Work and Energy

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  • Created by: Mal
  • Created on: 19-03-14 11:50
PDF Document 62.97 Kb

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Page 1

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Work
...is the product of a
force and the
displacement in the
direction of this force.
W=Fx

Page 2

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Work
If F and x are not
collinear, then:
W = F x Cos
Where is the angle
between the directions
of F and x.
Units: Nm or J

Page 3

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Work & Energy
v2 = u2 + 2as
If we multiply through
by mass m and
rearrange:
Fs = ½mv2 - ½mu2
Work done = Change of
Energy (in this case,
Kinetic)

Page 4

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Work & Energy
So Work is NOT
equal to Energy.
Instead, Work causes
a change of energy.

Page 5

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Work & Power
P = E/t
Power is the rate of
change of energy.

OR, since
P = W/t
Power is the rate of
doing work.

Page 6

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Work & Power
Since W = Fx and if F is
constant:
P = F x/t = Fv
P=Fv
Here the power moves
the object at a constant
velocity, v against a
constant resistive force,
F.

Page 7

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Energy forms
Kinetic energy:
Ek = ½mv2
Gravitational potential
energy:
Ep = mgh

Page 8

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Energy forms
Elastic strain energy:
Es = ½kx2
(where k is the spring
constant ­ i.e. this
equation can only be
applied when Hooke's Law
is valid for extension x)

Page 9

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Test
If P = E/t and
Ek = ½mv2
Show that if a conveyor
belt must move at 5ms-1
and also deliver 10000kg
of rock per minute then it
must consume energy at
the rate of 2kW
(minimum)

Page 10

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Conservation of
Energy
The total energy of a
closed system is
constant and can only
be converted between
different energy types.

E.g. so you can assume
loss of gravitational
potential energy = gain
in kinetic energy etc.

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