Transcript Energy, Work, and Machines

Energy, Work, and
Machines
What is work?
 Put
a book over your head, are you
working?
 Hold a pencil out straight from your body,
are you working?
 From a physics standpoint, no. Work is
the product of the force and the objects
distance, W = Fd
 Force x Time changes momentum
 Force x Distance changes energy
What are we doing then?
 Energy
is being expended, a force is being
exerted but, without any net displacement,
no work is done, no energy is changed.
 What about a planet orbiting the sun? Is
work being performed?
 Remember, a force perpendicular to an
object does not change its speed, only its
direction.
What if?
 What
if you push against a wall?
 What if you push a flat cart along the
ground at a constant speed?
 What if you say forget it, jump out of a
plane and are in free fall?
 Ultimately, work can be defined as a
change in kinetic energy where
 KE = ½ mv2
Kinetic Energy
 From
the equation we can see that an
increase in mass or speed results in an
increase in kinetic energy, similar to
momentum.
 Notice that velocity is squared, therefore a
small increase in speed results in a large
increase in kinetic energy.
So?

The definition of energy is the ability to cause
change, in relation to an object either in itself or
its surroundings. Kinetic energy is the energy of
motion, potential energy at rest. Energy is
measured in quanta, we’ll get to that later.
 The work energy theorem states that work is a
change in kinetic energy or W = KE
 This states that work is accomplished when
there is a net change in kinetic energy
 Work is accomplished with any change in
energy, W = E
James Prescott Joule
 English
Dude, 1800’s, discovered
relationship between work and the change
in energy.
 Joule, also known as a Newton - Meter is
equal to a force of 1 Newton acting over a
distance of 1 meter. It is a unit of energy
 Lifting an apple a distance of 1 meter is
approximately 1 Joule
What if?
 What
if I am pushing a car at an angle of
25 degrees from the horizontal.
 Remember a force acting in the direction
of motion does work. A force acting
perpendicular to the direction of motion
does no work.
 In this case work is a function of the angle.
Still What if
W
= Fdcos0
 The sine of the angle is the y component
and vertical or perpendicular forces do no
work
 Gravity and normal forces do not
contribute to work. The cosine of 90 is 0
 Friction opposes motion and 180 degrees
would be opposite of motion, the cosine of
180 is -1
More work
 It
should be noted that the area under a
graph of force versus displacement curve
illustrates work.
What about time?
 You
lift 100lbs over your head but take 60
seconds to get it there. Your buddy lifts
the same weight the same distance in 1
second. Work performed is the same.
However your buddy is obviously stronger
or he has more power.
 Power is work done divided by the time it
takes to do it of P = w/t
Power
 Power
is measured in watts, 1 joule of
energy in 1 second. Watts are typically
measured in kilowatts
 Energy and power are not the same thing
the smallest device can use a tremendous
amount of energy given enough time.
Power is how fast a device can use or
transfer energy
More Power!!!!!
 If
power is equal to work divided by time
then it can also be said that:
 P = Fd/t of P = Fv since v equals d/t
 Machines
incorporate these variables to
maximize power. You need the proper
amount of force and speed
Machines
 A machine
is any device that changes
either the direction or magnitude of the
applied force or both.
 If the machine increases the magnitude of
the applied force it gives us a mechanical
advantage, the resistance force divided by
the effort force.
 MA = Fr/Fe
Mechanical Advantage
 If
the MA is greater than 1, the machine
provides a mechanical advantage or
increase in the applied force.
 Machines can increase force but it cannot
increase energy. All machines lose energy
through heat, friction, etc.
 An ideal machine transfers all the energy
Ideal Mechanical Advantage
 While
MA can be written as resistance
force over effort force it can also be written
as output work Wo / input work Wi or
 FrDr / FeDe
 Ideal mechanical advantage is calculated
 IMA = de/dr
 Notice, MA is Fr/Fe and IMA is de/dr
Machines
 Machines
are never ideal, this is a
measure of the maximum possible amount
of work.
 Efficiency of machines can be measured
in two ways:
 Efficiency = Wo/Wi x 100 or
 Efficiency = MA/IMA x 100
More Machines
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A simple machine is one that does work with
only one movement.
There are six basic types: Lever, inclined plan,
wedge, screw, pulley, and wheel & axle.
Lever, increases applied force Le/Lr and
changes direction
3 types 1st class fulcrum in the middle
2nd class, fulcrum resistance and effort
3rd class, fulcrum effort and resistance
The human body is composed of several levers.
Simple machines
 Pulleys,
single pulley changes direction, a
movable pulley or multiple pulleys
decrease the effort force, increasing the
applied force.
 Wheel and axle, similar to a lever attached
to a shaft, increases your applied force
 IMA = Rw/Ra
More simple machines
 Inclined
plane, allows you to cover more
distance with less force.
 IMA = length of slope/height of slope
 Screw, is an inclined plane wrapped
around a post
 Wedge, is an inclined plane with two or
more sloping sides, change the direction
of the force
Machines, Machines, Machines
 A compound
machine is a combination of
any two or more simple machines
 Most “machines” are compound
 The MA for a compound machine is the
combination of MA’s for each simple
machine MA = MA1 + MA2