Energy Transfer (When work is done)

When an object is dropped from above the ground, work is done as the object is pulled to the ground. As the object is falling and work is done, the potential energy of the body is changed to kinetic energy. In principle, the quantity of potential energy stored in a body is always equal to kinetic energy produced when the body is released to do work. In other words, when energy changes, for example from potential to kinetic, there is always an accompanying work done.


What work is done when a mass of 6kg is raised through a vertical height of 3.5m(acceleration due to gravity is 10m/s2)


Acceleration (due to gravity) =10m/s2

Workdone W=Mass x acceleration x distance

= 6kgx10x 3.5

=210 joules.


Power is also related to the concepts of energy and work. Power is defined as the rate of doing work, i.e. workdone divided by time.

Power=Workdone/Time taken

The unit of power is Watt(w)

Example 1

What is the power of a child that has done work of 60 J in 20 seconds.


Power P=Work/Time

=60/2=3 watts

Example 2

Calculate the power of a pump which can lift 500kg of water through a vertical height of 12m in 0.3 minutes. Assuming g=9.8m/s2

Force=mass x acceleration(g)

Force=500kg x9.8N


Workdone =500 x9.8 x12m

Time taken= 0.3minutes x60

=18 secs

Workdone=mass x acceleration x distance

Workdone=Force x distance

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Since force= mass x acceleration

Workdone=500kg x 9.8N x12m


 Power = work done/Time taken


Power = 3266.67 watts

Convert this to kilowatts by dividing by 1000




1.If the kinetic energy of a ball moving at a velocity of 5m/s is 60kj.What is the mass of the ball?

2.The mass of a ball is 10g and the height is 4m. Calculate the velocity of the ball just before it touches the ground. (g=10m/s)

3.If the mass of an object is 6kg, and the height is 5m. Calculate the potential energy. (assuming=10m/s).

4.If a force of 10Newton is applied by a car over a distance of 4metres, calculate the workdone by the car.

5.Calculate the work done by a ball of mass 40g which falls freely from a height 0.8m above the ground. (g-10m/s)

6.Calculate the power of a pump which can lift 500kg of water through a vertical height of 12m in 0.3minutes. Assuming g-9.8m/s2

See also




Electrical Energy


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