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ENERGY, WORK
&
SIMPLE MACHINES
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The Nature of Energy
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Energy — the ability to cause change
Forms of energy include:
Electrical
Thermal
Chemical
Radiant
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Kinetic Energy — energy in the form of motion
KE depends on mass and velocity of moving object.
Example of KE:
Going down a slide
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Potential Energy — energy that is stored
Example of PE:
Waiting at top of slide
Elastic Potential Energy – energy
stored by something that can
stretch or compress, such as
rubber band or spring
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Chemical Potential Energy – energy stored in chemical
bonds
A glass of milk has CPE until you
drink it then calories are used as
energy for your body
Gravitational Potential Energy –
anything that can fall has stored GPE
A ball on a ledge has GPE
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Conservation Of Energy
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Mechanical Energy — total amount of kinetic
energy and potential energy in a system
Mechanical Energy = PE + KE
POINT A: PE MAXIMUM, KE = O
POINT B: KE , PEx
POINT C: KE , PE
POINT D: PE MAXIMUM, KE = O
SWING
: TOTAL MECHANICAL ENERGY
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The Law of Conservation of Energy – energy may change
form but it cannot be created or destroyed under ordinary
conditions.
No energy is lost, just friction
converts mechanical energy into
thermal energy
Air resistance converts
mechanical energy into
thermal energy
The breakfast you eat converts
chemical energy into
mechanical energy, heat energy
so your muscles can help you
pump your legs.
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Work
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Work — transfer of energy that occurs when a force
makes an object move
For work to be done, something has to move and the
motion must be same direction as force.
When work is done, a transfer of energy always occurs.
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Calculating Work
Work (joules) = Force (Newtons) x Distance (meters)
W=Fxd
Ex. You push a wheelbarrow with a force of 100N.
You moved the wheelbarrow 5m.
How much work did you do?
W=Fxd
W = (100N) x (5m)
W = 500 J
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Power – the rate at which work is done.
Calculating Power
Power (watts) = work(J)
time (s)
P=W
t
Ex. You do 900 J of work pushing a fridge. I t took
5 seconds. What was your power?
P = W = 900J = 180J/s = 180 watts (w)
t
5s
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Using Machines
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Machine — device that makes doing work easier
Machines make work easier by increasing the force
that can be applied to an object.
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Two forces are involved when machine does work
(FIN) Input Force
The Force applied
to machine
Output Force (FOUT)
The force applied by
machine
Input Force
Output Force
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Mechanical advantage – the ratio of the output force to
the input force
Mechanical advantage = output force (newtons)
Input force (newtons)
MA = FouT
FIN
Efficiency – measure of how much work put into a
machine is changed into useful output work by the
machine.
Machines can be made more efficient by reducing
friction.
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Simple Machines
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Simple Machine — machine that does work with only one movement of the
machine
LEVER
Bar that is free to pivot
around fixed point (fulcrum)
1st Class Lever – fulcrum is
between the input force and
output force
2nd Class Lever – output force is
between fulcrum and input force
3rd Class Lever – input force is
between fulcrum and output force
PULLEY
Grooved wheel with a rope, chain
or cable running along groove
Fixed pulley – attached to
something that doesn’t move
Moveable pulleys – one end of the
rope is fixed and the wheel is free
to move
Block and Tackle – system of
pulleys consisting of fixed and
moveable pulley
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WHEEL AND AXLE
Consists of an axle attached to
center of a larger wheel so that the
wheel and axle rotate together
INCLINED PLANE
Sloping surface, such as a ramp
that reduces the amount of force
required to do work
THE SCREW
Inclined plane wrapped in a spiral
around a cylindrical post
THE WEDGE
An inclined plane with one or two
sloping sides
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