Download 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