WORK, ENERGY &
POWER
LEVEL 1
(CHARITY BAZIWELO)
WORK
• If a body moves as a result of a force being applied to it, the
force is said to do work on the body.
Imagine you go home, then you decide to push hard on one of the
solid walls in your house, are you doing work?
• The amount of work done is the product of the applied force
and the distance, i.e.;
• What is the SI unit of work?
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• The is given the name Joule (J).
WORK CONT…
Example
1. Calculate the work done when a mass of 20 kg is lifted vertically
through a distance of 5 m. Assume that the acceleration due to
gravity is 9.81 .
Sol.
Force to overcome when lifting the mass:
Work done against gravity:
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WORK CONT…
Example
2. A spring, initially in a relaxed state, is extended by 100 mm.
Determine the work done if the spring requires a force of 0.6 N
per mm of stretch.
Sol.
Unlike the example before, here, the force is steadly changing (as
you pull on the spring).
In a relaxed state (extension), the force applied on the spring is .
At an extension of 100 mm, the force applied on the spring is
So which force do we use? Answer: the average force
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ENERGY
• Energy is the capacity (ability) to do work.
• The SI unit of energy is the same as that of work: Joule (J).
• Energy is expended when work is done.
• There are several forms of energy, including:
Mechanical energy
Heat or thermal energy
Electrical energy
Chemical energy
Nuclear energy
Light energy
Sound energy 5
ENERGY CONT…
• Energy may be converted from one form to another.
The Principle of conservation of energy states that the total amount of
energy remains the same in such conversions i.e.; energy cannot be
created or destroyed.
• For example, consider:
Mechanical energy being converted to electrical energy by a generator
Electrical energy being converted to light energy by a light bulb
Can you thing of other energy conversions?
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POWER
• Power is a measure of the rate at which work is done or at which
energy is converted from one form to another:
• The SI unit of Power is
Can you derive that?
• The is called a .
• Power can also be expressed in terms of force;
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POWER CONT…
Example
A car hauls a trailer at 90 km/h when exerting a steady pull of 600
N. Calculate
a) The work done in 30 minutes
b) The power required
Sol.
(Note that 30 minutes = 0.5 hour)
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PRACTICE PROBLEMS
1. Calculate the work done when a mass of 20 kg is lifted vertically
by a crane to a height of 100 m.
2. A spring, initially relaxed, is extended 80 mm. Determine the
work done if the spring requires a force of 0.5 N/mm of stretch.
3. At the instant of striking, a hammer of mass 30 kg has a velocity
of 15 . Determine the kinetic energy in the hammer.
4. A lorry having a mass of 1.5 tonne is travelling along a level road
at 72 . When the brakes are applied, the speed decrease to 18 .
Determine how much the kinetic energy of the lorry is reduced.
5. A 25 kJ of work is done by a force in moving an object uniformly
through 50 m in 40 s. Calculate a) the value of the force, and 9b)
the power.
THANK YOU!
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