1. Chapter 5: Work, Power, and Energy

2. 5.1 Objectives Understand the concepts of work and power.
Be able to make work and power calculations.

3. Work work: a force applied through a distance (not a displacement!)
Work is a scalar.
force (F) f
distance (x)
W = (F·cosf)·x
James Joule: studied
the relationship between
work and thermal energy
units: N·m = J (joules)
shake can!

4. 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 = F·x·cosf

5. 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

6. 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?

7. 5.2 Objectives
Understand the concepts of potential energy and kinetic energy.
Make GPE and KE calculations.

8. 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 = F·x·cos f
W = FW·h·cos(0o) = FW·h
height
(h)
W = m·g·h
GPE = m·g·h
units: N·m = J

9. GPE Problem A 425 N 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?

10. Kinetic Energy A moving object is capable of doing work if it runs
into something else—it has stored energy.
kinetic energy: the energy held by a moving object
(due to relative motion)
F = m·a
KE = ½·m·v2
F·d = m·a·d
Why?
W = m·a·d
force
W = m·a·½·a·t2
distance
W = ½·m·a2·t2
W = ½·m·v2

11. KE Problem How much energy does the space shuttle have
as it travels along in its orbit? The shuttle has a
mass of 2 x 106 kg and its orbital speed is 8 km/s.

12. Objectives Understand the work-energy theorem.
Be able to make work-energy theorem calculations.

13. 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·m·v2, or d ~ v2

14. Work-Energy Theorem Problem
Brakes apply a SF when applied. How much SW
is done to stop a 1450 kg car traveling at (a) 15.6 m/s
and (b) 31.2 m/s? [ 35 mph and 70 mph ]
What is the stopping distance in each case if
the brakes apply 7.5 kN of force?

15. Stopping Distance Chart

16. 5.3 Objectives Understand the law of conservation of energy.
Use the law of conservation of energy to solve assorted dynamics problems.

17. 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 + KEi = SPEf + KEf
Why?
vf2 = vi2 + 2·a·d
vf2 = 2·g·h
GPE
KE = ½·m·v2
KE = ½·m·2·g·h
height (h)
KE = m·g·h
(= GPE = m·g·h)
KE = ?

18. Conservation of Energy Problem
With what speed must a ball be thrown upward to
reach a height of 28 meters?

19. 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

20. Bullseye Lab h1
razor h2
dx = ?
It is an extremely
simple equation! dx

21. 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 (IMA), actual mechanical (AMA) advantage, input work (WI), output work (WO), and efficiency (e) of a simple machine.

22. 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).
input work:
WA = FA·dA
output work:
WO = FO·dO
If no friction: WA = WO
If friction is present: WA > WO

23. Simple Machines mechanical advantage (MA): factor by which
input force is multiplied by the machine
“ideal”
“actual”
efficiency: ratio of output work to input work
(indicates amount of friction in machine)