NCERT Solutions Class 9th Science Chapter – 8 Force and Laws of Motion Notes

May 26, 2022

NCERT Solutions Class 9th Science Chapter – 8 Force and Laws of Motion

Textbook	NCERT
Class	9^th
Subject	Science
Chapter	8th
Chapter Name	Force and Laws of Motion
Category	Class 9th Science
Medium	English
Source	Last Doubt

NCERT Solutions Class 9th Science Chapter – 8 Force and Laws of Motion Note What are the laws of motion and force Class 9? What is force in motion? What is Newton’s 1st law equation What are Newton’s 1st 2nd and 3rd laws of motion? What is 3 law of motion class 9? What is Newton’s 2nd law called? What is Newton’s law formula? What is called inertia? force is friction? What is the law of acceleration? What is called inertia? What force is friction? What is the law of acceleration? How is gravity a force? What is gravity formula? Who found gravity? Is gravity 9.8 Newtons?is 1 kgf equal to? Why is gravity divided by 2? At what speed is gravity?

NCERT Solutions Class 9th Science Chapter – 8 Force and Laws of Motion

Chapter – 8

Force and Laws of Motion

Notes

Force: It is the force that enables us to do any work. To do anything, either we pull or push the object. Therefore, pull or push is called force.

Example, to open a door, either we push or pull it. A drawer is pulled to open and pushed to close.

Effect of Force

(i) Force can move a stationary body or object.
For example, a football can be set to move by kicking it, i.e., by applying a force.

(ii) Force can stop a moving body.
For example, by applying brakes, a running cycle or a running vehicle can be stopped.

(iii) Force can change the direction of a moving object.
For example, by applying force, i.e., by moving handle, the direction of a running bicycle can be changed. Similarly by moving steering, the direction of a running vehicle is changed.

(iv) Force can change the speed of a moving body.
For example, By accelerating, the speed of a running vehicle can be increased or by applying brakes the speed of a running vehicle can be decreased.

(v) Force can change the shape and size of an object.
For example, by hammering, a block of metal can be turned into a thin sheet. By hammering, a stone can be broken into pieces.

Forces are mainly of two types:

(A) Balanced forces: If the resultant of applied forces is equal to zero, these are called balanced forces.

Example: In a tug of war if both the team apply similar magnitude of forces in opposite directions, rope does not move in either side. This happens because of balanced forces in which resultant of applied forces become zero.

(i) Balanced forces do not cause any change of state of an object.

(ii) Balanced forces are equal in magnitude and opposite in direction.

(iii) Balanced forces can change the shape and size of an object.
For example: when forces are applied from both sides over a balloon, the size and shape of balloon is changed.

(B) Unbalanced Forces : If the resultant of applied forces are greater than zero, the forces are called unbalanced forces. An object in rest can be moved because of applying Unbalanced forces.

Unbalanced forces can do the following:

(i) Move a stationary object.

(ii) Increase the speed of a moving object.

(iii) Decrease the speed of a moving object.

(iv) Stop a moving object.

(v) Change the shape and size of an object.

Laws of Motion :

Galileo Galilei: was the first to say that objects move with a constant speed when no force acts on them. This means if an object is moving on a frictionless path and no other force is acting upon it, the object would be moving in uniform motion forever. That is, there is no unbalanced force working on the object.

But practically it is not possible for any object. As opposing forces. We need to apply an external force to encounter the opposing forces. Force of friction, force of air and many other forces are always acting upon an object.

Newton’s Laws of Motion :

Newton studied the ideas of Galileo and gave the three laws of motion. These laws are known as Newton’s laws of motion.

Newton’s First Law of Motion (Law of Inertia) : Any object remains in the state of rest or in uniform motion along a straight line, until it is compelled to change the state by applying external force.

Explanation: If any object is in the state of rest, then it will remain in rest until a external force is applied to change its state. Similarly, an object will remain in uniform motion in a straight line until any external force is applied over it to change its state. This means all objects resist a change in their state. The state of any object can be changed by applying external forces only.

Newton’s First Law of Motion in Everyday Life:

(a) A person standing in a bus falls backward when bus starts moving suddenly. This happens because the person and bus both are in rest while bus is not moving, but as the bus starts moving, the legs of the person start moving along with bus but rest portion of his body has the tendency to remain in rest. Because of this, the person falls backward; if he is not alert.

(b) A person standing in a moving bus falls forward if driver applies brakes suddenly. This happens because when bus is moving, the person standing in it is also in motion along with bus. But when driver applies brakes the speed of bus decreases suddenly or bus comes in the state of rest suddenly in this condition the legs of the person which are in contact with the bus come in rest while the rest part of his/her body has the tendency to remain in motion. Because of this person falls forward if he/ She is not alert.

(c) Before hanging the wet clothes over laundry line, usually many jerks are given to the clothes to get them dried quickly. Because of jerks, droplets of water from the pores of the cloth falls on the ground and reduced amount of water in clothes dries them quickly. This happens because when suddenly clothes are made in motion by giving jerks, the water droplets in it have the tendency to remain in rest and they are separated from clothes and fall on the ground.

(d) When the pile of coin on the carom-board is hit by a striker, only the coin at the bottom moves away leaving rest of the pile of coin at same place. This happens because when the pile is struck with a striker, the coin at the bottom comes in motion while rest of the coin in the pile has the tendency to remain in the rest and they vertically falls on the carom-board and remain at same place.

Mass and Inertia

(i) The property of an object because of which it resists a change in its state is called inertia. Inertia of an object is measured by its mass.

(ii) Inertia is directly proportional to the mass. This means inertia increases with increase in mass and decreases with decrease in mass.

