Transcript Chapter 5 – Work and Energy

Chapter 5 – Work and Energy
Physics
Coach Kelsoe
Pages 159–191
Section 5–1: Work
Physics
Coach Kelsoe
Pages 160–163
Objectives
• Recognize the difference between
the scientific and ordinary
definitions of work.
• Define work by relating it to force
and displacement
• Identify where work is being
performed in a variety of situations.
• Calculate the net work done when
many forces are applied to an
object.
Definition of Work
• Work is done on an object when a
force causes a displacement of the
object.
• Work is done only when
components of a force are parallel
to a displacement.
So Is This Work?
• Consider the following scenarios:
– Jared holds a heavy chair at arm’s
length for several minutes.
– Kim carries a bucket of water along a
horizontal path while walking at a
constant velocity.
• Is work being done on the chair or
the bucket in these situations?
• The answer is no, even though
effort is required in both cases.
Definition of Work
• Work is the product of the
component of a force along the
direction of displacement and the
magnitude of the displacement.
• The formula for work is W = Fd,
where W = work, F = force, and
d = displacement. Notice that we
are substituting the variable “d” for
our Δx.
• Work is measured in units of
Joules (J), which are equal to N·m.
Joules
• The joule is named for British
physicist James Prescott Joule,
who had major contributions in the
fields of energy, heat, and
electricity.
• To put joules into perspective, the
work it takes to lift an apple from
your waist to your head is about 1
joule.
Why Isn’t Work Done On The
Chair?
• The application of a force alone
does not constitute work.
• Even though Jared exerts a force
to support the chair, the chair
doesn’t move.
• Jared’s arm muscles will go
through many small displacements
within his body, and therefore do
work within his body, but NOT on
the chair.
Why Isn’t Work Done On The
Bucket of Water?
• Work is only done when
COMPONENTS of a force are
parallel to a displacement.
• Since Kim exerts an upward force
on the bucket of water, which is
perpendicular to the displacement,
there is no work done on the
bucket of water.
What About Angles?
• When the force on an object and
the object’s displacement are in
different directions, only the
component of the force that is
parallel to the object’s
displacement does work.
• As long as there is a component
parallel to the displacement
(usually horizontally), work is
taking place.
More Than One Force
• If there are more than one
constant forces acting on an
object, you can find the net work
done on the object by finding the
net force on the object:
• Wnet = Fnetd cos θ
Definition of Work
Sample Problem
• How much work is done on a
vacuum cleaner pulled 3.0 m by a
force of 50.0 N at an angle of 30.0º
above the horizontal?
Sample Problem Solution
• 1. Identify givens and unknowns:
– F = 50.0 N
– θ = 30.0º
– d = 3.0 m
• 2. Choose equation that fits the
problem
– W = Fd cos θ
• 3. Substitute for values.
– W = (50.0 N)(3.0 m)(cos 30.0º)
– W = 130 J
Work Is A Scalar Quantity
• Work is a scalar quantity and can
be positive or negative.
• Work is positive when the
component of force is in the same
direction as the displacement.
– Example – lifting a box
• Work is negative when the
component of force is opposite of
the displacement.
– Example – the force of kinetic friction
between a sliding box and the floor
Work is a Scalar Quantity
• If the work done on an object
results only in a change in the
object’s speed, the sign of the net
work on the object tells you
whether the object’s speed is
increasing or decreasing.
• If net work is positive, the object
speeds up and work is done ON
the object.
• If net work is negative, the object
slows down and work is done by
the object on something else.
Vocabulary
• Work