Work is only done by a force on an

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Work is only done by a force on an object if the
force causes the object to move in the direction
of the force.
Objects that are at rest may have many forces
acting on them, but no work is done if there is
no movement.
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Work, by definition, is the product of the force
exerted on an object and the distance the
object moves in the direction of the force.
W Fd
Work is a scalar quantity.
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The SI unit of work is the Joule, named in honor
of James Prescott Joule.
One Joule, J, of work is the work done when 1.0 N
of force is applied through a distance of 1.0 m.
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Graphically, work is the area under a Force vs.
Displacement graph.
displacement, m
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If the force and displacement are not in the
exact same direction, then work Fd(cosq), where
q is the angle between the force direction and
displacement direction.
F 40 N
d 3.0 m
The work done in moving the block 3.0 m to the
right by the 40 N force at an angle of 35 to the
horizontal is ...
W Fd(cos q) (40N)(3.0 m)(cos 35) 98 J
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Energy
the ability (capacity) to do work
Energy comes in many forms mechanical,
electrical , magnetic, solar, thermal, chemical,
etc...
The SI unit of energy is the Joule.
Energy, like work, is a scalar.
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Kinetic Energy
energy of motion
All moving objects that have mass have kinetic
energy.
KE 1/2 mv2
m - mass of the object in kg v - speed of the
object in m/s KE - the kinetic energy in J
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Work-Energy Theorem
the net work done on an object is equal to its
change in kinetic energy
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A net force causes an object to change its KE
because a net force causes an object to
accelerate, and acceleration means a change in
velocity, and if velocity changes, KE changes.
Learn more about the Work-Energy Theorem here and
here.
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Potential Energy
energy of position or condition
gravitational potential energy
PEg mgh
m - mass of object in kg g - acceleration of
gravity in m/s2 h - height of object, in m,
from some arbitrary reference point PE
gravitational potential energy in J
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Potential Energy
energy of position or condition
elastic potential energy
PEe ½ kx2
k elastic constant in N/m x - elongation
or compression in m PEe elastic potential
energy in J
Click here to investigate elastic constants.
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Law of Conservation of Energy
Energy can be neither created nor destroyed. It
may only change forms.
S all types of energy before the event
S all types of energy after the event

• Examples
• A dropped object loses gravitational PE as it
  gains KE.
• A block slides across the floor and comes to a
  stop.
• A compressed spring shoots a ball into the air.

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Power, by definition, is the time rate of doing
work or the time rate transfer of energy.
P W / t
Power is a scalar quantity.
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The SI unit of power is the Watt, named in honor
of James Watt.
One Watt, W, of power is the power achieved when
1.0 J of work is done or 1.0 J of energy is
transferred in a time of 1.0 s.
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Simple Machines a device that is used to
manipulate the amount and/or direction of force
when work is done A common misconception is
that machines are used to do a task with less
work than would be needed to do the task without
the machine. They do not! In fact (mainly
because of friction), you actually do more work
with a machine than without it (for the same
task). The major benefit of a machine is
that the work can be done with less applied
force, but at the expense of the distance through
which the force must be applied.
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Work Force x Distance large force x small
distance small force x large distance For
example, 1000 J of work is needed to lift 1000 N
onto a table 1.0 m high. If the object were
pushed up a 4.0 m ramp (inclined plane), a
minimum of 250 N of force would be needed (250 N
x 4.0 m 1000 J). In reality, friction
between the object and the ramp would make
the necessary force greater than 250 N.
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If a 10 m ramp were used, a minimum of 100 N of
force would be needed. The greater the distance,
the smaller the necessary force. Efficiency of a
Machine the ratio of useful work output to
useful work input It is impossible to get as
much useful work or energy out of a machine as
you put into it.
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Consider the lever, pulley, and inclined plane as
examples of simple machines Inclined plane
decreases necessary force because of an increase
in distance Lever used to decrease force by
increasing distance changes direction of force
(link) Pulley used to decrease force by
increasing distance
may change direction of force
(link)
Click here to perform an interesting activity on
simple machines.
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Click here to explore energy, work, and
the Work-Energy Theorem in more depth.