Energy

1. Energy

2. Work • An object that has energy has the ability to produce a change • Work is how we transfer energy,it is equal to the change in energy • In order to do work on an object, you must increase the energy within it • Any type of energy can do work

3. Kinetic Energy • Kinetic Energy (KE) is the energy of motion • More speed means more KE

4. Gravitational Potential Energy • Potential Energy is Stored Energy

5. Heat is energy that is created by friction Heat Energy • This could be from sliding, rubbing or deformation

6. Work

7. Work Examples • Let’s look at the following scenarios: • Work or Not? • Lifting a box above your head • Holding that box there for 2 hours • Sliding a box across a frictionless surface at constant speed work not work not work

8. Energy Equations: Work • Work is the product of the force applied in the directionof motion and the distance it is applied • When the force and the movement are parallel, work is simply F|| Force (F) θ

9. Energy Units • Notice: from the work formula, energy units are a combo of Force (Newtons) and distance (m) or Newton•meters(N•m) • The SI units for energy are Joules (J). • So, one Joule is equal to 1 Newton•meter.

10. Power • In physics, power just means the rate of doing work • So, faster work means more power • The units come out to Joules per second. • We call this a Watt (W)for short

11. Work Example In the 1950s, an experimental train, which had a mass of 2.5 x 104 kg, was forced across a level track by a jet engine that produced a thrust of 5.0 x 105 N for a distance of 509 m. Find the work done on the train. Equation: Given: Unknown:

12. Work Example In the 1950s, an experimental train, which had a mass of 2.5 x 104 kg, was forced across a level track by a jet engine that produced a thrust of 5.0 x 105 N for a distance of 509 m. Find the work done on the train. Equation: Given: Unknown:

13. Time to practice! Turn to pg. 409 Complete #5-11 If you finish early, try #2 on pg 408

14. Kinetic Energy • Energy is the ability to do work • Kinetic Energy (KE) is the energy of motion • More speed means more KE

15. Kinetic Energy

16. Energy Equations: Kinetic E • Let’sthrowa block • Work can transfer energy into the block • Work is done while the block is being accelerated by the hand a distance of d

17. Energy Equations: Kinetic E • So, the work done is: • This time the force is simply ma • Remember that acceleration equation?:

18. Energy Equations: Kinetic E • Let’s substitute: • The normal equation assumes starting from rest (vi = 0):

19. HW Q #1 pg 408 You will need to do some estimating for parts of this problem. I am purposely leaving these a little vague. Specify where you got information that you had to look up or explain how you arrived at estimates for mass and velocity. a. Estimate the Kinetic Energy of a Chihuahua moving as fast as it can.

20. Potential Energy • Potential Energy is storedenergy • Gravitational Potential Energy (GPE) is when energy is stored in an objects position(height) • The higher an object goes, the more GPE • (and the faster the speed it will have when it hits the ground)

21. Elastic Potential Energy (EPE) • The other type of Potential Energy we will look at is Elastic Potential Energy (EPE) • Instead of height, the energy is stored by stretching an object. • More stretching means more EPE • ex. rubber band, spring

22. Potential Energy

23. Energy Equations: GPE • For GPE, we still have force x distance, but this time the force is the objects weight, mg • This gives us the equation: • We use h instead of d since it will always be height for GPE F=mg m

24. Energy Equations: Elastic PE • EPE is trickier than GPE • force changes depending on how much you stretch the object • This force depends on both the distance stretched (x) and a spring constant (k) • This equation is known as Hooke’s Law

25. Energy Equations: Elastic PE • This k comes from how much force is needed to stretch a spring per a certain distance • What is the k for this spring?

26. Energy Equations: Elastic PE • Since the force at the beginning of the stretch is different than the end, we use an average to calculate the EPE: • Since we usually start the stretch from rest:

27. Energy Equations: heat • When pushing a block at constant speed across a surface, the friction force is turned into heat • Since added force is only working against friction (no a), all of the work done on the block is then turned into heat f

28. Energy Equations: heat • Remember that d is only during the friction

29. Proportionality Example By what factor does the Kinetic Energy of a car change if the speed doubles? Given: Unknown:

31. Labette pg 479-481 Everyone should calculate their own personal Power (in other words, everyone should get some exercise) There are 3 stations • Free Weights (Biceps) • Scales & Push ups (Triceps) • Stairs (legs)

32. Labette pg 479-481 You will need: • A group composed of 2-3 people • A stopwatch (use a cellphone) • A meter stick • A pencil & Your lab (duhhh!) When you are done with data collection, start your calculations

33. Conservation of Energy • Energy cannot be created nor destroyed, but only changed from one form to another • What does this mean?

34. Conservation of Energy • All of the energy that you start with… • you end with! • initial energy = final energy • Total energy at top • equals • Total energy at bottom • Total energy anywhere

35. Conservation of Energy • All of the energy that you start with… • you end with! • initial energy = final energy • Total energy at top • equals • Total energy at bottom • Total energy anywhere

36. Conservation of Energy All GPE GPE andKE All KE

37. Conservation of Energy Problems • Identify type of energy at beginning and end • Full law in equation form: • For most problems, many are zero

38. Conservation of Energy: Example • Rolling down a hill from rest • Top (initial): all GPE • Bottom (final): all KE • Left with: • or:

39. Conservation of Energy: Example A bow is used to shoot a .050 kg arrow into the air. If the average force used to draw the bow is 110 N and the bow is drawn 0.50 m, how fast is the arrow moving when it has risen 35 meters above the bow? (Assume air resistance is negligible) Define: initial : and final : What type of energy is it? (work) when bow is drawn (KE & GPE) when arrow is at 35 m

40. Conservation of Energy: Example Write out CoEeqn and cross out missing E’s moving at rest start at h = 0 goes higher finding through work (no k) nothing stretched/pressed no air resistance

41. Conservation of Energy: Example • A bow is used to shoot a .050 kg arrow into the air. If the average force used to draw the bow is 110 N and the bow is drawn 0.50 m, how fast is the arrow moving when it has risen 35 meters above the bow? • (Assume air resistance is negligible) • Givens: 0.050 kg 110N 0.50 m how fast 35 meters above Unknown m = 0.050 kg v = ? (speed in /s) F = 110N d = 0.50 m h = 35 m

42. Conservation of Energy: Example rewrite and expand solve for v

43. Conservation of Energy: Example plug and chug

44. Time to practice in pairs Turn to pg. 495 If you finish early, start pg 496

48. Hopper Popper Lab Strategy • Write down everything you could measure with the resources you haves • Write down your unknowns

49. Brain Break! What’s wrong with Energy in this movie?

50. Discussion Question • A bowling ball attached to a wire is released one inch away from someone’s face. It swings across the room, and then back towards the person. It will… • Gain speed on the way back and hit the person in the face. • Stop one inch from the person’s face. • Lose speed and not make it all the way to the person’s face.