Monday, 27 June 2011

Form 4 - Chapter 2 : 2.10 Understanding Work, Energy, Power and Efficiency.


Work is done when a force cause an object to move in the direction of the force.

The work done, W is defined as the product of the for ce, F and the distance, s in the direction of the force.

Case 1 : The displacement. s of the object is in the direction of force.

                                                     a)                                                b)

Picture a) : The boy not do any work, as the wall did not move though he applied a force on it.
Picture b) The girl does  work when she pushed the trolley of the books.

Case 2 : The displacement. s of the object is not in the direction of force.

Consider, a man carrying a bag.


So, the component of force that contribute to the work is W = F cos Ɵ

Case 3 : The displacement, s of the object is perpendicular to the direction of force, F.

Consider a waiter carrying a plate of food.


The direction of the applied force and the displacement are not the same. They are perpendicular to each other. Since cos 90o is zero, the work done is zero.


Energy is defined as the capasity to do work.

Energy can exist in various forms : mechanical energy, heat energy, electrical energy, chemical energy, sound energy, light energy and nuclear energy.

The units of energy is the joule, J. It has the same unit as work.

Energy is transfered from one object to another when work is done.



1. Potential Energy

Potential energy is defined as the energy stored in the object because of its position.

Two types of potential energy:
a) gravitational potential energy

Example : A rollercoster at its highest position.

Potential energy,
m=mass, g=gravitational acceleration, h=height

b) Elastic potential energy

Example :  A wound coil-spring in a toy car.

2. Kinetic energy

Kinetic energy, Ek is the energy possessed by an object due to its motion.

Kinetic energy,

Prinsiple of conservation of energy,

Energy can be converted from one form to another, but it cannot be created or destroyed.


Power, P is the rate at which work is done or the rate at which energy is transformed.

                                              1 W = 1 J/ 1 s = 1 J s-1

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