CONSERVATION OF ENERGY Sec 3 Express, (Pure) Physics Work, Energy, Power (Part 2)

1. CONSERVATION OF ENERGYSec 3 Express, (Pure) PhysicsWork, Energy, Power (Part 2) Prepared by Goh Han Pin

2. Recall … We have learnt in the previous lesson … • KE = • PE = mgh • work done = force × distance moved in the direction of the force

3. Lesson Overview … Today, we will learn about … • Conservation of Energy • Case studies • Ball: up and down • Inclined plane • Pendulum

4. DEMONSTRATION

5. Spiderman knows his physics too!

6. What’s conservation of energy? Andy: • “Energy is always conserved? But when a car brakes to a stop, how come the initial kinetic energy of the car is not equal to the final kinetic energy?” Beng Huat: • “Mother says must conserve energy by turning off the lights when they are not in use. So I do the same.”

7. What’s conservation of energy? • Is energy “destroyed” during energy conversions? • Does the word “conserve” have its usual meaning of “trying not to waste something” in physics?

8. Energy vs. Money • There are many different forms of $. • Money in the bank = Potential spending money • Cash in your pocket = Spending Money

9. Energy vs. Money • $ can be owned by different people. • Money can be owned by you, your friends, your parents, the shop that you patronise, …… • $ can be converted from one form to another, or transferred from one person to another. • Withdrawing from the ATM: • Buying a plate of fried rice from the canteen

10. Conservation of Money • The total amount of $, after adding all the different forms owned by everybody, is always constant. • Withdrawing from the ATM: • This doesn’t change the amount of money you have. • Buying a plate of fried rice from the canteen: • You own less money, the hawker has more $, but the sum of $ owned by both of you remains the same.

11. An example of COE

12. Comparing Energy with Money

13. Comparing Energy with Money

14. Principle of Conservation of Energy • Energy can neither be created nor destroyed in any process. • It can be converted from one form to another or transferred from one body to another, but the totalamount remains constant.

15. Do you believe in COE? • Suppose you had a total of $100 in cash and in the bank. • One day, you counted your money and, despite neither earning nor spending anything, you were $50 short! • Would you assume that the money just disappeared?

16. Yes, the COE is true. • Scientists have concluded that conservation of energy is a description of nature. • They have discovered forms into which energy can be converted, or places where the energy has been transferred to.

17. Homework … For the first question in the homework, • Re-examine the statements made by Andy and Beng Huat • Explain how energy is conserved in their scenarios.

18. Case Study 1: Ball up and down … Examine • Height • PE • Speed • KE • Mechanical Energy (PE + KE) Complete Pg. 4 of Handout 1 5 stages of motion • Ball just leaves your hands • Ball moving upwards • At the highest point • Ball moving downwards • Ball returns to your hands

19. On the way up, KE is converted into PE. • KE decreases and PE increases. • On the way down, PE is converted into KE. • PE decreases and KE increases. • Without air resistance, mechanical energy (PE + KE) is the same throughout the motion.

20. Note, however, that PE does not immediately become all KE, vice versa.

21. Consider the example of a skateboarder on the right. • Does all the initial PE immediately becomes all KE? • What is his mechanical energy? • Is it constant throughout the motion?

22. Computer Animation http://physics.weber.edu/amiri/director/dcrfiles/energy/FallingBallS.dcr (2 kg, 45 m) Observe the changing energy bars …

23. Complete the table

24. Complete the table: Check answers

25. Decrease in PE is equal to the increase in KE • Mechanical energy, the sum of PE and KE, remains constant.

26. END OF MICROTEACHING SEGMENT

27. Group Activity 1 • Groups of 3 • Specific roles • Taskmaster • Secretary • Presenter • Complete Worksheet 1 in 10 minutes • To be followed by Group presentation • Be prepared for Q&A

28. What happens if mass is doubled? • 2 kg ball, released 5 m high • If mass is doubled to 4 kg, what happens to speed of ball? • Let’s vote! • http://jersey.uoregon.edu/vlab/PotentialEnergy/index.html

30. KE depends on mass too!

31. What if air resistance is present? • Suppose the mass of the ball remains at 2 kg, and it is released from rest at the same height. However, air resistance is not negligible. • What would be the speed of the ball just before it hits the floor?

32. Lets shade some energy bars!

33. Effect of air resistance If air resistance is not negligible, • mechanical energy (KE + PE) of the ball decreases with time, • mechanical energy will be converted into other forms of energy such as heat (internal energy), sound, etc, • but, the total energy of the ball is still conserved.

34. Group Activity 2 and 3 • Groups of 3 • Specific roles • Taskmaster • Secretary • Presenter • Complete Worksheet 2 or 3 in 15 minutes • Group presentation at next lesson • Be prepared for Q&A

35. Its QUIZ time!

36. Question 1 A ball is thrown up from the bottom with an initial KE of 2 500 J in the absence of air resistance. What is its PE at the top? A. 0 J B. 1 250 J C. 2 500 J D. 5 000 J [ ]

37. Question 2 Consider a ball thrown up from the bottom in the absence of air resistance. At the instant when the ball is at half of the maximum height reached, its PE is 1 200 J. What is the mechanical energy of the ball at this instant? A. 0 J B. 600 J C. 1 200 J D. 2 400 J [ ]

38. Question 3 A ball of mass m is dropped from rest. It reaches the floor 10 m below with a final speed of v. Another ball of mass is also dropped from rest from the same position. What is its speed when it reaches the floor? Assume that air resistance is negligible. A. v / 4 B. v / 2 C. v D. 2 v E. 4 v [ ]

39. Question 4 A ball of mass m is dropped from rest. In the absence of air resistance, it will reach the floor 10 m below with a final speed of v. Suppose air resistance is actually not negligible. Deduce the final speed of the ball in this case. A. Slower than v B. Same final speed, v C. Faster than v [ ]

40. Reflections • Write down 3 things that you intend to remember. • Write down 2 things that you found interesting in today’s lesson. • Write down 1 thing that you didn’t quite understand today.