1. Work and Energy
Come in and get out your notes Journals and a paper off the blue bookshelf !

2. Energy is the ability of a physical system to cause change
All Energy is measured in Joules (J)

3. Energy is the capability for doing work.
Energy is anything that can be con-verted into work; i.e., anything that can exert a force through a distance.
Energy is the capability for doing work.

4. Types of Energy * Kinetic energy – the energy of motion
* Potential Energy – the energy of position
* Chemical Potential Energy – energy stored in molecules
* Thermal Energy – heat energy
* Nuclear Energy – energy related to matter itself
* Radiant Energy – light, microwaves, radio waves, x rays

5. Work = F d Work Work is measured in Joules
Work energy required to make something move.
WORK is ENERGY!
Force (N)
Work = F d
Distance (m)
Work Energy (J)
Work is measured in Joules

6. W = F d Joule = Newton Meter What is a Joule? Work Force Distance
1 Nm = 1 Joule (J)

7. W = F * d W = (mg) * d W = (8* 9.8) * 2 W = 156.8 J
Mass is NOT Force!!
Many times the Force that an object exerts is not known. We can find that Force by using the Formula F = mg (g =9.8)
W = F * d
W = (mg) * d
Example:
m = 8 kg
d = 2 m
W=?
W = (8* 9.8) * 2
W = J

8. Three things are necessary for the performance of work
1. Force “F”.
2. distance “d”.
3. The force and distance need to be parallel

9. If a force does not affect displacement, it does no work.
The force F exerted on the pot by the man does work. F W
The earth exerts a force W on the pot, but does no work even though there is displacement.

10. WORK DONE F d F W NO WORK F d
When you lift something your force is in the upward direction and your distance is going up
When you carry something, you move to the side, but your force is up
3 things to have Work!
Force
Distance
A component of force in the direction of distance
WORK DONE F d F W
NO WORK F d

11. Power Power is defined as the rate at which work is done: P = W = Fd
Work (J)
Force (N)
Distance (m)
P = W = Fd
t t
Power (watts)
Time (sec)
Power is measured in Watts

12. Power They both have the same amount of WORK!
Two physics students, Bob and Sam are in the weightlifting room. Bob lifts the 50-kg barbell a distance of .5 meters one minute; Sam lifts the 50-kg barbell the same distance in 10 seconds.
A). Which student does the most work?
B). Which student delivers the most power?  
Work of Bob = Fd
Wb = mgd
Wb = (50 kg) (9.8 m/s2) (.5m)
Wb = 245 J
Work of Sam = mgd
Ws = (50 kg) (9.8 m/s2) (.5m)
Ws = 245 J
They both have
the same amount
of WORK!

13. Power Sam has more POWER because he did the same amount of work
Two physics students, Bob and Sam are in the weightlifting room. Bob lifts the 50-kg barbell a distance of .5 meters one minute; Sam lifts the 50-kg barbell the same distance in 10 seconds.
A). Which student does the most work? SAME FOR BOTH!
B). Which student delivers the most power?  
Power of Bob =W/t
P of bob= 245 J / 60 sec
Power = 4.08 WATT
Power of Sam = W/t
P of sam = 245 J/ 10 sec
Power = 24.5 WATT
Sam has more
POWER because
he did the same
amount of work
in less time!

14. Potential Energy measured in Joules
Potential Energy: Energy of Position.
PE = mgh
Potential Energy (J)
Height of object (m)
Mass (Kg)
Gravity (9.8 m/s2)
Potential Energy measured in Joules
If h = 0 , then PE = 0

15. Kinetic Energy measured in Joules
Kinetic Energy: Energy in motion
KE = ½ mv2
Kinetic Energy (J)
Velocity (m/s)
Mass (kg)
Kinetic Energy measured in Joules
If v = 0 , then KE = 0

16. Kinetic and Potential Energy are EQUAL!!
Total PE = Total KE
There is height and velocity
There is height but no velocity
There is velocity
Height is 0
P.E. = max
K.E. = 0
P.E. = K.E.
K.E. = max
P.E. = 0

17. Energy is converted but not lost! Total Energy in this system = 100 J
P.E. = 100 J
K.E. = 0
P.E. = 50 J
K.E. = 50 J
K.E. = 100 J
P.E. = 0

18. What is the kinetic energy of a 1000-kg car traveling at 14.1 m/s?
Examples:
What is the Potential energy of a 1000-kg car resting at the top of a 20m hill?
P = mgh = (1000 kg)(9.8)(20 m)
P = 196,000 J
What is the kinetic energy of a 1000-kg car traveling at 14.1 m/s?
K = 99,405 J

19. Beware of the evil force!
Sometimes the problem will tell you the force of the object. This refers to the WEIGHT.
F = W = mg
PE = F h
You can also use the Weight to find the mass.
m = W/g or m = F/g

20. How are P.E. and K.E. related?
There is height and velocity
There is height but no velocity
There is velocity
Height is 0
P.E. = max
K.E. = 0
K.E. = max
P.E. = 0
P.E. = K.E.

21. Law of Conservation of Energy
@ highest point
P.E. = max
K.E. = 0
@ midpoint
P.E. = K.E.