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• Please set up the next page in your journal for
Cornell notes (you may need two pages):
–Page: 25
–Date: 1-4-17
–Title: Work, Power, and
Energy
–Essential Question: How are work, power,
and energy related?
First, the pertinent vocabulary words…
• WORK: the amount of energy transferred from one object to another
by a force
– measured in Joules (J) or newton-meters (Nm)
– only occurs when an object moves in a direction parallel to the force
–W = Fd
• POWER: the rate at which work is done
– measured in Watts (W) or Joules per second (J/s)
𝐖
–P =
𝐭
• ENERGY: the ability to cause change or do work
– measured in Joules (J)
What is work?
• In physics, work has a very
specific meaning.
• Work represents a
measurable change in a
system, caused by a
force.
Work - horizontal
• If you push a box with a force (F) of one newton
for a distance (d) of one meter, you have done
exactly one joule (J) of work.
Work
• It is important to consider all the forces acting on an object separately.
Consider you are pushing a box on a frictionless surface while your
friend is trying to prevent you from moving it.
• What forces are acting on
the box and how much work
is being done?
F=3N
F = 1.5 N
Work
• The total work done on the box would be:
• W = Fnetd = (3 N – 1.5 N)(1 m) = (1.5 N)(1 m)
= 1.5 J
F=3N
F = 1.5 N
Work against gravity
mass (kg)
Work (joules)
W = mgh
height object raised (m)
gravity (m/s2)
The path doesn’t matter…why not?
A 63.4 g facial tissue box is lifted at constant
speed from the ground to height 1.08 m. Find the
work done on the box.
Fapp & d
W=mgh
= (0.0634 kg)(10 m/s2)(1.08 m)
=
Fw = m g
0.685 J
A student carries the 63.4 g tissue box 3.2 m
at a constant height of 1.08 m above floor. How
much work is done on the box?
ZERO! F and d are not in the same direction!
Fapp
d
Fw = m g
What is power?
• The same amount of work done in a shorter
time = MORE POWER!
•𝐏 =
𝐖
𝐭
=
𝐅𝐝
𝐭
=
𝐦𝐠𝐡
𝐭
• may be in kilowatts (kW)
–1 kW = 1000 W
• may be in horsepower
–1 horsepower = 746 W
Power
• Consider you and your friend pushing the box.
–Remember,1.5 J of work was done in this scenario.
How much power was generated if it took
3 seconds to move the box?
P=
W
t
=
=
1.5 J
3s
0.5 W
F=3N
F = 1.5 N
So, what
about
energy?
The workings of the universe can be viewed
as energy flowing from one place to another
and changing back and forth from one form
to another.
• In this unit, we will focus on mechanical energy.
• KINETIC ENERGY: energy of motion
– depends on mass and speed (velocity)
• directly proportional to mass
– if speed remains constant and mass doubles, then kinetic energy also doubles
• directly proportional to the SQUARE of speed
– if mass remains constant and speed doubles, then kinetic energy increases four times
Kinetic Energy
Kinetic Energy
(joules)
mass (kg)
𝟏
𝟐
𝐊𝐄 = 𝐦𝐯
𝟐
speed (m/s)
What is the kinetic energy of a
650 kg racecar moving at 67 m/s?
𝟏
𝐊𝐄 = 𝐦𝐯𝟐
𝟐
= ½ (650 kg)(67
2
m/s)
= 21775 J = 22000 J
• POTENTIAL ENERGY: stored energy
– GRAVITATIONAL POTENTIAL ENERGY: energy due to an object’s
location
• location is measured relative to an arbitrary reference line (usually the ground)
mass (kg)
Potential Energy
(joules)
PE
= mgh
height (m)
acceleration due to
gravity (m/s2)
A waiter stands at the top of a 2.3 m high
staircase, holding a 3.1 kg platter of food 1.5
m above the landing. What is the gravitational
potential energy of the platter?
PE = mgh
= (3.1 kg)(10 m/s2)(2.3m + 1.5m)
= (3.1 kg)(10 m/s2)(3.8 m)
= 117.8 J = 120 J