1. Chapter 13 WORK & ENERGY

2. TN Standards CLE 3202.4.3 – Demonstrate the relationship among work, power, and machines CLE 3202.2.6 – 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/s 2 )

5. 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 Work, Power, and Machines

6. 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. 1 st class ( most common ) – hammer 2 nd class – wheelbarrow, nutcrackers, hinged doors 3 rd class – human forearm

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

10. 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 % ) 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

11. Mechanical Advantage / Efficiency

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

13. 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

14. 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)mv 2 = (1/2)(1500 kg)(20 m/s)^2 KE = 300,000 J

15. 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 ) mv 2 – m is mass ; v is velocity

16. 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 )

17. 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 )

18. Roller Coaster Example

19. 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