Exploring Personal Horsepower and Mechanical Work in Physics

1. class notes; 4.25.11.Personal Horsepower Lab Quiz tomorrowHW 195 = 100%Twin Tower Video Sheet 30 =100% Personal Horsepower Lab Data Lab Due Wednesday Thursday Twins Physics Day Early bus stuff Dress for the weather Leave at 8:30 http://www.ftexploring.com/energy/energy-1.htm

2. Mechanical Work How muchmechanical work is done by the person pictured If he carries the load one mile over level ground at a constant speed.

3. Mechanical Work • NO MECHANICAL work • Only bio-chemical work • MECHANICAL WORK affects motion or height

4. Work = F * d

6. Repeat timing going UP the stairs 3 times at different speeds

7. Use the Stairs in the FRONT HALL COUNT THE STAIRS!! Record : the # of stairs above the data table and the timeson the data and the table

8. Find the total height of the stairs Count the risers to find the number of steps

9. Record the # of stairs Record the times Measure the times and count the stairs. Data and analysis: Height of one step: ______ m Number of steps: ______steps

10. Multiply the height of one step by the # of steps. Data and analysis: Height of one step: ______ m Number of steps: ______steps

11. -W = Fg = m*ag Data and analysis: Height of one step: ______ m Number of steps: ______steps

13. Calculations:

14. Watts and Horsepower • James Watt patented the steam engine in 1769. • To sell it, he needed to tell people how many horses it would replace. • He measured how quickly farm horses could do work. • There are few horses that actually produce exactly one horsepower of power.

15. POWER

16. Assignment. Each student makes two trips at different speeds. The 3rd line can be another student’s data. In class: Go to the center stairs and Time ourselves doing all the way up the stairs at three different speeds Count the Number of stairs Back in the room: Calculations Use the information on the lab sheet Answer the questions Write the conclusion Report is due: Monday. . . .

17. Measurement of Horsepower • The maximum horsepower developed by a human being over a few seconds time can be measured by timing a volunteer running up the stairs in the lecture hall. • If a person of weight W runs up height h in time t, then h.p. = Wh/t X 1/550 ft-lbs/sec. • A person in good shape can develop one to two horsepower. It will be entertaining to the students if the professor tries it too. • Should the person be allowed a running start? http://www.physics.ucla.edu/demoweb/demomanual/mechanics/energy/faith_in_physics_pendulum.html http://www.physics.ucla.edu/demoweb/demomanual/mechanics/uniform_circular_motion/index.html

18. The amount of work done is dependent height

19. Horizontal displacement does not affect the gravitational PE • Knowing that the potential energy at the top of the tall pillar is 30 J, what is the potential energy at the other positions shown on the hill and the stairs.

20. Use: To convert from Newtons to kg or from kg to Newtons

22. Total mechanical energy • As discussed earlier, there are two forms of PE discussed in our course - gravitational potential energy and elastic potential energy. Given this fact, the above equation can be rewritten: • TME = PEgrav + PEspring + KE

23. Total mechanical energy is constantconservative force  gravity transfers PE-KE • The diagram below depicts the motion of Li Ping Phar (esteemed Chinese ski jumper) as she glides down the hill and makes one of her record-setting jumps.

24. Height at A = 60m The car's mass is 500kg. • A roller coaster with two loops and a small hill, see diagram below • In the diagram A is the highest point of the coaster, B is 3/4 height of A, C is 1/2 of A, D is 1/4 of A, E is the ground level, and F is 1/8 of A.

25. HA = 60m m =500kg PE = mgh Speed use KE = ½ m v2 KE = TME (previous) – PE A 60 (500(9.8)(60) = 294,000-294,000= 294,000 J 0 Joules 294,000J 0 m/s B 45 C 30 D 15 E 0 F 7.5 PE = mgh KE = ½ m v2

26. HA = 60m m =500kg PE = mgh Speed use KE = ½ m v2 KE = TME (previous) – PE A 60 (500(9.8)(60) = 294,000-294,000= 294,000 J 0 Joules 294,000J 0 m/s B 45 (500(9.8)(45) = 294,000-220,500 = 294,000J 220,500 J 73,500J 17.1 m/s Equation: KE = ½ m v2 Substitute: 73,500J = ½ 500kg v2 X 2 …/ by 500…take √ .. v= 17.1 m/s

27. Work and EnergyHow High Will It Go?The motion of the sled in the animation below is similar to the motion of a roller coaster car on roller coaster track. • As on a roller coaster, energy is transformed from potential energy to kinetic energy and vice versa. Provided that external forces (such as friction forces and applied forces) do not do work, the total amount of mechanical energy will be held constant.

28. Energy Conservation on an Incline • If air resistance is neglected, then it would be expected that the total mechanical energy of the cart would be conserved. The animation below depicts this phenomenon (in the absence of air resistance).

29. TotalMechanical energystays the same until it hits the water.

30. Mechanical Energy Equations Page 7 section #3

31. W1Force of Gravity pulls down Mechanical Work  PE  KE TME does not change W1 W4 W2 W4

32. The transfers of energy during the 1st Bounce W1 W4 W2 W3

33. W2Force of Gravity pulls down Mechanical Work  PE  KE TME does not change W1 W4 W2 W3

34. W3ball compressed Mechanical Energy lost to HEAT TME does change W1 W2 W4 W3

35. W4Force of Gravity pulls down Mechanical Work  KE  PE TME does not change W1 W4 W2 W3

36. Notice the speed change

38. Missing mechanical energy?? Energyinitial – Energyfinal = Energylost

39. Frictional Work • According to the Cedar Point website the maximum speed of the Magnum XL-200 is 72 mph not 76 mph as we calculated above. • The difference is due to frictional forces acting on the roller coaster cars. • Assuming that the mass of a loaded roller coaster car is 600 kg what is the frictional (non-conservative) work done on the car by the track?

40. Analyze the transfers of energy during the 1st Bounce

41. Work on incline • Answer the following about the above picture: • Draw the three forces acting on the object. • If the object slides down the incline, what work was done with gravity? • What work is done against the motion? • What is the net work done? • Predict the final velocity of the object.

42. Mechanical Energy

46. Follow the bouncing ball

48. Bouncing balls • When a ball is dropped, it transfers its GPE to kinetic energy.  As the ball hits the floor, its kinetic energy is turned into elastic potential energy (and some heat, and noise).  High speed photography can show how the ball gets deformed. • The elastic potential energy is transferred to kinetic energy as the ball bounces.  Some energy is lost as heat as the ball bounces, so it does not achieve the height from which it was dropped.

49. different types of balls at room temperature and when they are frozen. • When a ball is dropped on a surface, molecules in the ball can deform or absorb the kinetic energy of the fall. If they return to their original shape they push the ball away from the surface. If the energy is absorbed, the ball does not bounce.