1. Work, Power, and Energy

2. Work W = F * d Work = Force x distance
Work: the result of a force applied to an object which causes the object to experience a displacement.
unit: Joule
Work = Force x distance
W = F * d
(Joule) = (Newton) x (meter)
1 meter A B C
1 N
Only the Force component in the same direction as the motion can count as the “work”!
1 meter

3. What if I double the distance?
What if I double the force?
Since …… Work = Fd,
Then ……… (2F) x d = 2Fd
And that equals twice the work!
2 W = (2)Fd
What if I double the distance?
Since …… Work = Fd,
Then ……… F x (2d) = 2Fd
And that equals twice the work!
2 W = F (2)d

4. Work Practice
Susan used 7.5 Newtons of force to lift an object 18.4 meters. How much work did Susan do?
W = 140 J
You did 170 joules of work lifting a 140 N backpack. How high did you lift the backpack?
d = 1.2m
If Sam performs 4,000 joules of work while dragging a tarp 10 meters, how much force did he use?
F = 400 N

5. Power P = t W Power: the rate at which work is done unit: watt
Power = Work / time
(watt) = (joule) / (seconds)

6. If I double the power, How much time will it take?
Remember P = W / t
2P is the same work in half as much time or
2P is the same amount of time
but doing twice as much work

7. Power Practice
What is the power of a man that lifts 356J in 42.0 seconds?
P = 8.48 watts
You can lift your own weight (500 N) up a staircase that is 5 meters high in 30 seconds. How much power do you use?
P = 83 watts
How does this compare to a 100 watt light bulb?
less than a light bulb

8. Work and Power Practice (10 min) Human Power Stair Lab (30 min)
Assignments:
Work and Power Practice (10 min)
Human Power Stair Lab (30 min)

9. Energy Energy: ability to do work stored work Potential Energy
create a force over some distance
stored work
measured in joules
E = W
Potential Energy
Kinetic Energy
PE = mgh
KE = ½ mv2
J = (kg)(m/s2)(m)
J = (kg)(m/s)

10. Potential Energy
potential energy: stored energy that comes from position
“possible” energy
due to gravity
PE = mgh
J = (kg)(m/s2)(m)
You need to put a 1 kg mass that is on the floor way on a shelf that is 3 meters high. How much energy will it have while sitting on the shelf?
PE = 29.4 joules

11. Kinetic Energy kinetic energy: energy of motion (has a velocity)
KE increases with increasing speed
KE increases with increasing mass
…… takes more F to move the object
KE = ½ mv2
J = (kg)(m/s)2

12. Energy Practice (10min) Phet Skate Park (30 min)
Assignments:
Energy Practice (10min)
Phet Skate Park (30 min)

13. Conservation of Energy
law of conservation of energy: energy is neither created nor destroyed … just transformed from one form into another
some energy may be “lost” to heat from friction, but not destroyed
MEtotal = PE + KE PE KE
50 J PE KE
15 J
35 J
mgh = ½ mv2
PEtop = KEbottom PE KE
50 J

14. Conservation of Energy
v = 6.3 m/s

15. Conservation of Energy
2. A ball slides down a frictionless track shown below. The ball has no velocity at position A.
___________ a. To what point does the ball rise on the opposite incline?
___________ b. At what point(s) does the ball have a maximum speed?
___________ c. At what point(s) is the kinetic energy of the ball a maximum?
___________ d. At what point(s) is the potential energy of the ball a maximum?
___________ e. At what point(s) is the potential energy of the ball a minimum? C A B D E F E C C
A , E C

16. Conservation of Energy2.
KE= 66 J

17. Conservation of Energy
3 m 8m
12 m C B A D 4.
At point A the 68.0 kg skier is moving at 6.3 m/s. Remember TME is “total mechanical energy”.
At point A, find : KE = ___________ PE = _____________ TME = ______________
At Point B, find: KE = ___________ PE = _____________ TME = ______________ J J J J J J

18. Conservation of Energy
3 m 8m
12 m C B A D 4.
At Point C, find: KE = ___________ PE = _____________ TME = ______________
At point C, find the skier’s velocity.
At point D, what is the TME? J
0.0 J J
v = 16.57… m/s J

19. Roller Coaster of Doom (20 min) Quiz Tomorrow (W, P, E)
Assignments:
Roller Coaster of Doom (20 min)
Quiz Tomorrow (W, P, E)

20. Conservation of Energy
law of conservation of energy: energy is neither created nor destroyed … just transformed from one form into another
some energy may be “lost” to heat from friction, but not destroyed
MEtotal = PE + KE PE KE
50 J PE KE
15 J
35 J
mgh = ½ mv2
PEtop = KEbottom PE KE
50 J

21. A Mexican jumping bean jumps with the aid of a small worm that lives inside the bean. The bean jumps meters from your hand into the air. What is the velocity of the bean as it lands back on your palm?
v = 0.54 m/s

22. Energy Energy: ability to do work stored work Potential Energy
create a force over some distance
stored work
measured in joules
E = W
Potential Energy
Kinetic Energy
PE = mgh
KE = ½ mv2
J = (kg)(m/s2)(m)
J = (kg)(m/s)

23. Work – Energy Theorem
The work-energy theorem says… the work done equals the change in energy.
PE = mgh
PE = (2)(9.8)(8)
PE = J
PE = 160 J
Work was done to move the box to the new position because a force was applied to move the box some distance.
W = Fd
W = Fgd
W = (2*9.8)(8)
W = J
W = ΔPE
W = PEf – PEi
W = – 0
W = 156.8J
PE = 0 J

24. Work – Energy Theorem
The work-energy theorem says… the work done equals the change in energy.
KE = ½ mv2
KE = ½ (0.8)(5)2
KE = (.4)(25)
KE = 10 J
KE = 10 J
Work was done to give the ball a new KE when a Force was used to accelerate the ball, giving it a new velocity.
W = ΔKE
W = KEf – KEi
W = 10 – 0
W = 10 J
W = Fd
W = (7.8)(1.3)
W = 10 J
vi = 0 m/s
KE = 0 J

25. Work – Energy Theorem (30 min)
Assignments:
Work – Energy Theorem (30 min)