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WORK & ENERGY
Unit 6.1
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WORK
In the everyday sense, the term “work”
means to do something that takes
physical or mental effort.
In physics, work has a distinctly different
meaning.
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WORK
Work is the product of the magnitudes
of the component of a force along the
direction of displacement and the
displacement.
W = Fd
* d represents displacement instead of
∆x
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WORK
Work is not done on an object unless
the object is moved because of the
action of a force.
The application of force alone does not
constitute work.
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WORK
So even if a person holds a chair at arm’s
length, the person exerts a force to
support the chair but since the chair
does not move, no work is done.
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WORK
When the force on an object and the
object’s displacement are in different
directions, only the component of the
force that is in the direction of the
object’s displacement does work.
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WORK
Components of the force perpendicular
to a displacement do not do work.
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WORK
For example, imagine pushing a crate
along the ground.
If the force you exert is horizontal, all of
your effort moves the crate.
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WORK
If your force is other than horizontal,
only the horizontal component of your
applied force causes displacement and
does work.
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WORK
If the angle between the force and the
direction of displacement is θ, then
W = Fd(cos θ)
θ
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WORK
If θ = 0°, then cos 0 = 1
which results in W = Fd
If θ = 90°, then cos 90 = 0
which results in W = 0
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WORK
Example:
A bucket of water being carried by a
student walking horizontally.
The upward force exerted to support
the bucket is perpendicular to the
displacement of the bucket which results
in no work done on the bucket.
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WORK
Work has dimensions of
force times length
N * m or J (joules)
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WORK
Work is a vector quantity and can be
negative or positive.
F
F
d
d
F
d
F
d
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WORK
Work is positive when the
component of force is in the same
direction as the displacement.
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WORK
Work is negative when the
component of force is in the
direction opposite of the
displacement.
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WHICH OF THE FOLLOWING ARE
EXAMPLES OF WORK?
1.
2.
3.
A book falls off a table and free falls
to the ground.
A rocket accelerates through space.
A waiter carries a tray full of meals
above his head by one arm straight
across the room at constant speed.
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ANSWERS:
1. Yes, this is an example of work. There is a
force (gravity) which acts on the book
which causes it to be displaced in a
downward direction (i.e., "fall").
2. Yes. This is an example of work. There is a
force (the expelled gases push on the
rocket) which causes the rocket to be
displaced through space.
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ANSWERS:
3. There is a force (the waiter pushes up on
the tray) and there is a displacement (the
tray is moved horizontally across the
room).Yet the force does not cause the
displacement. To cause a displacement,
there must be a component of force in the
direction of the displacement.
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https://www.youtube.com/watch?v=w4QFJb9a8vo
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