Force and law of motion CBSE CLASS 9

Force and law of motion

CONTENT LIST

Introduction to Force

A push or pull on an object can be described as a force.
A push or pull on an object can arise due to gravity, magnetism, electricity ,human effort or by other mean etc.
Forces(a push or pull) can change an object's speed, its direction or even shape of object.
Pushing a door to open, pulling it to closed, stretching a rubber band —all of these actions are force.
In this chapter We define force as an action that changes or maintains the motion of a body or object.

Effect of force

Force can make a stationary object into motion. Example: pushing a box at rest on the table brings the box in motion.
Force can stop a moving object . Example: Catching a moving ball and making it stop on your hand.
Force can change the speed of a moving object(Increase or decrease the speed). Example: increasing the speed of a bicycle by exerting more force on the peddle.
Force can change the direction of a moving object. Example: changing the direction of motion of the cricket ball by the batsman after he hits the ball.
Force can change the shape or size of an object. Example: squeezing toothpaste and changing its shape.

Balanced and Unbalanced Forces

Balance Force

Balanced forces are equal in magnitude and opposite in direction.
Due to balanced forces, there is no change in motion of object.
Thus We can define 'When the forces on an object are equal in magnitude and in opposite directions, the forces are said to be balanced, and there is no change in motion.'

Unbalance Force

When two or more forces act on an object in such a way that net force is not zero, they are known as unbalanced forces.

Unbalanced forces result in a change in state of motion or rest of any object.

Net Forces

The net force means the sum of all the forces acting on an object.

It is the combined effect of all the forces acting on the object.

Friction Forces

Frictional force is defined as a force that opposes motion when the surface of a body comes into contact with that of another body.

It is the force that resists the motion of any two surfaces that are in contact.

Friction between our feet and the ground allows us to move on the ground.

First law of motion

Newton's first law of motion is stated as

An object at rest remains at rest or an object in motion remains in motion with the same speed and in the same direction unless an unbalanced force acts upon it.

An object at rest remains at rest or an object in motion remains in motion with the same speed and in the same direction. This virtue of an object is called inertia .

Newton's first law of motion - is also called the law of inertia.

Inertia

An object at rest remains at rest or an object in motion remains in motion with the same speed and in the same direction. This virtue of an object is called inertia .

Inertia of Rest

The tendency of the body to remain in rest or inability of the body to change the state of rest is called inertia of rest.

When the bus starts suddenly, passengers move backward due to inertia of rest.

Inertia of Motion

Inertia of motion is the tendency of a body to oppose any change in its state of uniform motion .

When the bus stops suddenly, people fall forward.

We experience difficulty to move a heavier body than the lighter one. In the same way, it is difficult to stop a moving heavier body that a lighter body moving with the same velocity. Therefore we can say that mass of the body is the measure of inertia, more the mass, more the inertia.

Definition of force based on concept of inertia as "Force is the external cause that changes or tries to change the state of rest or of uniform motion of a body in a straight line".

Momentum or Linear momentum

Momentum of an object gives an idea about quality of motion possessed by an object.

Momentum is determined by multiplying the mass of an object by its velocity.

The formula to calculate momentum is as below:

Momentum = Mass × Velocity

p is the momentum, m is the mass and v is the velocity.

Then p = mv

The SI unit of momentum is kilogram-metre per second (kg·m/s).

Momentum is a vector quantity because it has both direction and magnitude. Its direction is the same as that of velocity.

Second Law of Motion

Newton's second law of motion states that the force exerted by a body is directly proportional to the rate of change of its momentum.

Let an object of mass ‘m’, whose velocity changes from u to v in time t, when a unbalanced force ‘F’ is applied on it.

Initial momentum p1 = mu

Final momentum p2 = mv

Therefore,

Change in momentum = p2 – p1 = mv – mu = m(v – u)

F∝(mv−mu)/t

F∝m( v−u)/t

⇒F∝ma⇒F=kma(∵a= (v−u)/t

⇒F=ma(∵k=constant=1)

The second law motion gives idea about how to calculate the value of a force.

Conservation of Momentum

The conservation of momentum principle describes that if two objects collide, the total momentum before and after the collision will be the same if no external force acts on the colliding objects.

Final momentum (P2) = Initial momentum (P1)

Let us consider two objects A and B are collining and initial velocity of A be u1 and after Collision velicity become v1 and initial velocity of B be u2 and after Collision velicity become v2.

Now, calculate the change in momentum for body A and it is represented by Pa and is given by,

Pa = m1 (v1 – u1)

Now, calculate the change in momentum for body B and it is represented by Pb and is given by,

Pb = m2 (v2 – u2)

Now, from Newton’s Third Law of Motion it can be written as,

Fab = -Fba ------Eqn 1

where Fab is the force acting on A due to B and Fba is the force acting on B due to A.

Now, from Newton’s Second Law of Motion it can be written as,

Fab = m1 a1 -----Eqn 2

Fba = m2 a2 ------Eqn 3

where a1 and a2 are the acceleration of bodies A and B.

Therefore, substitute equation (2) and (3) in equation (1) as,