Transcript Work and Power - Broadneck High School

Physics: Work and Power
This presentation was developed at Oak
Ridge High School
There are many ways to look at the
application of a force. Rather than
considering the time, we could look
at the distance over which a force
is applied.
The product of the force applied over
a distance is called WORK.
W=Fd
Describing Work
There are three requirements for
work:
1. A force must be applied
2. Something must be moved by the
force.
3. The force and the motion must be
in the SAME direction.
No Direction
•
•
•
•
Acceleration has a direction
Velocity has a direction
Force has a direction
Momentum has a direction
• Work has no direction – scalar!
• Temperature has no direction
• Gallons of gas in my car has no
direction.
Thinking about work…
A person carrying a backpack up four
flights of stairs does ___________
the work as a person climbing two
flights of stairs
a) half
b) twice
c) four times
d) the same
Thinking about work…
A person carrying a backpack up four
flights of stairs does ___________
the work as a person climbing two
flights of stairs
a) half
Since W = F d, if you
b) twice
DOUBLE the distance,
c) four times you DOUBLE the work
d) the same
Thinking about work…
A weightlifter holding 500lbs over
his head is doing no work.
True or False?
True!
The weightlifter is not
moving the barbell over
any distance. Therefore
he is not doing any work.
Which picture?
• Which picture illustrates
work being done?
Neither one by
themselves!
A better question…
• How much work did
the weightlifter do to
move the weights
distance Y?
Distance Y
Question?
If the distance is
doubled, how
does that affect
the work?
Distance
Doubled!
Distance
Distance
What if…
• the weights were twice as heavy?
– Twice the work
• the weights were twice as heavy
and they were lifted twice as far?
– Four times as much work
Another thing about work…
The definition of work
requires that the
force you are
exerting be in the
SAME direction you
are moving an
object.
Thinking Physics
• Guy has to get a piano onto a 2.0 m
high platform. He can use a 3.0 m
long, frictionless ramp or a 4.0 m
long, frictionless ramp.
– Which ramp will Guy use if he wants
to do the least amount of work?
Work – a review
•
What is work?
– Work is the force applied over a
distance
•
Does work have a direction?
–
•
Work is a SCALAR and does NOT have a
direction.
What is the relationship between
the force applied and the work
done on an object?
– Work is directly proportional to force.
Calculating work
How much work does a student
do when she carries 30 kg of
books up a 5 meter staircase?
Knowns
Unknowns
m = 30 kg
d=5m
a = g = 9.8 m/s2
W = ???
F = ???
Knowns
m = 30 kg
d=5m
a = g = 9.8 m/s2
Unknowns
W = ???
F = ???
Now that you have your information
organized, decide what relationships
are important to solving the problem.
Relationships
F=mg
W=Fd
Step 1: Find the force
The force you must find is the
weight of the books.
F=mg
F = (30 kg) (9.8 m/s2)
F = 294 N
Step 2 : Find the work
Work is the product of the force
applied over a distance. In this
case, she carries 294 N up 5
meters of stairs.
W=Fd
W = (294 N) (5 m)
W = 1470 N m = 1470 J
Try this…
A net force of 20 N is needed to push
a rock 1.5 m with a constant
velocity.
• How much work is done on the rock?
30 J
• What does this look like graphically?
Work = area under the curve
Force (N)
40
Force vs. Displacement
20
10
0
1.0
2.0
3.0
Displacement (m)
4.0
5.0
Work can be calculated by the area under the curve.
The area of a rectangle = base * height
Area = (1.5 m)(20 N) = 30 J
Problem?
• If Jane pushes the lawn mower with
120 N of force over a distance of 5
meters, how much work is done?
W=Fd
W = (120 N)(5 m)
W = 600 J
Work and Direction of Force
• Which direction is the force applied?
• Which direction is the mower going?
Break Force into Components
W = F d (cos 60o)
o
W = (120 N)(5 m) (cos 60 )
W = 300 Nm or 300 J
Only the horizontal
component of the force
does work!!!
Fv
60o
Fh
Practice Problem
A rope is used to pull a metal box
15.0 m across the floor. The rope is
held at an angle of 30.0o with the
floor using a force of 628 N.
• How much work does the force on
the rope do?
W = F d (cos θ)
W = (628 N) (15.0 m) (cos 30o)
W = 8157.96 J ~ 8160 J
What is work?
• Work is the transfer of energy by
mechanical means.
Needing something more…
Impulse does a good job of helping
you to know the time over which a
force is applied.
Work does a good job of telling you
the distance over which a force is
applied.
Wouldn’t it be nice to find a way to
combine force, time, and distance
into one relationship?
I’ve got the POWER!!!
We invent POWER as a combination
of WORK (force x distance) and
TIME.
Power = Work done
time interval
P=W/t
Units of Power
Power is measured in joules per unit time.
This is often rewritten as WATTS, in
honor of James Watt, the developer of
the steam engine.
• 1 Watt = 1 Joule/second = 1 N m/s
• Because a Watt is small, it is
usually written in terms of kW or
kilo-Watts.
• 750 Watts = 1 Horse Power
Practice Problem
An electric motor lifts an elevator that
weighs 12 000 N a distance of 9.00
m in 15.0 s.
• What is the power of the motor in
watts?
P = 7200 W
• What is the power in kilowatts?
P = 7.2 kW
Try this
• If little Nellie Newton lifts her 40-kg
body at a velocity of 0.125 m/s then
what is the power delivered by little
Nellie's biceps?
I Rock…!
P = W/t
P = (Fd)/t
P = F(d/t)
P=Fv
P = (40*9.81)(0.125 m/s)
P = 49 Watts