Work

The Concept of Work
If a person pushes a wall and the wall does not move, though the person may sweat and physically become tired, he would not have done any work. But if the person pushes a trolley and the trolley moves it is said work is done.

The S.I Unit of Work
Work is the product of force and distance moved in the direction of the force.
Thus,
Work = Force (f) * distance (d) moved in the direction of the force.
SI unit of work is Joules.

The Work Done by an Applied Force

Energy

The Concept of Energy
Energy can be defined as capacity of doing work.
Energy has the same SI unit like that of work, and that is Joules (J)

S.I Unit of Energy
Energy has the same SI unit like that of work, and that is Joules (J)

Different Forms of Energy

There are different forms of energy such as:
1. Chemical energy
2. Heat energy
3. Light energy
4. Sound energy
5. Electrical energy
6. Nuclear energy
7. Solar energy

Difference between Potential Energy and Kinetic Energy

There are two types of chemical energy, which are:
1. Potential energy: It is the energy possessed by a body mass in its position or state.
2. Kinetic energy: It is the energy possessed by a body due to its motion.

Consider  when  the  body  is  vertically  thrown  upwards  with  an  initial  velocity  „u‟  from  the ground.



At the ground: The height is zero and initial velocity is at maximum so as to attain maximum height.
Therefore K.E = ½ MV2 will be maximum

K.Emax = ½ mv2
Where K.E = Kinetic energy
M = Mass of the object/body
V=Velocity

P.E = Mgh
Where
P.E = Potential energy M= Mass of the object H = Height of the object g = gravitation force
P.E = Mgh will be zero because P.E= M*g*0 (body at the ground where k=0)


Neglecting the air resistance, as the body moves upwards its velocity decreases it also experiences gravitational force (g) pulling downwards towards the earth's center.

The maximum Height Attained
The final velocity of the body will be zero (V=0) Therefore K.E = ½ mv2
K.E = ½ m(0)2
K.E = 0
P.E = MgHmax


Note: That the object drops from Hmax that is; it leaves with zero Kinetic Energy. At position A in fig. 8. The conservation of mechanical energy (M.E) is given as:
P.E + K.E = Constant
(The sum of P.E and K.E is constant throughout the motion of the object if the air resistance is neglected)


The Transformation of Energy

The notion of energy is that energy is changed from one form into different forms using transducers.
Transducer is a device used to transform energy from one form to another.

For example:
1. Battery converts chemical energy into electrical energy.
2. A generator converts mechanical energy into electrical energy.
3. A motor converts electrical energy into mechanical energy.


MOTION OF SIMPLE PENDULUM




Observation
When the bob is at position A, it possesses potential energy only due to the height “h” which is equal to “Mgh”.

As it swings downwards to position B, the height decreases, and as the result it loses potential energy.
 The bob has Vmax and hence K.Emax at B. The height at B is zero, thus the P.E is zero.
 As it swings towards C, the P.E increases and reaches its maximum again in position C, where the Kinetic Energy is zero. At position D, the energy of the bob is party potential and party Kinetic.

The Principle of Conservation of Energy

The law of conservation of energy state that “ Energy can neither be created nor destroyed but
can only be converted from one form to another.”

This means the amount of energy is constant all the time.

Uses of Mechanical Energy

The mechanical energy can be used to produce electric power using generators. Some uses of mechanical energy are: It enables our body to do work, it makes work easier and faster, it is used to transport goods and people from one place to another, many transport vehicles uses the knowledge of mechanical energy. Examples of vehicles which uses mechanical energy are airplanes and motor cars.

Power

Power is the rate of which work is done.
 It is a measure of the rate at which energy changes.
 This means that whenever work is done energy changes into a different form.

The S.I Unit of Power
The SI unit of power is Joules per second J/S or watts, W.
1 Joules per second = 1 watt

When 1 Joules of work is done per second the power produced is a watt. Watt is the unit for measuring electrical power.

The Rate of Doing Work

Suppose that two cranes each lift objects having masses of 200 kg to a height of 12m. Crane A lifts its object in 10 sec while crane B requires 15 sec to lift its object. Assume they lift the objects at a constant velocity they do the same amount of work.
Work done = GPE
= Mgh
= (200kg) (9.8m/s2)(12m)
= 23520J
Each did a work that was equivalent to 23520J.

What is different for the two cranes is the rate at which they did the work or their generation of power.
The power of crane A can be calculated by;

PB = 1568 watts.