Force and laws of motion

Made by uttam kumar

class – 9th ‘a’1

force

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A force can be a push or a pull. For example, when you push

open a door you have to apply a force to the door. You also

have to apply a force to pull open a drawer.

You cannot see a force but often you can see what it does.

Forces can change the speed of something, the direction it is

moving in or its shape. For example, an elastic band gets

longer if you pull it.

Balance force• When two forces acting on an object are equal in size

but act in opposite directions, we say that they

are balanced forces.

If the forces on an object are balanced (or if there are

no forces acting on it) this is what happens:

• an object that is not moving stays still

• an object that is moving continues to move at the

same speed and in the same direction

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Example

• Hanging objects

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The forces on this

hanging crate are

equal in size but

act in opposite

directions. The

weight pulls down

and the tension in

the rope pulls up.

The forces on this

hanging crate are

balanced.

Unbalanced force

• When two forces acting on an object are

not equal in size, we say that they

are unbalanced forces.

If the forces on an object

are unbalanced this is what happens:

• an object that is not moving starts to

move

• an object that is moving changes speed

or direction5

example

• Resultant forces• The size of the overall force acting on an object

is called the resultant force. If the forces are

balanced, this is zero. In the example above, the

resultant force is the difference between the two

forces, which is 100 - 60 = 40 N.

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LAWS OF MOTION

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NEWTONSir Isaac Newton PRS MP (25 December

1642 – 20 March 1727) was an

English physicist and mathematician who is

widely regarded as one of the most influential

scientists of all time and as a key figure in

the scientific revolution

. Newton also made seminal contributions

to optics and shares credit with Gottfried

Leibniz for the invention of the infinitesimal

calculus.

Newton's Principia formulated the laws of

motion and universal gravitation that

dominated scientists' view of the physical

universe for the next three centuries. It also

demonstrated that the motion of objects on the

Earth and that of celestial bodies could be

described by the same principles.

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NewtON’s Laws Of MOtiON1. 1st Law – An object at rest

will stay at rest, and an

object in motion will stay in

motion at constant

velocity, unless acted upon

by an unbalanced force.

2. 2nd Law – Force equals

mass times acceleration.

3. 3rd Law – For every action

there is an equal and

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First law of motion

law of inertia

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First law the law of inertia

According to Newton's first law, an

object in motion continues in motion

with the same speed and in the same

direction unless acted upon by an

unbalanced force. It is the natural

tendency of objects to keep on doing

what they're doing. All objects resist

changes in their state of motion. In

the absence of an unbalanced force,

an object in motion will maintain its

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According

to

Newton's

first

law, the

marble on

that bottom

ramp

should just

keep going.

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MATHEMATICALLYFIRST LAW

The first law can be stated mathematically

as:-

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‹#›

Factors which determine the Moment of Inertia of a

body

The mass of the body. Experiments show

that Inertia is directly proportional to the

mass.

The distribution of mass in the body.

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Affect of inertia on change Of Object ‘s Mass

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A real car has a large

mass ,so it has a

large mass , so it has

a large inertia, and

hence quite difficult to

move by pushing

A toy car has a

small mass, so it

has a small inertia

,and hence can be

moved easily by

pushing

Affect of inertia if object is at rest

Initially, both the coin

and card ,are in state

of rest. Now when we

hit the card with our

fingers , a force acts

on the card and

changes its state of

rest to that of motion.

The force of flicker

however, does not

acts on the coin and it

falls into the tumbler.17

Affect of inertia if object is moving

This is what happens if passengers do not wear

seat belts while travelling in a car and the car

stops suddenly due to an accident. The large

force of inertia on the body of passengers can

throw passengers violently in forward direction

causing serious injuries.18

Second law of motion

(F= m x a)

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momentumMomentum can be defined as "mass in motion." All objects

have mass; so if an object is moving, then it has

momentum - it has its mass in motion. The amount of

momentum that an object has is dependent upon two

variables: how much stuff is moving and how fast

the stuff is moving. Momentum depends upon the

variables mass and velocity. In terms of an equation, the

momentum of an object is equal to the mass of the object

times the velocity of the object.

Momentum = mass • velocity

In physics, the symbol for the quantity momentum is the

lower case "p". Thus, the above equation can be rewritten

as

p = m • v

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• The units for momentum would be mass

units times velocity units. The standard

metric unit of momentum is the kg•m/s.

While the kg•m/s is the standard metric

unit of momentum, there are a variety of

other units that are acceptable (though not

conventional) units of momentum.

Examples include kg•mi/hr, kg•km/hr, and

g•cm/s. In each of these examples, a

mass unit is multiplied by a velocity unit to

provide a momentum unit. This is

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Momentum In Everyday Life

• A karate player

is able to break

so many tiles,

because he

strikes with his

hand very, very

fast, producing

a extremely

large

momentum.

