion will continue in motion in a straight line with a uniform speed, unless it is compelled by an external force to change its state of rest or of uniform motion.

Inertia is that property of a body due to which it resists a change in its state of rest or of uniform motion. Greater the inertia of a body, greater will be the force required to bring a change in its state of rest or of uniform motion. In fact, mass is a measure of the inertia of a body. If a body has more mass, it has more inertia. That is, heavier objects have more inertia than lighter objects.

the inertia of a body depends on its mass.

we know that to overcome the inertia and make a body move from rest, we must apply an external force.

We will now consider the second part of the first law of motion which says that a body in uniform motion will continue to move unless a force compels it to change its state of uniform motion in a straight line.

Applications of Newton’s first law of Motion

• When a hanging carpet is beaten with a stick, the dust particles start coming out of it.
• When a tree (having flexible stem) is shaken vigorously, its fruits and leaves fall.
• when a car or bus starts suddenly, the passengers fall backward.
• When a running car or bus stops suddenly, the passengers are jerked forward.
• When a car or bus turns a corner sharply, we tend to fall sideways.
• It is dangerous to jump out of a moving bus because the jumping man, who is moving with the high speed of the bus, would tend to remain in motion (due to inertia) even on falling to the ground and get hurt due to the resistance offered by ground.

Newton’s first law of motion gives us a definition of force. It says that a force is something which changes or tends to change the state of rest or of uniform motion of a body. In other words, a force is an influence which can produce an acceleration or retardation in a body. Force is a vector quantity having magnitude as well as direction.

Momentum

We know that the force required to stop a moving body is directly proportional to its mass.

the force required to stop a moving body is also directly proportional to its velocity. Thus, the quantity of motion in a body depends on the mass and velocity of the body. This gives us another term known as “momentum”. The momentum of a body is defined as the product of its mass and velocity.

if a body is at rest, its velocity is zero and hence its momentum is also zero. Thus, the total momentum of the gun and bullet before firing is zero because their velocity is zero. Momentum is a vector quantity and takes place in the direction of velocity.

the SI unit of momentum is kilogram metres per second which is written as kg.m/s or kg.m s^-1.

Sample Problem. What is the momentum of a man of mass 75 kg when he walks with a uniform velocity of 2 m/s?

Solution. We know that:

Newton’s Second law of Motion

According to Newton’s second law of motion: The rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction in which the force acts. The rate of change of momentum of a body can be obtained by dividing the ‘Change in momentum’ by ‘Time taken’ for change. So, Newton’s second law of motion can be expressed as:

Newton’s second law of motion gives us a relationship between ‘force’ and ‘acceleration’.

Since the acceleration produced is inversely proportional to the mass of a body, therefore, if the mass of a body is doubled, its acceleration will be halved. And if the mass is halved then acceleration will get doubled (provided the force remains the same).

The SI unit of force is newton which is denoted by N. A newton is that force which when acting on a body of mass 1 kg produces an acceleration of 1 m/s² in it.

Newton’s second law gives us a relationship between the force applied to a body and the acceleration produced in the body.

If a minus sign comes with the force, it will indicate that the force is acting in a direction opposite to that in which the body is moving (just as the force of friction acts in a direction opposite to that of the moving body).

Applications of Newton’s second law of Motion

Catching a cricket ball

A cricket player (or fielder) moves his hands backwards on catching a fast cricket ball. This is done to prevent injury to the hands.

The case of a high jumper

During athletics meet, a high jumping athlete is provided either a cushion or a heap of sand on the ground to fall upon. This is done to prevent injury to the athlete when he falls after making a high jump.

the cushion or sand, being soft, reduces the athlete’s momentum more gently.

The use of seat belt in cars

These days all the cars are provided with seat belts for passengers to prevent injuries in case of an accident. 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.

Newton’s third law of Motion

According to Newton’s third law of motion: Whenever one body exerts a force on another body, the second body exerts an equal and opposite force on the first body.

