## 2009-05-20

### Work and Energy

Work

1. Work is done when a force is exerted to move an object through a distance in the direction of the force.
2. Work, W, is defined as product of the force and the displacement of an object in the direction of the force.
Work=Fs
Where,

F= the force acting
S= the displacement (or distance traveled in the direction of the force)

3 .Work is a scalar quantity and its unit is joule (J) or N m. 1 joule =1Nm

Example:
A block which is at rest is acted on by force of magnitude 3 N in different direction. Determine the wok done by the block in each case.
a) The force act from the left, the object move to the right for 2 m.
b) The force act from the right, the object to the left for 2 m.

Solution

a) F=3 N
Moving to the right for 2m.

Work done, FS = 3 x 2
= 6Nm

b) F= -3 N
Moving to the left for 2 m (negative

Work done = FS = -3 X -2
= -3 N x (-2m)
=6 Nm

5 .1 joule is the work done when a force of 1  Newton moves of an object for 1 m in the direction of the force .
6 . Work is not done when a force is exerted on an object but the object does not move.
7 . In conclusion, work is not done when:

a) The direction of motion is perpendicular to the direction of the force exerted
b) Force is exerted on the object but the object does not move.

Energy

1. We need energy to do work.
2. Energy is defined as the Potential or the ability to do work.
3. Energy is scalar quantity and its unit is the joule (J) or N m.
4. Energy can exist in various form. Examples potential energy, kinetic energy, heat energy, electrical energy and sound energy.
5. Energy cannot be created or destroyed. The work done related to the change of the form of the energy.
Example

A student of mass 50 kg walks up a flight of stairs 1.5 m high. What is…

a) the work done by the student?
Work = Fx s
=mg x s
=(50 x 10) N x 1.5 m
=750 J
b) energy needed = work done
=750 J

rahul said...

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Anonymous said...

for the proof section ( power ), isn't it force must be divided by time as well?

FREE PHYSICS NOTES FOR SECONDARY SCHOOL said...

Anonymous:
yes..need to divide by time as well but since
Power = work divide by time = W / T

and

Work = Force x displacement

so Power = (force and displacement) divide by time = (F x D) / T

mathematically: u only have to divide one of them...so we choose displacement / time , which is velocity,v

which brings us to Power = Force x Velocity

:)

Anonymous said...

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