1. Chapter 13
WORK & ENERGY

2. TN Standards
CLE – Demonstrate the relationship among work, power, and machines
CLE – Investigate the Law of Conservation of Energy

3. Bellwork
What is Work?

4. Work, Power, and Machines
Work is conducted only when an applied ( net ) force results in the change in position of an object
Work is measured in Joules ( J )
1 Joule = 1 N x 1m
Work is calculated by multiplying force and distance ( over which the force is applied )
W = F x d ( if given mass of an object – find weight by multiplying by gravity which is 9.81 m/s2 )

5. Work, Power, and Machines
Power is the amount of work conducted over a certain time interval
The rate at which work is conducted
P = W / Δt
Power is measured in Watts

6. Work, Power, and Machines
Machines help to do work by changing the size of an input force, the direction of a force, or both
Different forces can do the same amount of work
Figure 3 ( a box lifted vs a box pushed up a ramp )

7. Simple Machines The six types of simple machines are: Simple lever
Wheel and axle
Pulley
Simple inclined plane
Wedge
Screw

8. Classes of Machines What are the two families of simple machines?
Inclined plane and lever
Levers are divided into 3 classes based on the location of the fulcrum and of the input and output forces.
1st class ( most common ) – hammer
2nd class – wheelbarrow, nutcrackers, hinged doors
3rd class – human forearm

9. Compound Machines A combination of any of those six simple machines
Example – SCISSORS
Combination of lever and wedge

10. Bellwork – 10/25/16
How efficient is a machine that uses 150 J to lift a 60 N load 2 m?

11. Bellwork – 11/17/15
How efficient is a machine that uses 150 J to lift a 60 N load 2 m?
Input = 150 J ; Output = 60 N x 2 m = 120 J
Efficiency = out/in = 120 J / 150 J = 0.8
For percentage, multiply by 100 ( 80 % )

12. ‘Nuther Efficiency Example
What is the work output by an 85% efficient wheel and axle when 65 J of work is input?

13. Nuther Efficiency Example
What is the work output by an 85% efficient wheel and axle when 65 J of work is input?
0.85 = out / 65 J  output = 55.3 J

14. Mechanical Advantage / Efficiency

15. Bellwork – 11/11/15
What is Energy?

16. What is Energy?
Energy is a property of an object due to its motion or its position
When work is done, energy is transferred or transformed from one system to another
Carry a tennis ball up in the stadium
You add potential energy to it by carrying it up

17. Bellwork – 11/11/15
An 1500 kg car is driving at 20 m/s. What kind of energy does it have? How much of this energy is there?
Kinetic
KE = (½)mv2 = (1/2)(1500 kg)(20 m/s)^2
KE = 300,000 J

18. What is Energy? Energy is measured also in Joules ( J )
Potential energy – energy of position
PE = hmg
h is height ; g is gravity; m is mass
Height is relative
Kinetic Energy – energy of motion
KE = ( ½ ) mv2
m is mass ; v is velocity

19. Other Forms of Energy Mechanical energy ( sum of
Potential and Kinetic )
Non-mechanical energy
( atomic level )
Chemical reactions involves chemical energy ( a form of potential energy )
Sun gets energy from nuclear reactions
Electrical energy ( stored in electric fields )

20. Conservation of Energy
Energy easily changes from one form to another ( battery )
PE can become KE ( tennis ball dropped from the top of stadium )
Vice-versa ( projectile shot / reaches max height )
Law of Conservation of Energy?
( neither created nor destroyed )

21. Roller Coaster Example

22. Thermodynamics
For any system, the net change in energy equals the energy transferred as work and heat
Efficiency ( how much energy/work you get out of a machine or process compared to how much is put in )
Internal Combustion Engine