Work, Energy, & Power
(Chapter 4)
Student Learning Objectives
• Compare and contrast mechanical work,
energy, and power
• Name examples of simple machines
• Describe renewable energy sources.
What is mechanical work?
Mechanical work is the use of energy to move an
object a distance.
Depends on total displacement, not path
W = Fd
http://www.physicsclassroom.com/mmedia/energy/au.cfm
Practice
1) Is mechanical work a scalar or vector quantity? Explain.
2) How much work would be done on a 150 lb (667 N)
barbell?
a) When it is lifted 2 ft (0.61 m)?
b) While it is held overhead?
c) When it is lowered 2 ft (0.61 m)?
3) What is the work done to lift a 15 kg child up one meter?
What is mechanical energy?
Mechanical energy is associated with objects
that are in motion or may become in motion.
Object has ability to do mechanical work.
Kinetic Energy
Kinetic energy is the energy an object has when it
is in motion.
KE = (½)mv2
The work done to change the motion of a moving
object = DKE
Practice
Which has the most kinetic energy?
Which has the most momentum?
Which has the greatest inertia?
18-Wheel Truck 10,000 lbs (4535 kg)
Parked
Football Player
300 lbs (136 kg)
Running 10 mph (4.47 m/s)
Olympic Sprinter
150 lbs (68 kg)
Running 22 mph (9.83 m/s)
Small Car
2,640 lbs (1,200 kg)
Rolling (1 m/s)
Potential Energy
Gravitational potential
energy is the energy an
object has that may fall
or travel downward
from a height.
GPE = mgh
The work done by or
against gravity = DGPE
Practice
A 60 kg person stands on
top of a 4 m tall ladder.
a) What is the gravitational
potential energy of the person?
b) How much work would the
person do on a lazy observer
lying on the floor if the person
falls from the ladder?
An object can have both GPE and KE,
at the same time.
Energy may change form.
http://www.physicsclassroom.com/mmedia/energy
Image Credit: Physics Classroom
How is conservation of mechanical energy used?
 Energy cannot be
created or destroyed.
The total amount of
energy in the universe is
conserved.
 Mechanical energy is
conserved when no
energy is lost or gained by
the system.
 Mechanical energy and
work are interchangeable.
Energy  Work
For Systems on a Hill
GPEtop = KEbottom
Practice
(Always assume mechanical energy is conserved)
1) A 10 kg steel ball is dropped 12 meters from the roof
of a building onto a concrete sidewalk.
a) Calculate the initial energy.
b) What is the KE the instant before impact?
c) What happens to this KE?
More Practice
2) A 68 kg skier begins to ski at the top of a hill that is
20 meters high.
a) How much kinetic energy does the skier have 75% of
the way down?
b) When are GPE and KE equal?
c) If the skier runs into Teresa at the bottom of the hill,
how much mechanical work may be done on Teresa?
d) What is the skier's speed at the bottom of the hill?
Elliptical Orbits
 Elliptical orbits are maintained by conservation of
energy.
 Planets move in elliptical orbits with the Sun at one focus of
the ellipse.
Perihelion
Aphelion
Equal Areas in Equal
Time Intervals
The orbital periods of
the planets depend on
the semi-major axis of
the orbit.
P2 = a3
Orbits Practice
Which planet has the
longer orbital period?
Saturn: a = 9.54 AU
Jupiter: a = 5.2 AU
a
http://www.exploratorium.edu/ronh/age/
What is Mechanical Power?
Power is the rate at which work is done or energy
is used.
P=W
t
P=E
t
Car Engines & Light bulbs
Practice
1) How much power is needed to lift a 150 lb (667 N)
barbell 2 ft (0.61 m), in 0.25 seconds? How many
horsepower is this? 1 hp = 746 Watts
2) What exactly is a kWh?
3) The energy used by a 60 Watt light bulb in 2 hours, is
432,000 Joules. How does this compare to a kWh?
What does a machine do?
Machines transform mechanical energy into
mechanical work.
Simple machines can multiply input force.
Lever
Ramp (Inclined Plane)
Pulley
Torque is work that
results in rotational
motion. (lever)
 Force acts through a distance
 Torque is equal on each side
of the fulcrum (pivot)
t = Fd
Practice
1) If a 667 N person sits 3 meters from the pivot, where
would a 533 N person need to sit to balance a seesaw?
More Practice
2) A wrench measures 12 inches on the long side and 2
inches on the short side. If a person applies 300 N to
the long side of the wrench, how many Newtons are
exerted on the other end of the wrench?
12 in
2 in
Pivot
What is a renewable energy source?
Renewable energy sources can be
replenished in a short period of time.
Wind
Water
Solar
Geothermal
Biofuel