# Study Notes On Physics: Work,Power And Energy

Here we are providing the short notes on Physics which will be help to crack science part in may examinations like SSC, Railway and other competitive exams.

## Work

Work is a scalar quantity. Its SI unit is joule and CGS unit is erg. 1 joule = erg.

Work done by a force is zero when

-Body is not displace actually, i.e. s = 0

-Body is displaced perpendicular to the direction of force i.e. θ = 90°.

## Work done by a variable force

If we throw a ball upward, work done against gravity is given by, W = mgh

where, m = mass of the body,

g = acceleration due to gravity and

h = height through which the ball is raised.

The centripetal force acts on a body perpendicular to the direction of motion. Therefore, work done by or against centripetal force in circular motion is zero.

If a coolie is carrying a load on his head and moving on a horizontal platform, then work done by force of gravity is zero as displacement is perpendicular to the direction of force of gravity.

## Energy

Energy of a body is its capacity of doing work. It is a scalar quantity and its SI unit is joule.

Energy can be transformed into work and vice-versa with the help of some mechanical device.

There are two types of Mechanical Energy, which are as follows

## Kinetic Energy

The energy possessed by a body by virtue of its motion is called its kinetic energy.

Kinetic energy of the body of mass m moving with velocity v is given by K =

## Potential Energy

The energy possessed by any object by virtue of its position or configuration is called its potential-energy.

Gravitational potential energy, U = mgh

## Einstein’s Mass-Energy Relation

According to this relation, the mass can be transformed into energy and vice-versa.

When ∆m mass is disappeared, then produced energy

E= ∆

where, c = speed of light in vacuum.

## Conservative and Non-conservative forces

Conservative forces are non-dissipative forces like gravitational force, electrostatic force etc.

For the conservative forces, work done during a round trip is always zero.

Non-conservative forces are dissipative in nature like frictional force, viscous force etc.

## Law of Conservation of Energy

Energy can neither be created nor be destroyed, only one type of energy can be transformed into other form of energy.

Only for conservative forces, (total mechanical energy)

initially = (total mechanical energy) finally

 Some Equipments used to Transform Energy S. Equipment Energy Transformed 1. Dynamo Mechanical energy into electrical energy 2. Candle Chemical energy into light and heat energy. 3. Microphone Sound energy into electrical energy. 4. Loud Speaker Electrical energy into sound energy. 5. Solar Cell Solar energy into electrical energy. 6. Tube light Electrical energy into light energy. 7. Electric Bulb Electrical energy into light and heat energy. 8. Battery Chemical energy into electrical energy. 9. Electric motor Electrical energy into mechanical energy. 10. Sitar Mechanical energy into sound energy.

## Collision

Collision between two or more particles is the interaction for a very short interval of time in which they apply relatively strong forces on each other. For a collision, physical contact of two bodies is not necessary.

A collision in which momentum of the system as well as kinetic energy of the system remains conserved, is called an elastic collision. In an elastic collision, all involved forces are conservative forces.

A collision in which only momentum remains conserved but kinetic energy of the system does not remain conserved, is called an inelastic collision.

If after collision two colliding bodies gets sticked with each other and moves with a common velocity, then collision is said to be perfectly inelastic.

In perfectly inelastic collision, the loss of kinetic energy during collision do not recover at all and two bodies stick together after collision.

## Gravitation

Each and every massive body attracts each other by virtue of their masses. This phenomenon is called gravitation.

## Newton’s Law of Gravitation

The gravitational force acting between two point objects is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

Gravitational force (F) =

where, G is universal gravitational constant.

Its value is 6.67 × .

Gravitational force is a central as well as conservative force.

## Acceleration Due to Gravity of Earth

The uniform acceleration produced in a freely falling body due to the earth’s gravitational pull, is called acceleration due to gravity, g =

where, M = mass of the earth, R = radius of the earth.

The value of g changes slightly from place to place but its value near the earth’s surface is 9.8 .

Gravitational force is the weakest force in nature. It is times smaller than electrostatic force and times smaller than nuclear force.

## Factors Affecting Acceleration due to Gravity

Shape of Earth Earth is not completely spherical its radius at equator is approximately 42 km greater than its radius at poles.

The value of g is maximum at poles and minimum at equator.

There is no effect of rotation of the earth at poles and maximum at equator.

Effect of Altitude Therefore, g decreases with altitude.

Effect of Depth g decreases with depth and becomes zero at centre of the earth.

## Mass and Weight

The mass of a body is the quantity of matter contained in it. It is a scalar quantity and its SI unit is kg.

Mass is measured by an ordinary equal arm balance.

Mass of a body does not change from place to place and remains constant.

The weight of a body is the force with which it is attracted towards the centre of the earth. Weight of a body (w) = mg

The centre of gravity of a body is that point at which the whole weight of the body appears to act.

The centre of gravity of a body can be inside the material of the body or outside it.

It is a vector quantity and its SI unit is newton (N). It is measured by a spring balance.

Weight of a body is not constant, it changes from place to place.

## Weight of a Body in a Lift

When lift is rest or in uniform motion The weight recorded in spring balance (i.e. apparent weight) is equal to the real weight of the body w = mg.

When lift is accelerating upward The weight recorded in spring balance is greater than then real weight of the body w’ = m(g + a)

When lift is accelerating downward The weight recorded in spring balance is smaller than the real weight of the body w’ = m(g – a).

When lift is falling freely under gravity The apparent weight of the body

w’ = m (g – g) (∵ a = g)

w’ = 0

Therefore, body will experiences weightlessness.

## Weight of a Body at the Moon

As mass and radius of moon is lesser than the earth, so the force of gravity at the moon is also less than that of the earth. It’s value at the moon’s surface is .

## Satellite

A heavenly body revolving around a planet in an orbit is called a satellite. Moon is a natural satellite of the earth. The satellite may be artificial. Artificial satellites are of two types.

## Geostationary Satellites

It revolves around the earth in equatorial orbits which is also called Geostationary or Geosynchronous orbit. The time period of these satellites is 24 hour.

## Polar Satellites

These satellites revolve around the earth in polar orbits at a height of approximately 800 km.

Weather monitoring which is predicted on the basis of information about moisture present in air, atmospheric pressure etc, obtained through a polar satellite.

We are able to see a live telecast of cricket world cup match or other programme with the help of a communication satellite which is a geostationary satellite.

Launching vehicles – PSLV & GSLV

## Time Period of a Satellite

It is the time taken by a satellite to complete one revolution.

If satellite is near the earth’s surface, then T = 2π 84.6 min.

## Escape Velocity

Escape velocity: Escape velocity is that minimum velocity with which a body should be projected from the surface of earth so as it goes out of gravitational field of earth and never return to earth.

Escape velocity is independent of the mass, shape and size of the body and its direction of projection.

Escape velocity is also called second cosmic velocity.

For earth, escape velocity = 11.2 km/s.

For moon, escape velocity = 2.4 km/s.

## Orbital Velocity

Orbital velocity of a satellite and escape velocity where R = Radius of earth. i.e. i.e. escape velocity is times the orbital velocity.

There if the orbital velocity of a satellite is increased to times (increased by 41%), the satellite will leave the orbit

and escape. 