(iii) A heavy object will have more inertia than the lighter one. In other words, the natural tendency of an object that resists the change in state of uniform motion or rest of the object is called inertia.

(iv) Since a heavy object has more inertia, thus it is difficult to push or pull a heavy box over the ground than the lighter one. Similarly, a greater opposing force is needed to stop a heavy body than a light body in the same time, if they are moving with the same speed.

Momentum: Momentum is the quantity of motion an object has. The product of velocity and mass is called the momentum. Momentum is denoted by ‘p’.

Therefore, Or, Momentum of the object = Mass × Velocity
p = mxv
Where, p = momentum, m = mass of the object and v = velocity of the object.

Momentum and Mass and Velocity: Since momentum is the product of mass and velocity (p = m × v) of an object. This means momentum is directly proportional to mass and velocity.

(i) Momentum increases with increase of either mass or velocity of an object. This means if a lighter and a heavier object is moving with same velocity, then heavier object will have more momentum than the lighter one.

(ii) If a small object is moving with great velocity, it has tremendous momentum. And because of momentum, it can harm an object more severely. For example, a small bullet having a little mass even kills a person when it is fired from a gun.

(iii) Usually, road accidents prove more fatal because of high speed than in slower speed. This happens because vehicles running with high speed have greater momentum compared to a vehicle running with slower speed.

Momentum of an object which is in the state of rest:
Let an object with mass ‘m’ is in the rest.
Since, object is in rest, therefore, its velocity, V = 0
Now, we know that

Momentum = mass x velocity
p = m x 0 = 0
Thus, the momentum of an object in the rest i.e., non-moving, is equal to zero.

Unit of momentum :

SI unit of mass = kg
SI unit of velocity = meter per second i.e., m/s
Momentum (p) = m x v
Therefore, = p = kg x m/s
p = kg m/s Therefore,
SI unit of momentum = kg m/s

Statement of Second Law : Rate of change of momentum of an object is proportional to applied unbalanced force in the direction of force.

Mathematical expression
Suppose,
Mass of an object = m kg
Initial velocity of an object = u m/s
Final velocity of an object = v m/s
So,

Initial momentum, P₁ = mu, Final momentum, p₂ = mv
Change in momentum = Final momentum – Initial momentum
= mv – mu
= m (v – u)

Change in momentum = Rate of change of momentum/Time taken
= m(v – u)/t

According to IInd law: this rate of change is momentum is directly proportional to force.
F ∞ m(v – u)/t
We know that, V – u/t = a (From Ist equation of motion)
So,

F = kma
Where k is a constant. Its value = 1.
F = 1 x m x a = ma
SI unit of force = kg m/s² or Newton

1 Newton : When an acceleration of 1 m/s² is seen in a body of mass 1 kg, then the force applied on the body is said to be 1 Newton.

Proof of Newton’s First Law of Motion from Second Law

First law states that if external force F = 0, then a moving body keeps moving with the same velocity, or a body at rest continues to be at rest.
So, F = 0

We know F = m(v – u)/t = v – u = 0

(a) A body is moving with initial velocity u, then
0 = m(v – u)/t
= v – u = 0
So, v = u
Thus, final velocity is also same.

(b) A body is at rest i.e., u = 0.
Therefore, from above u = v = 0
So, the body will continue to be at rest.

Third law of Motion: To every action there is an equal and opposite reaction.
Note: Action and reaction act on two different objects.

Applications:

(i) Walking is enabled by III rd law.
(ii) A boat moves back when we deboard it.
(iii) A gun recoils.
(iv) Rowing of a boat.

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.

Initial momentum (before collision) = Final momentum (after collision)

Suppose, two objects A and B each of mass m₁, and mass m₂ are moving initially with velocities u₁, and u₂, strike each other for time, t and start moving with velocities v₁ and v₂ respectively.
Now,
Initial momentum of object A = m₁u₁
Initial momentum of object B = m₂u₂

Final momentum of object A = m₁v₁
Final momentum of object B = m₂v₂

So, Rate of change of momentum in A, F₁ = m₁v₁ – m₁u₁/t
= m₁(v₁ — u₁)/t

And Rate of change of momentum in B, F₂ = m₂v₂ – m₂u₂/t
= m₂ (v₂ – u₂)/t
m₁ (v₁ – u₁)/t

And Rate of change of momentum in B, F₂ = m₂v₂ – m₂u₂/t
= m₂ (v₂ – u₂)/t

We know from IIIrd law of motion,
F₁ = – F₂
So, m₁ (v₁ – u₁)/t = m₂ (v₂ – u₂)/t [From equations (i) & (ii)]
Or = m₁v₁ – m₁u₁ = – m₂v₂ + m₂u₂
So = m₁v₁ + m₁u₁ = – m₂v₂ – m₂u₂
Thus, Initial momentum = Final momentum

Q1. Name the scientist who proved for the first time that objects move with constant speed when no force acts on them.

Ans: Galileo.

Q2. Why do bicycles begin to slow down when we stop pedalling?

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

Q3. Which law of motion gives the measure of force?

Ans: Newton’s second law of motion.

Q4. What did Galileo conclude on the basis of his experiments on the motion of objects?

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

Q5. Can every force produce motion in every object?

Ans: No.

Q6. When a force is applied to a body, what are the two essential effects it can produce?

Ans: It can bring about the change in the state of motion of a body or It can deform a body, i.e., it can change its shape.

Q7. Define 1 newton force.

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

Q8. What do you mean by an impact force?

Ans: The force produced by the impact of a fast moving object on another is called impact force.

Q9. Define force of friction.

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

Q10. Define electrostatic force.

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

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