To every action there is an equal and opposite reaction.

Applications of Newton’s third law of Motion

How do we walk?

When we walk on the ground, then 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.

the acceleration produced will be more in the body having less mass whereas the acceleration produced will be less in the body having more mass.

Why the gun recoils?

When a bullet is fired from a gun, the force sending the bullet forward is equal to the force sending the gun backward (Figure 33). But due to high mass of the gun, it moves only a little distance backward and gives a backward jerk or kick to the shoulder of the gunman. The gun is said to have recoiled.

The Flying of Jet Aeroplanes and Rockets

Jet aeroplanes utilise the principle of action and reaction. In the modern jet aircraft, the hot gases obtained by the rapid burning of fuel rush out of a jet (a nozzle) at the rear end (back end) of the aircraft at a great speed. The equal and opposite reaction of the backward going gases pushes the aircraft forward at a great speed.

The Case of a Boat and the Ship

During the rowing of a boat, the boatman pushes the water backwards with the oars. The water exerts an equal and opposite push on the boat which makes the boat move forward.

The Case of Hose Pipe

The backward movement of the hose pipe is due to the backward reaction of water rushing through it in the forward direction at a great speed.

The Case of Horse Pulling a Cart

So, to make the cart move, the horse bends forward and pushes the ground with its feet. When the forward reaction to the backward push of the horse is greater than the opposing frictional forces of the wheels, the cart moves.

In some cases, the two bodies can also exert force on each other even when they are not in contact with each other. That is, the interaction can also take place even when the two bodies are not in contact. In all such cases the forces of action and reaction are equal and opposite.

Conservation of momentum

According to the law of conservation of momentum: When two (or more) bodies act upon one another, their total momentum remains constant (or conserved) provided no external forces are acting. The law of conservation of momentum means that whenever one body gains momentum, then some other body must lose an equal amount of momentum.

This law can also be stated as: Momentum is never created or destroyed. The law of conservation of momentum is also known as the principle of conservation of momentum. The principle of conservation of momentum is in accord with Newton’s third law of motion which says that action and reaction (forces) are equal and opposite.

Force and Laws of Motion Class 9 Notes Questions Answers

Q1. What is the definition of force?

Answer- A push or pull on an object is called force.

Q2. Which physical quantity like the rate of change of momentum?

Answer- Force

Q3. If the mass of a body and the force acting on it are both doubled, what happens to the acceleration?

Answer- Acceleration remains the similar.

Q4. Title the physical quantity whose unit is ‘newton’.

Answer- Force

Q5. Which physical principle is involved in the working of a jet aeroplane?

Answer- Principle of conservation of momentum.

Q6. Is the following statement true or false: A rocket can push itself in a vacuum?

Answer- True

Q7. What is the force which produces an acceleration of 1 m/s2 in a body of mass 1 kg?

Answer- 1 newton (1N)

Motion Class 9 Notes

Motion Class 9, What is Motion Class 9, Motion Chapter Class 9 Notes pdf, Physics notes for motion 9th

Topics in this chapter 

1. Movement of a body 
2. Distance and Displacement
3. Uniform and Non-uniform motion
4. Speed and velocity
5. Acceleration
6. Uniform circular motion

Movement of a Body

A common characteristic of all the moving bodies is that they change their position with time. 

A body is said to be in motion (or moving) when its position changes continuously with respect to a stationary object taken as a reference point.

For example, when the position of the car changes continuously with respect to stationary objects like houses and trees. We say that the car is moving or that the car is in motion.

Distance 

The distance travelled by a body is the actual length of the path covered by a moving body irrespective of the direction in which the body travels.

For example, in this case, the actual length of the path covered by the man is 5 km + 3 km = 8 km , so the distance travelled by the man is 8 km.

Displacement

When a body moves from one point to another , the distance travelled refers to the actual length of the indirect path whereas displacement refers to the straight line path between the initial and the final positions.