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Second law of motion

• According to the second law of

motion :- The rate of change of

momentum of a body is directly

proportional to the applied

force, and takes place in the

direction in which the force acts.

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•So, newton's second law of motion can be expressed as :-

Force ∝ change in momentum / time taken

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MATHEMATICAL FORMULATION OF SECOND LAW OF MOTION

Suppose an object of mass, m is moving along a straight

line with an initial velocity, u. It is uniformly accelerated to

velocity, ν in time, t by the application of a constant force, F

throughout the time, t. The initial and final momentum of

the object will be, p1 = mu and p2 = mν respectively.

The change in momentum

α p2 – p1

α mν – mu

α m (ν – u).

The rate of change of momentum α m (ν −u) / t

Or, the applied force, F α m (ν −u) / t

Or, the applied force, F = km (ν −u) / t (2)

= kma (3)25

Here a [ = (ν-u) / t] is the acceleration, which

is the rate of change of velocity. The

quantity, k is a constant of proportionality.

The SI units of mass and acceleration are kg

and m s-2 respectively. The unit of force is so

chosen that the value of the constant, k

becomes one. For this, one unit of force is

defined as the amount that produces an

acceleration of 1 m s-2 in an object of 1 kg

mass. That is,

1 unit of force = k (1 kg) (1 m s-2).

Thus, the value of k becomes 1. From Eq. 26

Demonstration of second law of motion

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Since the acceleration produced is inversely proportional to the mass of

the object, it is easier to move (or accelerate) a small ball (having small

mass) than a big truck (having large mass ) by the force of our push.

• The SI unit of force is newton which

is denoted by N. A newton is that

force which when acting on a body

of mass 1 kg produces an

acceleration of 1m/s2 in it. We

have just seen that

F=ma

Putting m=1kg and a=1m/s2, F

becomes 1 newton.

So 1 newton = 1kg 1m/s2.28

Application of second law of

motion

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In a cricket match a fielder moves his arms back while trying to catch

a cricket ball because if he tries to stop the fast moving ball suddenly

then the speed decreases to zero in a very short time. Therefore the

retardation of the ball will be very large. As a result the fielder has to

apply a larger force to stop the ball. Thus, if he tries to stop a fast

moving cricket ball the fielder may get hurt as the ball exerts a great

pressure on the hands but if he tries to stop it gradually by moving his

arms back then the velocity decreases gradually in a longer interval

of time and hence retardation decreases. Thus the force exerted by

ball on the hand decreases and the fielder does not get hurt.

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A cushion like surface is made for a „high jump athlete‟. This reduces the large momentum of falling athlete more gently. Due to this, less opposing force acts on the athlete's body and injuries are prevented. 31

NewtON’s 2nd Law proves that different masses accelerate to the earth at the

same rate, but with different forces.

• 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

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third law of motion

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NewtON’s third Law Of motion

According to Newton‟s Third Law of Motion

:-

To Every Action There is an

Equal and Opposite Reaction

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Newton‟s third law of motion says :

Whenever one body exerts a force on

another body , the second body exerts an

equal and opposite force on the first body.

The force exerted by the first body is known

as “action” and the force exerted by the

second body on the first body is known as

“reaction”.

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Examples to illustrate

third law of motion

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How Do We Walk

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• When we walk on ground , then our foot

pushes the ground backward. The forward

reaction exerted by the ground on our foot

makes us move forward.

Recoiling of gun

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• When a bullet is fired from a gun, the force sending

the bullet forward is equal is equal to the force

sending the gun backward. But due to the high

mass of the gun, it moves only a little distance

backward and gives a backward jerk or kick to

shoulder of the gunman. Gun is said to be recoiled.

Flying of jet aeroplanes and rockets

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Modern jet aeroplanes and rockets work on the principle of

action and reaction. In aeroplanes engines exert a

backward force on the exhaust gases; the backward

rushing exhaust gases exert a forward force on the plane

which makes it move forward.

The case of a boat and the ship

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• Diagram shows “action” and

“reaction” when a man steps out

of a boat.

• The men push the water

backwards with the

oars. The backward

going water exerts an

equal and opposite push

on the boat, which

makes the boat move

forward.

Conservation of momentum

Momentum is never

created or

destroyed.When two(or more) bodies act

upon one another, their total

momentum remains constant(or

conserved) provided no external

force are acting.

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A Newton's cradle demonstrates

conservation of momentum.

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•According to law of

conservation of mass

total momentum before

collision=total

momentum after collision

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Application of law of conservation of momentum

A rocket works on

the principle of

conservation of

momentum44

A jet aeroplane also works on

the principle of conservation of

momentum

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