Work, Energy And Power SS1 Physics Lesson Note

WORK

Work is said to be done whenever a force moves a body through a certain distance in the direction of force.  Work done can be defined mathematically as the product of the force and the displacement. It is a scalar quantity & measured in Joules

W = F X d 

W = mgh    

If a force is applied on a body at an angle Ø to the horizontal.

Work done to move the body on the horizontal plane = Fcos Ø x d 

Work done to raise the body to an appreciable height = Fcos Ø x d 

Examples– A boy of mass 50kg runs up a set of steps of total height 3.0m. Find the work done against gravity

Solution

m = 50kg, h = 3m, g = 10m/s2

Work done = mgh

= 50 x 10 x 3

= 1500 Joules

Energy

Energy is defined as the ability to do work. It is a scalar quantity & measured in Joules. There are many forms of energy. These include:

i. Mechanical energy 

ii. Thermal energy

iii. Chemical energy

iv. Electrical energy

v. Nuclear/Atomic Energy

vi. Solar/Light energy

vii. Sound Energy

Types of Mechanical Energy

Mechanical energy is classified as:

a) Potential energy

b) Kinetic energy

a) Potential Energy:- is simply stored energy i.e. energy possessed by a body by virtue of its states:

P.E = mgh  

b) Kinetic Energy: is the energy possessed by a body under its motion. Examples: area student running a race, wind or air motion, electrical charges in motion, a moving bullet

K.E = ½ mv2  

Example – An object of mass 5kg is moving at a constant velocity of 15mls. Calculate its kinetic energy.

Solution:

K.E = ½ mv2  = ½ x 5 x 15 x 15 = 562.5 J

Example – Find the potential energy of a boy of mass 10kg standing on a building floor 10m above the ground level. g = 10m/s2 

Solution:

P.E =mgh = 10 x 10 x10 =1000 J

POWER

Power is defined as the rate of doing work or the rate of transfer of energy. It is a scalar quantity & measured in watt

Power = work done     

        Time                

P = (F X d)/t  = F X d/t = FV  

Example: – : 

A boy of mass 10kg climbs up 10 steps each of height 0.2m in 20 seconds. Calculate the power of the boy.

Solution

Height climbed = 10 x 0.2 = 2m

Work done = mgh = 10 x 10 x 2 = 200 Joules

POWER = work  

                   Time

=        10 x 10 x 2 = 10watts

                   20     

Work Done In A Force Field & Energy Conversion 

Work done in Lifting a Body & Falling Bodies:

The magnitude of work done in lifting a body is given by 

Work = force x distance = mg x h = mgh

Also, the work done on falling bodies is given by

Work = force x distance = mg x h = mgh                            

Conservation & Transformation of Energy

Energy can be converted from one form to another in a closed system. The law of conservation of energy states that in an enclosed system, energy can neither be created nor destroyed during transformation.

 Examples of such conversions include: 

(a) Motor converts electrical to mechanical energy 

(b) Generator converts mechanical to electrical energy 

(c) Electric pressing iron converts electrical to heat energy 

World Energy Resources

World energy resources can be classified as:

1. Renewable Energy Resources: They are energy that can be replaced as they are used e.g. solar energy, wind energy, water energy & biomass
2. Non-renewable Energy Resources: Energy that cannot be replaced after use e.g. nuclear energy, petroleum & natural gas