1. Work What kind of work do you do?

2. Work Scientists say work is… –force applied through a distance Think of it this way, –Are you doing homework if you are sitting with your book? –Are you doing homework if you are reading information and writing answers?

3. Work Work is ONLY done when… –an object is moved. Even if I apply force, unless the object moves there is NO work being done.

4. Work- Mathematically In non- parallel systems… –Work= applied force x distance x cosine of  where  is the angle between the force and the motion. –W=Fd cos  In parallel systems… –Work= applied force x distance –W=Fd

5. Work- Mathematically What are the units of work? Force is in Newtons (N) Distance is in meters (m) Work is in joules (J) –How can we show this in basic SI units?

6. Example You push a refrigerator with a horizontal force of 100N. If you move the refrigerator a distance of 5 m while you are pushing, how much work do you do?

7. Your Turn A couch is pushed with a horizontal force of 80 N and moves a distance of 5 m across the floor. How much work is done in moving the couch? The force needed to lift an object is equal in size to the gravitational force on the object. How much work is done in lifting an object that has the mass of 5 kg a vertical distance of 2 m?

8. Ponder Suppose you used a force of 50 N to shoot an arrow. The arrow flew 25 m. As you shot the arrow, the bowstring moved the arrow 1 m. Did you do 1,250 J of work or 50 J of work on the arrow?

9. Machines What is a machine? –Device that changes the force or increases the motion from work What do machines do for you?

10. Types of Machines Simple –Only one movement of the machine Compound –Combination of two or more simple machines

11. Types of Machines Lever Wheel and Axle Pulley Inclined Plane Wedge Screw

12. Types of Gears Gears Bevel Gears Worm Gear

13. Anatomy of a Machine Lab You and your partner will get a machine. Observe how the machine works. Observe how each part moves. Answer the following questions: –What is the function of this machine? –How many moving parts does it have? –How are the moving parts connected to each other? –What does each moving part do in the machine? –Which parts are elements of machines? Place the machine between each of you.

14. Anatomy of a Machine Sketch your machine from your point of view. –No touching the machine. Once both people are done sketching, switch places and draw the machine from another angle. After you have two sketches… –add arrows and written notes to indicate directions of motion for each part –label the elements of machines involved –explain connections

16. Efficiency Machines can increase… –Force –Speed Force and speed cannot increase at the same time.

17. Efficiency What is efficiency? –The work we get out divided by the work we put in. We can calculate this

18. Example You do 20 J of work in pushing a crate up a ramp. If the output work from the incline plane is 11 J, then what was the efficiency of the inclined plane?

19. Your turn Find the efficiency of a machine that does 800 J of work if the input work is 2000 J. Workers do 8000 J of work on a 2000 N crate to push it up a ramp. If the ramp is 2 m high, then what is the efficiency of the ramp?

20. Machines What can machines do for you? –Think about our simple machines. –Increase speed –Change direction of force –Increase force Machines give us a mechanical advantage.

21. Mechanical Advantage What do you think mechanical advantage is? Break it down!

22. Mechanical Advantage Mechanical advantage- –Ratio of the output force to input force

23. Mechanical Advantage and Efficiency Lab Objectives: –Model lifting devices based on an inclined plane. –Calculate the work needed to lift a weight straight up. –Calculate the work needed to pull a weight up an inclined plane. –Calculate mechanical advantage and efficiency for an inclined plane.

24. Mechanical Advantage and Efficiency Lab Materials –Wooden board –Textbooks –0.2 kg mass –Spring scale –Meter stick

25. Mechanical Advantage and Efficiency Lab Set up your inclined plane. It should be 40 cm long and about 10 cm high (measure exact height). Using the spring scale, find how much force is needed to lift the 200 g mass straight up. This is the output force. Calculate the work needed to lift the mass the height of the stacked textbooks. This is the inclined plane’s output work. Using the spring scale, find how much force is needed to pull the 200 g mass up the inclined plane. This is the input force. Calculate the work done on the mass as it is pulled up the inclined plane. This is the input work.

26. Mechanical Advantage and Efficiency Lab Results: –Calculate the mechanical advantage of the inclined plane. –Calculate the efficiency of the inclined plane. Discussion: –Explain how you might improve the efficiency of your inclined plane. –Predict how you might increase the mechanical advantage of your inclined plane. –Identify situations in which an inclined plane would be useful.