In simple words, when a body moves from one position to another, the shortest (straight line) distance between the initial and final position of the body , along with direction is known as its displacement.

The quantities like distance, displacement etc. are known as physical quantities. The magnitude of a physical quantity means size of the physical quantity. A physical quantity having only magnitude is known as scalar quantity. A scalar quantity has no direction. On the other hand, a physical quantity having magnitude as well as direction is known as vector quantity.

1. Distance is a scalar quantity (only magnitude)
2. Displacement is a vector quantity (have magnitude and direction)

The distance travelled by a moving body can not be zero but the final displacement of a moving body can be zero.

Sample problem : a man travels a distance of 1.5 m toward east , then 2.0 m towards south and finally 4.5 m towards east.

1. What is the total distance travelled? 
2. What is his resultant displacement? 

Solution :  1. Total distance travelled = 1.5 + 2.0 + 4.5 = 8.0m

2. By the help of graph, 

Thus, the final displacement as represented by AD is 6.3m

Uniform and Non-uniform motion

A body has a uniform motion if it travels equal distances in equal intervals of time.

For example, a car running at a constant speed of 10 m per second, will cover equal distances of 10 m, every second, so its motion will be uniform. 

The distance- time graph for uniform motion is a straight line.

A body has a non-uniform motion if it travels unequal distances in equal intervals of time.

For example, a falling body is an example of non-uniform motion.

The distance- time graph for a body having non-uniform is a curved line.

Speed 

The speed of a body gives us an idea of how slow or fast that body is moving.

In other words, the speed of a body is the distance travelled by it per unit time.

If a body travels a distance s in time t, then its speed v is given by : 

For example, a car travels a distance of 100 km in 4 hours, then the speed of this cr is given by  : 

Speed = 100 km / 4 hours

Speed = 25 km per hours (25 km/hr)

The SI unit of speed is metres per second (m/s)

For small values, the SI unit is cm/s.

For large values, the SI unit is km/hr.

Speed has magnitude only, it has no specified direction , therefore speed is scalar quantity .

Average speed

The average speed of a body is the total distance travelled divided by the total time taken to cover the distance.

Average speed = total distance travelled / total time taken

The SI unit of average speed is the same as speed m/s.

Uniform speed

A body has a uniform speed if it travels equal distance in equal intervals of time, no matter how small these time intervals may be.

For example, a car is said to have a uniform speed of say, 60 km per hour, if it travels 30 km every half hour, 15 km every quarter of hour, 1 km every minute and 1/60 km every second. As we have already discussed above, in actual practice the speed of the body rarely remains uniform (or constant) for a long time. 

Question : a scooterist covers a distance of 3 km in 5 min. Calculate his speed in : 

Solution : (1) in order to calculate the speed in cm per second , we should convert the given distance of 3 km into cm and the given time of 5 min into seconds. 

Please note that 1 km is 1000 m and 1 metre is 100 cm.

Now, 

Thus, the speed of a scooterist is 1000 cm per second.

Solution : (2) in order to express the speed in m per second we should convert the given distance of 3 km into m and then the given time of 5 min into seconds . 

Thus in this case,

Distance travelled = 3 km

                              = 3 × 1000 m 

                              = 3000 m 

So the speed of a scooterist is 10 m per second.

Solution : (3) in order to calculate the speed in kilometres per hour, we should express the given distance in km and the given time in hours. 

So in this case :

Thus the speed of a scooterist is 36 km per hour. 

Velocity 

Velocity of a body is the distance travelled by it per unit time in a given direction.

If a body travels a distance s in time t in a given direction, then its velocity v is given by : 

v = s / t

We know that the distance travelled in a given direction is known as displacement.

So in other words, velocity of a body is the displacement produced per unit time. 

Velocity = displacement / time taken 

v = s / t 

The SI unit of velocity is the same as that of speed, metres per second (m/s).

