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

Chapter 8 Force and Laws of Motion Class 9 Science CBSE NCERT Notes

Force may be defined as an influence, which tend to change state, speed, direction and shape of a body. It has both magnitude and direction and is therefore, a vector quantity. It may also be defined as an external agency, which changes the speed and direction of a body. It can also change the shape of a body. Force is a vector quantity. Example, to open a door, either we push or pull it. A drawer is pulled to open and pushed to close.

Effect of Force

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

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

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.

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

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:

Balanced Force
Unbalanced Force

Balanced force

When all the applied forces in different directions, nullify each other then it is said to be balanced forces. Body at rest remains at rest and moving object keeps moving.

Shape or size may change. As in a rope game, the rope remains at rest as the magnitude of force applied on each ends are equal.

Unbalanced force

If the resultant of all the applied force becomes greater than zero then it is said to be unbalanced forces.

Stationary object can be brought into motion. A moving object can acquire more speed on applying unbalanced force.

It can decrease the speed of moving speed. It can stop a moving object.

Unbalanced forces change the shape and size of an object.

Laws of Motion

Galileo Galilei: Galileo first of all said that object move with a constant speed when no foces act 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 forever. That is, there is no unbalanced force working on the object.

But practically it is not possible for any object. Because to attain the condition of zero, unbalanced force is impossible. 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 motion until any external force is applied over it to change its state. This means all objects resist to in changing their state. The state of any object can be changed by applying external forces only.

Newton First law of motion in everyday life

When a bus starts moving the person in the bus falls backward. This condition arises because when bus starts moving the legs also move with it but the rest part of the body has tendency to remain at rest.

When driver of a bus applies brake suddenly then person standing in it falls in forward direction. This condition arises because the person standing in the moving bus is also in motion along the bus, but when brake is applied suddenly the legs in contact with the bus stops with the bus but rest part of the body has the tendency to be in motion.

On a carom board if we hit the pile of coin, then only the pile present at bottom moves leaving rest of the coin of pile at its place. This happens because when striker hits the bottom coin, induce motion in it but rest of the coin of pile has tendency to remain at rest.

Wet clothes when jerked before putting in sunlight makes it easy to dry. This happens because the droplets present in the cloth pores has tendency to remain at rest but when jerked the cloth acquires motion. This makes water droplets to move out of it so clothes easily dry out.

Mass and Inertia

The property of an object because of which it resists to get disturb its state is called inertia. Inertia of an object is measured by its mass. Inertia is directly proportional to the mass. This means inertia increases with increase in mass and decreases with decrease in mass. 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 motion or rest of the object is called inertia.

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.

Momentum

The force required to stop a moving body is directly proportional to its velocity. Thus the quantity of motion in a body depends on mass and velocity of the body. This quantity of motion defines a new physical quantity called momentum.

The momentum of an object is defined as the product of its mass and its velocity. Momentum is a vector quantity and its direction will be same as that of velocity. It is represented by p.

p = mv where, m is the mass of the object, v is its velocity.

SI unit of momentum is kg m/s.

Consider the following explanations to understand the momentum

A person get injured in the case of hitting by a moving object, such as stone, pebbles or anything because of momentum of the object.

Even a small bullet is able to kill a person when it is fired from a gun because of its momentum due to great velocity.

A person get injured severely when hit by a moving vehicle because of momentum of vehicle due to mass and velocity.

Newton’s Second Law of Motion

Newton’s second law of motion states that rate of change of momentum is directly proportional to applied force and takes place in the same direction as the applied force.

Explanation

Momentum is the force possessed by a body at any particular instant during its course of motion. Mathematically momentum is the product of mass and velocity.

Momentum and Mass and Velocity

Since momentum is the product of mass and velocity (p = m x v) of an object. This means momentum is directly proportional to mass and velocity. 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.

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.

Consider a body of mass m, having an initial velocity u. Let the body be acted upon by some force F for time t, such that its final velocity is v.
Initial momentum = mu
Final momentum = mv
Change in momentum in time t = m (v – u)
Change in momentum in unit time t = m (v – u)/t
But v-u/t = a
Change in momentum in unit time = ma

Or, Rate of change of momentum = ma
According to Newton’s second law Rate of change of momentum F

F m.a
F = Km.a (K is the constant of proportionality)
If a body has unit mass and unit acceleration, such that force possessed by it is also one unit then 1= K ✕ 1 ✕ 1
or K = 1

One Newton force is that force which produces an acceleration of 1 m/s2 on an object of mass 1 kg. Force is a vector quantity. Newton’s second law of motion gives a quantitative definition of force.

Impulse

Mathematical representation of Newton’s second law of motion is
F = mv – mu/t

Or, Ft = mv – mu

When the time of application of force is short then Ft is defined as impulse. Impulse is large force acting for a short duration.

SI unit of impulse = N s or kg m/s.

When we kick a football, the kick lasts only for a fraction of a second. The force, which we apply on a football, is an example for impulsive force.

Newton’s Third Law of Motion

It states that, “If a body exerts a force on some other body, then the second body exerts a force of some magnitude on it but in opposite direction”. Thus, in other words it states that, “To every action there is an equal and opposite reaction”. For example:

When a ball is thrown on to a wall, it comes back with the same force and speed.
When we pull a spring, it pulls us back with the same force.
When we hit a ball to the ground, it comes back at the same speed.

Law of Conservation of Momentum

From Newton’s third law of motion we know that whenever a force is applied on a body there will be an equal and opposite reaction. Action and reaction forces result in change in velocities of both the bodies which in turn change the momenta of these bodies.

In an elastic collision the initial momentum of the bodies before collision is found to be equal to the final momentum of the bodies after collision. Thus Newton’s second and third laws of motion lead us to the very important law of mechanics, the law of conservation of momentum.

Law of conservation of momentum states that if a group of bodies are exerting force on each other, i.e., interacting with each other, their total momentum remains conserved before and after the interaction provided there is no external force acting on them.

The following example will help us to understand clearly the law of conservation of momentum. Two bodies A and B of masses m₁ and m₂ are moving in the same direction with initial velocities u₁ and u₂. They make a direct collision. Let us assume that after collision they continue moving in the same direction. Let the collision last for a very short interval of time ‘t’ seconds.

During collision, A exerts a force on B. At the same time, B exerts a force on A. Due to these
action and reaction forces the velocities of A and B get changed. After collision, let v₁ and v₂ be the velocities of the bodies A and B respectively.
The force exerted on A = m₁a₁

According to Newton’s second law of motion,
Acceleration produced produced in a = change in velocity/time
a1 = v₁-u₁/t

Therefore, force exerted on a, m₁ (v₁-u₁/t)
F₁ = m₁v₁-m₁u₁/t
Acceleration produced in B = change in velocity/time
F₂ = v₂-u₂/t
Therefore, force exerted on B = m₂a₂
= m₂ (v₂-u₂/t)
= m₂v₂ – m₂u₂/t
According to Newton’s third law of motion, these two forces are equal and opposite. i.e., F₁ = – F₂
m₁v₁ – m₁u₁/t = -(m₂v₂ m₂u₂/t)
i.e., total momentum before collision is equal to the total momentum after collision, which is nothing but law of conservation of momentum.