**Newton’s Laws of Motion**

We know that force is a push or a pull which tends to change
the state of motion of an object i.e. if an object is initially at rest then an
applied force tends to bring it in motion and if an object is already in motion
then a force acting on it can bring the object at rest or change its speed or
change the direction of motion.

The relationship between force and motion can be best
understood with the Newton’s Laws of motion.

It will be better if we try to understand the Galileo’s law
of inertia first.

**Law of Inertia:**The tendency of an object to resist any change in its state of rest or uniform motion along a straight line is called inertia. In other words, if an object is at rest it continues to stay at rest and if an object is in motion it continues to remain in motion.

The measure of inertia is mass of an object. Higher the mass
of an object, higher is its inertia i.e. higher is its tendency to resist any
change in its state.

**Inertia is of 3 types:**

(1) Inertia
of rest

(2) Inertia
of motion

(3) Inertia
of direction

**Newton’s First Law of Motion:**

Newton’s first law is simply the law of inertia or we can
say extension of law of inertia.

**An object continues to be in a state of rest or uniform motion in a straight line unless acted upon by an external unbalanced force.**

The difference between Galileo’s law of inertia and Newton’s
first law of motion is that Galileo was not able to identify the cause or agent
that can bring the change in state of motion or rest. Whereas Newton’s identify
that unless an external force acts on an object its state cannot be changed.

**Examples of Newton’s first law:**

(a) When
a bus starts suddenly from rest passengers tends to fall backwards.

(b) When
a moving bus stops suddenly passengers tends to fall forward.

(c) An
athlete continues to run further even after crossing the finishing line.

(d) When
a carpet is beaten with a stick dust particles comes out.

(e) If
we hit a striker on a pile of carom coins only the bottom most coin will move
with the striker.

Before we start second law of Newton let us discuss an
important physical quantity known as momentum.

**Momentum:**The product of mass and velocity of an object is known as momentum. It is denoted by letter p and it is a vector quantity.

P = mv

The momentum simply gives us the idea of quantity of motion
an object possesses. A bullet of small mass moving with very high velocity
possesses huge momentum because of its velocity. Whereas a truck moving with
slow speed possesses large momentum due to its huge mass.

**Newton’s 2**

^{nd}Law
The first law of Newton’s identifies the force that it is
the cause of motion or change in state of motion. The Newton’s 2

^{nd}law gives the mathematical expression for that force.**The rate of change of momentum of an object is directly proportional to the applied unbalanced force in the direction of force.**

Consider an object of mass m moving with velocity u. A
constant force F acts on an object for a small time t such that its velocity
increases from u to v.

\[Change\ in\ momentum = mv - mu \]

\[Rate\ of\ change\ of\ momentum = \frac{mv - mu}{t}\]

According to second law, the rate of change of momentum is directly proportional to the applied unbalanced force.

\[\frac{mv-mu}{t}∝F\]

\[F∝\frac{mv-mu}{t}\]

\[F∝m\frac{v-u}{t}\]

\[F∝ma\]

\[F=kma\]

Where k is a constant. The unit of Force is so chosen such that the value of k becomes 1. Thus

\[F=ma\]

\[Rate\ of\ change\ of\ momentum = \frac{mv - mu}{t}\]

According to second law, the rate of change of momentum is directly proportional to the applied unbalanced force.

\[\frac{mv-mu}{t}∝F\]

\[F∝\frac{mv-mu}{t}\]

\[F∝m\frac{v-u}{t}\]

\[F∝ma\]

\[F=kma\]

Where k is a constant. The unit of Force is so chosen such that the value of k becomes 1. Thus

\[F=ma\]

The SI unit of force is newton (N). 1N is that force which when applied to a mass of 1 kg produces an acceleration of 1 m/\(s^2\) in it.

**Examples of Newton’s 2**

^{nd}law
(a) In
cricket, while catching a fast moving ball, the fielder gradually lowers his
hands with the ball. This is done to increase the time in which the final
velocity of the ball becomes zero. This reduces the rate of change of momentum
thereby decreasing the force exerted by ball on the hands of the player.

(b) A
karate player moves his hands very fast while breaking a pile of tiles.

(c) Delicates
items are wrapped in thermocol.

(d) In
a high jump, the athletes are made to fall on a cushion bed.

**Newton’s Third Law of motion**

For every action, there is equal and
opposite reaction.

It means when one object exerts a force
on another object then the second object instantaneously exerts equal and
opposite force on the first. These two forces are equal in magnitude but
opposite in direction.

The action and reaction forces act on
different object (not on the same object).

**Examples of Newton’s Third law**

(1) When
a gun is fired the bullet moves in the forward direction and the gun recoils in
the opposite direction. Here gun applies force on the bullet in the forward
direction and the bullet apply equal and opposite force on the gun in the
backward direction.

(2) When
we walk on the road we apply force on the road in the backward direction and
the road pushes us in the forward direction.

(3) When
we jump we apply force on the ground in downward direction and the ground apply
equal and opposite force on us in the upward direction.

(4) During
the flight of a bird, the bird pushes the air downward with its wings and the
air applies equal and opposite force on the wings in the upward direction.

(5) While
swimming, we apply force on the water in the backward direction. It is reaction
force of water that pushes us in the forward direction.

Significant and appreciable work

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