For bigger values, the SI unit is km per hour (km/hr)

For smaller values, the SI unit is cm per second (cm/s)

The difference between speed and velocity is that speed has only magnitude, it has no specific direction, but velocity has magnitude as well as direction. 

The direction of the velocity is the same as the direction of displacement of the body.

Uniform velocity 

A body has a uniform velocity if it travels in a specified direction in a straight line and moves over equal distance in equal intervals of time, no matter how small these time intervals may be.

The velocity of a body can be changed in two ways : 

1. By changing the speed of the body and
2. By keeping the speed constant but by changing the direction.

Acceleration 

Acceleration of a body is defined as the rate of change of its velocity with time 

Acceleration = change in velocity /  time taken for change 

Now, the change in velocity is the difference between the final velocity and the initial velocity. that is 

Change in velocity =  final velocity – initial velocity 

So , acceleration = final velocity – initial velocity / time taken 

  And t = time taken for the change in velocity

 The SI unit of acceleration is metres per second per second or metres per second square (m / s²)

Acceleration is a vector quantity. 

When a body is moving with uniform velocity, its acceleration will be zero.

Uniform acceleration 

A body has a uniform acceleration if it travels in a straight line and its velocity increases by equal amount in equal intervals of time.  

In simple words, a body has a uniform acceleration if its velocity changes at a uniform rate.

The velocity – time graph of a body having uniformly accelerated motion is a straight line.

Non-uniform acceleration 

A body has a non-uniform acceleration if its velocity increases by unequal amounts in equal intervals of time.

In simple words, a body has a non-uniform acceleration if its velocity changes at a non-uniform rate.  

The velocity – time graph for a body having non uniform acceleration is a curved line.

Retardation 

If the velocity of a body increases, the acceleration is positive and if the velocity of a body decreases, the acceleration is negative. 

A body is said to be retarded if its velocity is decreasing.

Retardation is measured in the same way a acceleration, retardation is equal to change in velocity / time taken and has the same unit of metres per second square (m/s²)

Retardation is actually acceleration with negative sign.

Average velocity 

The average velocity is given by the arithmetic mean of the initial and final velocity for a given period of time.

Average velocity = initial velocity + final velocity / 2

Uniform circular motion 

When a body moves in a circle, it is called circular motion.

In other words, motion in a circle is circular motion.

When a body moves along a circular path, then its direction of motion keeps changing continuously.

Since the velocity changes (due to change in direction), the motion along a circular path is said to be accelerated.

When a body moves in a circular path with uniform speed, its motion is called uniform circular motion.

A force needed to make an object travel in a circular path is called centripetal force.

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Examples of Uniform Circular Motion 

• Artificial satellites move in uniform circular motion around the earth. the motion of a satellite around the earth is accelerated.
• The moon moves in uniform circular motion around the earth.
• The earth moves around the sun in uniform circular motion.
• An athlete moving on a circular track with a constant speed exhibits uniform circular motion.
• The tip of a second hand of a watch exhibits uniform circular motion on the circular dial of the watch.

Some Important Questions

Q1. What is motion?

Answer- A body is said to be in motion when its position changes continuously with respect to a stationary object taken as reference point.

For example, we see a car at position A in front of a house and a tree at a particular time. Now, after 5 seconds, we see the car at position B which is quite far away from the house and the tree.

This mean that the position of this car is changing continuously with respect to a stationary object, house or tree. So we say that this car is moving or that this car is in motion.

Q2. What is uniform motion?

Answer- A body has a uniform motion if it travels equal distance in equal intervals of time ,no matter how small these time intervals.

Q3. What is non-uniform motion?

Answer- A body has a non-uniform motion if it travels an unequal distance in equal intervals of time.

Q4.  is speed can be zero?

Answer- We know that speed is a scalar quantity, so it can be either positive or zero.

Q5. Can velocity be negative?

Answer- We know that velocity is a vector quantity so it can be either positive, negative or zero.