1. Work, power, & Energy
Chapter 12

2. Work
Force which causes a change in the position or motion of an object in the direction of the applied force
An object must have moved for work to occur
Work = force X distance
W = Fd
SI Units:
kg x m2/s2 = Joules (J)

3. JOULE is a unit to describe work One JOULE = a NEWTON moving one METER
As you try to lift the object, it does not move… Is work being done?

4. Example: A weight lifter is holding a 200kg barbell above his head
Example: A weight lifter is holding a 200kg barbell above his head. What is the amount of work being done?

5. Running up the steps and walking take the same amount of work…
Running up the steps and walking take the same amount of work….but which requires more power?

6. Power The rate at which work is done power = work / time P = w/t
SI Units
Watts (W) = J/s

7. WATT (W) – a unit to describe power One WATT = one JOULE of work in one SECOND

8. Example: A girl lifts a 100-N load a height of 2. 0 m in a time of 0
Example: A girl lifts a 100-N load a height of 2.0 m in a time of 0.5 s. What power does the girl produce?

9. Machines
Require the same amount of work, but make it easier b/c less force is applied
Work input always equals work output

10. THE LEVER (THREE CLASSES)
FIRST – fulcrum in the middle.
SECOND – load in the middle.
THIRD – effort in the middle.

11. THE PULLEY (A MODIFIED LEVER)
One or more levers that changes dispersion of work

12. THE WHEEL AND AXLE (A LEVER OR PULLEY ON A SHAFT)
Mechanical Advantage depends on size of wheel and size of axle.
Some are designed for power.
Some are designed for speed.

13. INCLINED PLANE
A long gradual slope has a larger Mechanical Advantage than a short steep slope.

14. THE WEDGE (A DOUBLE INCLINED PLANE)
The mechanical advantage depends on the length of the slope and how wide the wedge is.

15. THE SCREW (AN INCLINED PLANE WRAPPED AROUND A CYLINDER)
Steepness of slope determines the M.A.

16. COMPOUND MACHINES

17. Energy All around us, organisms need energy to survive
Energy is the ability to do work
When work is done, energy is transformed or transferred to another system
Can be measured by how much work is done
Both energy and work are measured in Joules (J)

18. Potential Energy STORED energy (stretched rubber band)
‘energy of position’

19. Gravitational potential energy: any system where the 2 objects are separated by a distance
Example: apple falling from tree when stem breaks)
Depends on mass and distance b/w 2 objects
PE = mgh
Potential Energy = mass x acceleration due to gravity x height

20. Example: A 65 kg rock climber ascends a cliff
Example: A 65 kg rock climber ascends a cliff. What is the climbers gravitational potential energy at a point 35m above the base?

21. Kinetic Energy Energy of an object in motion
Is doing work b/c it is in motion
KE = 1/2mv2
Kinetic Energy = ½ mass x velocity2

22. Kinetic Energy depends on speed more than mass;
As speed increases, KE increases exponentially
2x the mass = 2x the KE
2x the velocity = 4 x the KE
Example: Car crashes are more dangerous at higher speeds b/c the car has a higher KE at higher speeds, thus it can do more work = damage.

23. Example: What is kinetic energy of a 44 kg cheetah running at 31m/s?

24. Other Forms of Energy
Mechanical energy – sum of potential & kinetic energy, amount of work an object can do
Before the apple falls it has PE, just before it it’s the ground it has KE, b/w it has some PE & some KE

25. Forms cont. Chemical energy – energy stored in the bonds between atoms
Energy is stored or released when bonds are broken
Example is cell gets energy from C6H12O6 when the bonds are broken during respiration

26. Forms cont. Nuclear energy – energy released from changing the nucleus
Sun’s energy comes from fusion – putting 2 H together to make He (H + H → He)
E = mc2 (mass converted to energy)

27. Forms cont.
Electrical energy – the energy of charged particles

28. Forms cont.
Light energy – energy that can travel through empty space in electromagnetic waves (blue has more energy than red)

29. Conservation of Energy
Energy can’t be created nor destroyed
The total energy remains constant
Energy only changes forms

30. Example: Rollercoaster
PE = 354 kJ
KE = 0 kJ
V = 0 m/s
KE = 177kJ
PE = 177 kJ
V = 26.2 m/s
Mass = 515 kg
h= 35m
PE = 0 kJ
KE = 354 kJ
V = 37.1 m/s

32. Example 2: Pendulum
All PE
No KE
All PE
No KE
All KE

33. Friction turns motion into heat Electric cords get hot
Potential to kinetic
Pendulum eventually will stop
Energy goes into the air
Friction turns motion into heat
Electric cords get hot

34. Efficiency Not all work is useful work
Always less than 100%
Perpetual motion machines are impossible b/c they would have to put more out than you put in and would require complete absence of friction.