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## Work, Power, and Energy

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### Title: Work, Power, and Energy Author: Jon Petersen Last modified by: Jon Petersen Created Date: 11/23/2007 2:20:58 PM Document presentation format – PowerPoint PPT presentation

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Title: Work, Power, and Energy

1
Chapter 5 Work, Power, and Energy
2
5.1 Objectives
• Understand the concept of work.
• Be able to perform work calculations.

3
Work
work a force applied through a distance (not
a displacement!)
Work is a scalar.
force (F)
f
distance (d)
W (Fcosf)d
James Joule studied the relationship
between work and thermal energy
units Nm J (joules)
4
Work
• positive () work is done if f lt 90o
• zero (0) work if f 90o
• negative (-) work if f gt 90o
• net work SW (SFcosf)d

5
Work Problem
A 107 N golf bag is dragged 125 meters (at
constant speed) with a force of 8.5 N. The
force is oriented 32o above the horizontal. How
much work is done by the golfer? By friction?
By gravity? By the normal force?
W Fcosfd
6
5.4 Power
power the rate at which work is done (work
time)
units J/s W (watts)
W in an equation is work W as units are Watts 746
W 1 hp
James Watt inventor of the steam engine
7
Power Problem
A weightlifter lifts a 275 kg mass from the floor
to a height of 1.92 m in only 1.48 seconds. How
much work is done by the weightlifter? How much
power is used?
8
5.2 Objectives
• Understand the concept of kinetic energy (KE) and
the closely-related work-energy theorem
• Solve KE and work-energy theorem problems.

9
Kinetic Energy
A moving object is capable of doing work if it
runs into something elseit has stored energy.
kinetic energy the energy held by a moving
object (due to relative motion)
F ma
KE ½mv2
Why?
force
W ma½at2
distance
W ½ma2t2
W ½mv2
10
KE Problem
• A major league baseball has a mass of 0.145 kg.
How much KE does a baseball have if it is thrown
at 44.4 m/s (100 mph)?

11
Work-Energy Theorem
If the sum of all the work done on an object (by
all the forces) is calculated, then the SW will
equal the change in kinetic energy of the object.
SW SF d DKE KEf - KEi
SW SF d ½ mDv2 therefore, d Dv2
12
Work-Energy Theorem Problem
• Brakes apply FFK ( SF)
• How much SW is done to stop a 1450 kg car
traveling at 20 m/s and 40 m/s? 45 mph 80 mph
• What is the stopping distance in each case if the
brakes apply 7.5 kN of force?

13
Stopping Distance Chart
14
5.2 Objectives
• Understand the concept of potential energy,
particularly gravitational potential energy
(GPE).
• Be able to solve gravitational potential energy
problems.

15
Potential Energy
Something is required to do work. That
something is called energy.
potential energy stored energy (due to the
presence of a force)
gravitational potential energy (GPE)
W Fdcos f
W FWhcos(0o) FWh
height (h)
W mgh
GPE mgh
units Nm J
16
GPE Problem
A 43 kg television is moved from the bottom to
the top of a flight of stairs that is 2.62 m
high. The stairs angle upward at 45o. How much
work is done? How much GPE does the TV have at
the top of the stairs?
17
5.3 Objectives
• Understand the law of conservation of energy.
• Use the law of conservation of energy to solve
assorted dynamics problems.

18
Conservation of Energy
Mechanical energy (ME) is the sum of KE and all
PE.
law of conservation of energy energy is
conserved when converted from one form to another
(the total ME remains constant).
SPEi SKEi SPEf SKEf
Why?
vf2 2gh
GPE
KE ½mv2
KE ½m2gh
height (h)
KE mgh
( GPE mgh)
KE ?
19
Conservation of Energy Problem
With what speed must a ball be thrown
directly upward to reach a final height of 28
meters?
20
Conservation of Energy Problem
A roller coaster traveling at 16 m/s drops down a
steep incline that is 25 meters high and then
moves up another incline. What is the height of
the second incline if the roller coaster is
moving at 12 m/s at its crest? Assume the effect
of friction is negligible.
website
21
Bullseye Lab
h1
razor
h2
What is the equation to find the range? dx ? It
is an extremely simple equation!
dx ?
22
Objectives
• Be able to identify simple machines.
• Be able to explain how simple machines make doing
work easier.
• Be able to calculate the ideal mechanical
advantage, input work (WI), output work (WO), and
efficiency (e) of a simple machine.

23
Simple Machines
4 kinds lever, inclined plane, pulley, wheel and
axle
Simple machines generally make doing work easier
by reducing applied force (but distance is
increased).