Chapter 6 Work & Energy
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Transcript Chapter 6 Work & Energy
Chapter 6 Work & Energy
6.1 Work Done By A Constant Force
When a force acts upon an object to cause a
displacement of the object, it is said that
work was done upon the object.
In order for a force to qualify as having done
work on an object, there must be a
displacement and the force must cause the
displacement.
6.1 Work Done By A Constant Force
Examples of WORK:
a horse pulling a plow through the field,
a person pushing a grocery cart
a student lifting a backpack, etc.
In each case described here there is a force
exerted upon an object to cause that object
to be displaced.
6.1 Work Done By A Constant Force
Work is not Effort:
Work is done upon an object whenever a
force acts upon it to cause it to be displaced
6.1 Work Done By A Constant Force
Is the following an example of work?
A student applies a force to a wall and becomes
exhausted.
This is not an example of work. The wall is
not displaced. A force must cause a
displacement in order for work to be done.
6.1 Work Done By A Constant Force
Is the following an example of work?
A book falls off the table and free falls to the
ground.
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").
6.1 Work Done By A Constant Force
Is the following an example of work?
A rocket accelerates through space.
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.
6.1 Work Done By A Constant Force
Is the following an example of work?
A student carries a heavy pile of books straight
across the room at constant speed.
This is not an example of work. The
student is exerting a normal force on the
books, but the normal force is not causing
the books to be displaced. More on this in a
few minutes…
6.1 Work Done By A Constant Force
The mathematics of work
W = force x displacement
Units of work = Nm
1 Nm = 1 J (joule)
Work is a scalar quantity
(magnitude only, no direction)
6.1 Work Done By A Constant Force
Some facts about work:
If the displacement is 0, no work was
done
Often force and displacement do not
point in the same direction
Only the component of force that is
parallel to the displacement is used in
defining work.
6.1 Work Done By A Constant Force
The work done by a constant force
F is
W = (𝐹 cos 𝜃)𝑠
Magnitude
of the force
Positive Work
Negative Work
Magnitude of
the displacement
Angle between
the force and the
displacement
NO Work
Example 1
Find the work done by a 45N force
in pulling the suitcase at an angle of
50° for a distance of 75m.
Example 2
A weightlifter is bench-pressing
a barbell whose weight is 710N.
He raises the barbell a distance
of 0.65m above his chest, and
then lowers it the same distance
at a constant velocity.
Determine the work done on
the barbell by the weightlifter
during (a) the lifting phase and
(b) the lowering phase.
Example 3
A 120kg crate lies on the flatbed
of a truck that is moving with an
acceleration of +1.5m/s2. The
crate does not slip as the truck
undergoes a displacement of
65m. What is the total work
down on the crate by all of the
forces acting on it?
Example 4 (#6 pg. 188)
A person pulls a toboggan for a distance of
35m along the snow with a rope directed
25° above the snow. Then tension in the
rope is 94N.
How much work is done on the toboggan by
the tension force?
How much work is done if the same tension
is directed parallel to the snow?
Assignment
p. 187 Focus on Concepts #1,2
p. 188 #1-11 odds