Physics project

Done by,R.Sri Krishna

IXth CBSERoll no :- 21

Nalanda Talent School

Newton’s Laws of Motion

A Brief Introduction…….Newton's laws of motion consist of three physical

laws that form the basis for classical mechanics. They describe the relationship between the forces acting on a body and its motion due to those forces. They have been expressed in several different ways over nearly three centuries.

The three laws of motion were first compiled by Sir Isaac Newton in his work ”Philosophiæ Naturalis Principia Mathematica”, first published on July 5, 1687.  Newton used them to explain and investigate the motion of many physical objects and systems

Newton's First and Second laws, in Latin, from the original 1687 edition of the Principia Mathematica.

Newton’s first law of

motion (law of inertia)

First law of motion

First law of motion :- Every body remains in a state of rest or uniform motion (constant velocity) unless it is acted upon by an external unbalanced force. This means that in the absence of a non-zero net force, the center of mass of a body either remains at rest, or moves at a constant speed in a straight line.

This tendency is also known as inertia!

Inertia Inertia is the

tendency of an object to resist changes in its

velocity. whether in motion or not

The ball doesn't move unless it is acted upon by an external unbalanced force.

Don’t let this be you. Wear seat belts.Because of inertia, objects (including you) resist changes in their

motion. When the car going 80 km/hour is stopped by the brick wall, your body keeps moving at 80 m/hour, so it moves forward!!!

Newton’s 1st law and you

Now we can get a doubt that –What is this unbalanced force that acts on an

object in motion?It is none other than friction!! Once

airborne or in motion, unless acted on by an unbalanced force (gravity and air – fluid friction), it would never stop!

Friction

For example --- Unless acted upon by an unbalanced force, this golf ball move forever. But because friction and gravity are acting on it the ball is stopping!

Newton’s second law of motion

Momentum

Momentum of a body is defined as the product of its mass and velocity

P(momentum) = M(mass) x V(velocity)

Second law of motionThe rate of change of momentum of an object

is proportional to the applied unbalanced force in the direction of the force

This tell us that :-

Force = Mass x acceleration

Units

When mass is in kilograms and acceleration is in m/s*s the unit of force is in Newton (N). One Newton is equal to the force required to accelerate one kilogram of mass at one meter/second square

If mass remains constant, doubling the acceleration, doubles the force. If force remains constant, doubling the mass, halves the acceleration.

We know that objects with different masses accelerate to the ground at the same rate.

However, because of the 2nd Law we know that they don’t hit the ground with the same force.

Newton’s 2nd Law proves that different masses accelerate to the earth at the same rate, but with different forces

F = ma

98 N = 10 kg x 9.8 m/s/s

F = ma

9.8 N = 1 kg x 9.8 m/s/s

Newton’s 3rd law

of motion

Third law of motionFor every action, there is an equal and

opposite reaction.This may be the most popular law in the

universe ! This law is sometimes referred to as

the action-reaction law, with F called the "action" and −F the "reaction". The action and the reaction are simultaneous.

Conservation of momentumAccording to Newton's third law every action

has an equalant reaction in the opposite direction .

So, when 2 bodies of masses M1 and M2 collide with initial velocities U1 and U2 in a time period t ,Then -

Force of A on B = -(Force of B on A )=> M1(V1-U1)/t = M2(V2-U2)/t

=> M1U1 +M2U2 = -(M1V1 + M2V2)

Examples According to Newton, whenever objects A and B interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body.

There are two forces resulting from this interaction - a force on the chair and a force on your body. These two forces are called action and reaction forces.

Consider the propulsion of a fish through the water. A fish uses its fins to push water backwards. In turn, the water reacts by pushing the fish forwards, propelling the fish through the water.

The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards).

Flying gracefully through the air, birds depend on Newton’s third law of motion. As the birds push down on the air with their wings, the air pushes their wings up and gives them lift. The wings of a bird push air downwards. In turn, the air reacts by pushing the bird upwards. The size of the force on the air equals the size of the force on the bird; the direction of the force on the air (downwards) is opposite the direction of the force on the bird (upwards).Action-reaction force pairs make it possible for birds to fly.

The reaction of a rocket is an application of the third law of motion. Various fuels are burned in the engine, producing hot gases.

The hot gases push against the inside tube of the rocket and escape out the bottom of the tube. As the gases move downward, the rocket moves in the opposite direction.

Bibliography :-

NCERT science text book of 9th class

Wikipedia S.chand super simplified physics for class 9

R.Sri kishnaIX CBSE