61 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 ID: 655921
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Slide1
Chapter 6 Work & EnergySlide2
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. Slide3
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
.Slide4
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 Slide5
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.Slide6
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").Slide7
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.Slide8
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…Slide9
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)Slide10
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.Slide11
6.1 Work Done By A Constant Force
The work done by a constant force
F
is
W =
Magnitude of the force
Magnitude of the displacement
Angle between the force and the displacement
Positive Work
Negative Work
NO WorkSlide12
Example 1
Find the work done by a 45N force in pulling the suitcase at an angle of 50° for a distance of 75m.Slide13
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.Slide14
Example 3
A 120kg crate lies on the flatbed of a truck that is moving with an acceleration of +1.5m/s
2
. 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?Slide15
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?Slide16
Assignment
p. 187 Focus on Concepts #1,2
p. 188 #1-11 odds