Transcript unit 3 lesson 1 – work

Work and Energy
Section 1
Unit 3
Lesson 1: Work
Goals:
•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.
Work and Energy
Section 1
Bellringer
• List five examples of things you have done in the last
year that you would consider work.
• Based on these examples, how do you define work?
Work and Energy
Section 1
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.
Work and Energy
Section 1
Work
• In physics, work is the magnitude of the force (F)
times the magnitude of the displacement (d) in
the same direction as the force.
• W = Fd
• What are the SI units for work?
– Force units (N)  distance units (m)
– N•m are also called joules (J).
• How much work is 1 joule?
– Lift an apple weighing about 1 N from the floor to the
desk, a distance of about 1 m.
Work and Energy
Work
Section 1
• Pushing this car is work
because F and d are in the
same direction.
• Why aren’t the following
tasks considered work?
– A student holds a heavy chair
at arm’s length for several
minutes.
– A student carries a bucket of
water along a horizontal path
while walking at a constant
velocity.
Work and Energy
Section 1
Practice Problem
• A 20.0 kg suitcase is raised 3.0 m above a
platform by a conveyor belt. How much work is
done on the suitcase?
Work and Energy
Section 1
Practice Problem
• A 20.0 kg suitcase is raised 3.0 m above a
platform by a conveyor belt. How much work is
done on the suitcase?
• 5.9 x 102 J
Work and Energy
Work
• How would you calculate
the work in this case?
– What is the component of F
in the direction of d?
• F cos 
– If the angle is 90°, what is
the component of F in the
direction of d?
• F cos 90° = 0
– If the angle is 0°, what is
the component of F in the
direction of d?
• F cos 0° = F
Section 1
Work and Energy
Work
Section 1
Work and Energy
Section 1
Practice 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?
Work and Energy
Section 1
Practice 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?
• 130 J
Work and Energy
Section 1
Practice Problem
• A tugboat pulls a ship with a constant net
horizontal force of 5.00 x 103 N and causes the
ship to move through a harbor. How much work
is done on the ship if it moves a distance of 3.00
km?
Work and Energy
Section 1
Practice Problem
• A tugboat pulls a ship with a constant net
horizontal force of 5.00 x 103 N and causes the
ship to move through a harbor. How much work
is done on the ship if it moves a distance of 3.00
km?
• 1.50 x 107 J
Work and Energy
Section 1
Practice Problem
• A weight lifter lifts a set of weights a vertical
distance of 2.00 m. If a constant net force of
350 N is exerted on the weights, what is the net
work done on the weights?
Work and Energy
Section 1
Practice Problem
• A weight lifter lifts a set of weights a vertical
distance of 2.00 m. If a constant net force of
350 N is exerted on the weights, what is the net
work done on the weights?
• 7.0 x 102 J
Work and Energy
Section 1
Practice Problem
• A shopper in a supermarket pushes a cart with a
force of 35 N directed at an angle of 25O
downward from the horizontal. Find the work
done by the shopper on the cart as the shopper
moves along a 50.0 m length of aisle.
Work and Energy
Section 1
Practice Problem
• A shopper in a supermarket pushes a cart with a
force of 35 N directed at an angle of 25O
downward from the horizontal. Find the work
done by the shopper on the cart as the shopper
moves along a 50.0 m length of aisle.
• 1.6 x 103 J
Work and Energy
Section 1
Practice Problem
• If 2.0 J of work is done in raising a 180 g apple,
how far is it lifted?
Work and Energy
Section 1
Practice Problem
• If 2.0 J of work is done in raising a 180 g apple,
how far is it lifted?
• 1.1 m
Work and Energy
Section 1
Practice Problem
• A bumble bee has a mass of about 0.3 grams.
Suppose the bee does 0.04 J of work against
gravity, so that it ascends straight up with a net
acceleration of 1.0 m/s2. How far up does it
move?
Work and Energy
Section 1
Practice Problem
• A bumble bee has a mass of about 0.3 grams.
Suppose the bee does 0.04 J of work against
gravity, so that it ascends straight up with a net
acceleration of 1.0 m/s2. How far up does it
move?
• Answer
Work and Energy
Work is a Scalar
Section 1
• Work can be
positive or
negative but
does not have
a direction.
• What is the
angle between
F and d in
each case?
Work and Energy
Section 1
Homework
Due Next Class
Work and Energy
Section 1
Ticket Out
• Determine whether work is being done in each
of the following examples
a. A train engine pulling a loaded boxcar initially at rest.
b. A tug of war that is evenly matched
c. A crane lifting a car
• Choose one of the examples above and explain
your answer.