Chapter 5, work, energy and power summary presentation for grade 12 science

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5. Work, energy and power
Physics
Grade 12

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Revision
Kinetic energy is the energy an object has due to its motion and can be calculated using:
Conservation of Energy: Energy is never created nor destroyed, but is only transformed from
one form to another.
Conservation of Mechanical Energy: in the absence of non-conservative forces mechanical
energy is conserved.
EK =
1
2
mv2

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Work
When a force acting on an object has a component along the line of motion, work is done.
Work is the process of transferring energy from object or system to another.
W=F Δ x cosθ
Work is done Work is doneWork is not done
If more than one force acts at the same time we need to take each force into account
when calculating work done. We call the work done after taking all the forces into
account the net work done.

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Work-energy theorem
The work-energy theorem states that the work done on an object by the net force is equal
to the change in its kinetic energy:
Conservation of energy
A conservative force is one for which work done by or against it depends only on the
starting and ending points of a motion and not on the path taken.
A non-conservative force is one for which work done on the object depends on the path
taken by the object.
Wnet=Δ Ek=EK ,f −EK ,i

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Power
Power is defined as the rate at which work is done or the rate at which energy is
transfered to or from a system. The mathematical definition for power is:
Using the definition of work done we can also write this as:
And, if F and Δx are in the same direction we get:
P=
W
t
P=
F Δ x cosθ
t
P=Fv

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