Best Notes Of Work Power and Energy Part-01
Work
Work When a body is displaced by applying a force on it, then work is said to be done. e.g., a girl pulls a trolley and the trolley moves through a distance. In this way, she has exerted a force on the trolley and it is displaced. Hence, work is done
. If a body is displaced by a distance (s) on applying a force (F) on it, then the work done
W = F.s
Thus, work can be defined as ‘work done by a force on an object is equal to the magnitude of the force multiplied by the distance moved in the direction of force’ The SI unit of work is newton-metre (N-m) or joule (J) and its CGS unit i quantity, it has only magnitude and no direction. If F = 1 N and s = 1 m, then work done by the force will be 1 J.
1 joule = 107erg
Work Done by a Force Acting at an Angle
When force Facts on an object in such a direction that it makes an angle ∅ with the direction of displacements, then work done by the force
Value of work will be maximum at 0=0°and minimum at 0= 90°
Types of Work
There are three types of work 1. Positive Work When the force and displacement are in the same direction, then wo positive (i.e., work is done by force)
. e.g., a boy pulls an object towards himself.
Negative Work
When the force and displacement are in opposite directions, then work done will be negative (i.e., work is done against force). eg., a body is made to slide over a rough horizontal surface, then frictional force acts in the direction opposite to the direction of displacement, so work done by friction will be negative.
Zero Work
When the force and displacement are in perpendicular direction, then work done is zero
. e.g.,
If a coolie is moving forward carrying load on his head, in this case force is acting vertically downward (weight of load) and displacement is along horizontal direction, so work done is zero.
• If a body is moving on circle, then after completing one circle work done is zero, because of zero displacement.
Power
The time rate of work done by a body is called its power.
Power = Work/Time
P = W/t
It is a scalar quantity and its SI unit is J/s or watt (W).
Some Other Units of Power
1 kilowatt = 103 watt
1 mega watt = 106 watt
1 horse power = 746 watt
1 kilowatt hour = 3.6 x 106 joule = 3.6 mega joule(HP)
Energy
Energy is measured as the capacity to do a work. It is a scalar quantity having SI unit same as the work i.e., joule (J) and CGS unit is erg. A larger unit of energy is kilojoule.
1 kilojoule (kJ) = 103 J
An object having energy can exert a force (and hence do a work) on another object. When it happens, energy is transferred from former object to latter object.
* The energy and power do not mean the same, these are different. Energy refers to the total amount of work done while power determines the rate of doing work
Some Practical Units Of Energy
| Unit | Symbol | Equivalence(Joule) |
| Erg | erg | 10-7 |
| Calorie | cal | 4.2 |
| Kilowatt hour | kwh | 3.6x 106 |
| Electron Volt | ev | 1.6×10-19 |
Different Forms of Energy
Energy can manifest itself in several forms. Some examples are
(i) Heat Energy The energy possessed by a body due to its temperature, is called heat energy.
(ii) Internal Energy The total energy possessed by a body by virtue of particular
configuration of its molecules and also their random motion, is called internal energy.
(iii) Electrical Energy The energy required to maintain the flow of current in an electric
appliance, is called electrical energy.
(iv) Chemical Energy The energy absorbed or released during The energy absorbed or released during a chemical reaction, is called chemical energy.
(v) Nuclear Energy The energy absorbed or released during a nuclear reaction (fission or nuclear fusion), is called nuclear energy
Kinetic Energy
he energy possessed by a body due to its motion is called kinetic Energy and is given by
KE = ½ MV2 = P2/2m
where, v = velocity of a body at any time,
m=mass of and p=linear momentum their opposite
thus, kinetic energy of an object increases with its speed
e.g
When a fast moving cricket ball hits a stationary wicket the wicket is thrown away .
When a raised hammer falls ona a nail placed on a piece of wood , it drives the nail into the wood .
Potential Energy
The energy possessed by a body due to change in its position or shape, is called potential energy.
e.g
* Water stored in dam has potential energy due to its position at the height •
A stone lying on the roof of the building has potential energy due to its height
- A wound spring of a watch has potential energy due to its shape.
- The sum of potential energy and kinetic energy of an object, is known as mechanical energy.