1. Work and Energy

2. Work and Energy Warm-up
What is our everyday definition of work? Do you think it has the same meaning in physics?
Name as many forms of energy as you can.
How does a rollercoaster take advantage of the Law of Conservation of Energy?
How does friction impact the Law of Conservation of Energy?

4. Work in Physics… A measure of energy transfer A force must be applied.
An object must move.
A component of the force must be in the same direction as movement.

5. Physicist’s definition of “work”
dist∥
A scalar
(not a vector)
dist
Work = F x dist∥= Fdcosq

6. Which requires more force, using 1 m rope or 3 m rope?

7. Atlas holds up the Earth
But he doesn’t move, dist∥ = 0
Work= Fx dist∥ = 0
He doesn’t do any work!

8. Garcon does work when he picks up the tray
but not while he
carries it around
the room
dist is not zero,
but dist∥ is 0

9. the Work-Energy Theorem
Work = change in KE
This is called:
the Work-Energy Theorem

10. Units again… Kinetic Energy = ½mv2 m2 s2 kg work = F x dist∥ =1Joule m
same!
=1Joule m s2 N m
=kg m

11. Gravitational Potential Energy
GPE= mgh
This is called:
“Gravitational Potential
Energy” (or PEgrav)

12. If gravity is the only force doing work….
Work-energy theorem:
-change in mgh = change in ½ mv2
0 = change in mgh + change in ½ mv2
change in (mgh + ½ mv2) = 0
mgh + ½ mv2 = constant

13. Conservation of energy
mgh + ½ mv2 = constant
Gravitational
Potential energy
Kinetic energy
If gravity is the only force that does work:
PE + KE = constant
Energy is conserved

14. Free fall (reminder) height t = 0s 80m V0 = 0 75m t = 1s V1 = 10m/s

15. m=1kg free falls from 80m mgh ½ mv2 sum t = 0s V0 = 0 h0=80m 800J 0
750J J
V1 = 10m/s; h1=75m
800J
t = 2s
V2 = 20m/s; h2=60m J J J
t = 3s
V3 = 30m/s; h3=35m J J J
t = 4s
V4 = 40m/s; h4= J J

16. Pendulum conserves energy
E=mghmax
E=mghmax
hmax
E=1/2 m(vmax)2

17. Roller coaster

18. Work done by a spring x F Relaxed Position F=0 I compress the spring
(I do + work;
spring does
-work)
PEspring = ½ kx2

19. If spring is the only force doing work….
Work-energy theorem:
-change in ½ kx2 = change in ½ mv2
0 = change in ½ kx2 + change in ½ mv2
change in ( ½ kx2 + ½ mv2) = 0
½ kx2 + ½ mv2 = constant

20. Mechanical Energy… The total energy of a system
Conservation of Energy…
MEi = MEf
½ mvi2 + mghi = ½ mvf2 + mghf

21. Conservation of energy springs & gravity
mgh + ½ kx2 + ½ mv2 = constant
Gravitational
potential energy
spring
potential energy
Kinetic energy
If elastic force & gravity are the only force doing work:
PEgrav + PEspring + KE = constant
Energy is conserved

22. (-)Work done by frictionheat

23. 1 calorie = heat energy required to raise the
Units again
Heat units:
1 calorie = heat energy required to raise the
temp of 1 gram of H2O by 1o C
Kg m2/s2
1 calorie= 4.18 Joules

24. Food Calories 1 Calorie = 1000 calories = 1Kcalorie
The Calories you read on food labels
1 Calorie= 4.18x103 Joules
7 x 106 J
2 x 106 J

25. electrical energy each second to produce light
Power
amout of energy
elapsed time
Rate of using energy:
Power =
Joule
second
Units: 1
= 1 Watt
A 100 W light bulb
consumes 100 J of
electrical energy each
second to produce light

26. Other units 1 Horsepower = 750 Watts Over a full day, a work-horse can
have an average work output of more than 750 Joules each second
1 Horsepower = 750 Watts

27. Kilowatt hours energy time Power =  energy = power x time
 power unit x time unit = energy unit
Kilowatts
(103 W)
hours
(3600 s)
Elec companies use: x
1 kilowatt-hour = 1kW-hr
= 103 W x 3.6x103 s = 3.6x106 Ws J

28. Review Questions…

29. Coaster Creator…
Use the simulator to design and build a rollercoaster. Observe energy transformations throughout the ride.