3. Warm up – 1. Sogand 2. Kevin A 3. Nadya 4. Courtney 5. Ilian 6. Kevin C 7. Jack 8. Dylan 9. Alexa 10. Taylor 11. Mark 12. Kylie Find your assigned seat 13. Catherine 14. Nick 15. Garrett 16. Alex 17. Megha 18. Justyna 19. Christopher 20. Danielle 21. Persia 22. Regan 23. Michael 24. Connor 25. Michelle 26. Jenny 27. Katie 28. Brianna 29. Dalton 30. Logan 31. Julia

4. New Unit – Work, Power & Energy These words mean very specific things in physics class. They have different definitions than when we use them in everyday language.

5. energy Discussion question: What types of energy have you learned about/heard of before?

6. Work Every day definition Physics definition A force does work on an object when the object undergoes a displacement. Forces can do positive or negative work on an object. Not all of the forces on the object do work. Beginning of the notes:

7. Energy Every day definition Physics definition/usage In this class we only have equations for: Kinetic Energy Gravitational potential Elastic potential

8. Power Every day definition Physics definition The rate at which work is done or the rate at which energy is expended.

9. Work in Physics Work is the result of applying a force to displace an object Work = Force ● Displacement W = F ● d

10. Units of Work W = F ● d From the equation, work is measured in Force units times Distance units. 1 N ● 1 m = 1 Joule The name was chosen to honor James Joule for his work in Physics.

11. Displacement Can Be Horizontal Work is done to overcome the force of friction caused by gravity making contact between the object and the surface. Direction of Displacement F r

12. Work is done by exerting a force equal to the weight of an object that lifts the object. F Displacement Can Be Vertical r

13. Positive Work FaFaFaFa Positive or Negative Work? Force and displacement in the same direction: Energy Added To System FfFfFfFf

14. Negative Work FfFfFfFf Positive or Negative Work? Energy Removed From System Force and displacement in the opposite direction:

15. FgFgFgFg F1F1F1F1 Forces do work? 1. If the box is moving horizontally? 2. If the box is moving vertically? F2F2F2F2 F3F3F3F3

16. Fg = 100 N F a = 20 N Sample Problems How much work is done to drag this box 4.0 m across a floor? W = F r = 20 N x 4.0 m = 80 N m = 80 J

17. F = 20 N r = 4 m W = 20 N x 4 m x cos 35 o = 65.5 J 35 o 35 o W net = F net r cos  Work Done at an Angle

18. Energy that is stored by the force of gravity or by the force of elastic energy Gravitational U g = mgh Elastic U s = ½kx 2 Where k is the spring constant and x is the amount of compression/elongation Potential Energy

19. An increase in elevation means an increase in potential energy. A 5,000 kg boulder atop a 5 m cliff has: U = 5,000 kg x 9.8 m/s2 x 5 m = 245,000 J Potential Energy

20. Energy that is the result of an object (mass) in motion (velocity). K of a 1200 kg car going 20 m/s K = ½ m v 2 = ½ 1200 kg (20 m/s) 2 K = 240,000 J K = ½ m v 2 Kinetic Energy

21. Energy is always conserved, but mechanical energy is not always conserved. If no energy is lost (to work done by friction) you can set the initial energy equal to the final energy. E i =E f Conservation of Energy

22. The potential energy at the top of a ski hill is converted to an equal amount of kinetic energy at the bottom of the hill. Conservation of Energy

23. Total Mechanical Energy E total = U + K E total = U g + U s + K http://www.youtube.com/watch?v=mhIOylZMg6Q

24. Conservation Example 1: If a 5 kg ball is dropped from a height of 30 m. How fast is it going when it hits the ground? (energy equations make this problem easier than when we used kinematics)

25. Conservation Example 2: If a 5 kg ball is dropped from a height of 30 m. How fast is it going when it is at a height of 10 m?

26. From the numbers and diagram below, determine the mass of the skier Conservation Example 3:

27. Conservation Example 4: A 5.0 kg box is pressed up against a spring (k=200 N/m) to compress it 10 cm. Once it is let go, on a frictionless surface, how fast does it slide when it leaves the spring?

28. The potential energy of a pendulum at the end of one swing is converted to an equal amount of kinetic energy at the bottom of the swing. PendulumsPendulums http://phet.colorado.edu/sims/pendulum-lab/pendulum-lab_en.html

29. The potential energy at the top of a roller coaster lift hill is converted to an equal amount of kinetic energy at the bottom. Conservation of Energy

30. The rate at which work is done is called power. A unit of power is a unit of work divided by a unit of time. P = W / t The unit of power is a J/s or a Watt. Power Power

31. Another way of defining power is to break down work into force units. P = F r / t P = F r / t But (r / t) is velocity so… (when velocity is constant) P = F v PowerPower

33. Kinetic Energy Kinetic energy is the energy of motion. What is the unit of energy? The kinetic energy of an object is equal to: K = ½ mv 2 K = kinetic energy, mass = m and v = speed Note the units!kg m 2 /s 2 = Nm = J

34. KE Problem #1 A 7.00 kg bowling ball moves at 3.00 m/s. How fast must a 2.45 g table-tennis ball move in order to have the same Kinetic Energy as the bowling ball?

35. Work - Kinetic Energy Theorem The net work done on an object is equal to its change in kinetic energy. W = ΔKE W = KE f – KE i W = ½ mv f 2 – ½ mv i 2 This is true if the only change on the object as a result of the work done on it is its speed.

36. Work - Kinetic Energy Problem #1 A 6.00 kg block is initially at rest. Then, it is pulled along a frictionless surface by a rope with a constant tension of 12.0 N. Find the speed of the block after it has moved 3.00 m.

37. Work - Kinetic Energy Problem #2 A 6.00 kg block is initially at rest. Then, it is pulled by a rope with constant tension of 12.0 N. The coefficient of kinetic friction, μ k, between the block and the surface it slides upon is 0.150. Find the speed of the block after it has moved 3.00 m. Hint: The work done on the block is reduced by F fk d. Recall that F fk = μ k F N.

38. a)How fast is it going as it hits the floor? b)What is the ball’s kinetic energy at this point? If a 10 kg ball falls from rest from 3.0 m. If the ball was 100 kg, how would this affect the kinetic energy & speed?

39. If a 0.5 kg soccer ball has 20.0 J of kinetic energy, how fast is it moving?

40. A 12 kg cannonball is launched straight up at 15 m/s. Neglecting air resistance, what is it’s maximum height? a)What type of energy does it have right after it leaves the cannon? b)What type of energy does it have at it’s max peak? c)What is it’s max peak? (neglecting air resistance)

41. Daniel wants to throw a ball so that it hits a Frisbee that is stuck 8.0 m up in a tree. How fast does Daniel need to throw the ball so that it reaches the Frisbee?

42. If Rowan throws a ball down at 1.5 m/s, how much faster is it going after falling 3.5 meters? What is it’s total speed?

43. Homework HW set has 1-8 Quiz 1 will be on #1-4 (next class) Quiz 2 will be on #5-8 (2 classes from now)