Work, Power, ENERGY

1. Work, Power, ENERGY

2. QOTD: Write a list of 10 examples your idea of work. Demo : create a work equation using a spring scale, string and a book.

3. Work is done only when a force moves an object • A force acting on an object and causing it to move a distance is work • Not every force is work..if you push against the wall it does not move..that is not work! • Work = force X distance • Work is measured in Joules • If you pick up a bag of groceries and walk across the room the work is picking up the groceries not the walking. What is work?

4. The object must move some distance as a result of your force • The force you exert must be in the same direction as the objects motion. • Ie: the groceries • You walk

5. What’s work? • A scientist delivers a speech to an audience of his peers. • A body builder lifts 350 pounds above his head. • A mother carries her baby from room to room. • A father pushes a baby in a carriage. • A woman carries a 20 kg grocery bag to her car?

6. What’s work? • A scientist delivers a speech to an audience of his peers. No • A body builder lifts 350 pounds above his head. Yes • A mother carries her baby from room to room. No • A father pushes a baby in a carriage. Yes • A woman carries a 20 kg grocery bag to her car? No

7. Work can be determined by calculating • Force used x distance moved = amount of work • Therefore what is the formula for work?

8. Work = force x distance  Joule – is the SI unit for work. Newton = force Meters = distance Therefore if you exert: 1 Newton of force for 1 meter of distance = 1 joule of work or 1N/m

9. Work is done when a force is exerted through a distance. A student lifts a bag of books that weighs 135 N. If the bag is lifted .75 m, how much work does the student do? F = 135 N d = .75 m W = Fd W = (135 N)( .75 m) W = 101.25 J

11. Negative Work If the object moves in the direction opposite the direction of the force (for instance if force and displacement are in opposite directions) then the work is negative: W < 0. The energy of the system is reduced.

13. A +24 N force is applied to an object that moves 10 m in the same direction during the time that the force is applied. How much work is done to the object?

14. http://www2.franciscan.edu/academic/mathsci/mathscienceintegation/MathScienceIntegation-1011.htm#item1015http://www2.franciscan.edu/academic/mathsci/mathscienceintegation/MathScienceIntegation-1011.htm#item1015

15. Find the equation for POWER 1. Attach a string and spring scale to a large book. 2. Pull the book .5m slowly. Use a stopwatch to determine time . 3. Record the time and distance on a data table. 4. Repeat 1- 3 – but this time pull the book faster. 5. Repeat 1-3 even faster. Force-N distance -m time- s

16. Power tells you how fast something is happening..how fast the work is being done • Power = work/time or Power = Force X Distance Time • Power is measured in watts (W) • One watt is equal to 1 joule per second of work divide joules/seconds Power

17. Power – the rate at which energy is transferred. P = W P = power Watts t W = work Joules t = time seconds 1 Watt (W) = 1 J/s

18. How much power must a motor have to operate a pump that raises 1500 kg of water every minute a distance of 12 m?

19. 1. m = 1500 kg t = 60 s d = 12 m  2. Equations- P = W/t W = Fd F = mg  3. Plug and chug- F = mg = (1500 kg)(9.8 m/s2) = 14,700 N W = Fd = (14,700 N)(12 m) = 1.76 x 105 J P = W/t = (1.76 x 105 J)/(60 s) = 2940 W

20. http://www.physicsclassroom.com

21. What is Energy? It turns out that energy is so fundamental, like space and time, that there is no good answer to this question. However, just like space and time, that doesn't stop us from doing very useful calculations with Energy We may not be able to define energy, but because it is a conserved property of nature, it's a very useful idea.

22. Potential Energy Potential Energy (PE):   Stored energy due to position Examples: rock on a cliff, battery, food, gasoline, stretched rubber band, apple hanging in a tree

23. Gravitational Potential Energy A barbell of mass "m" is lifted vertically upwards a distance 
"h" by an outside force. How much work does that outside 
force do on the barbell? Fapp W = Fdparallel  Since a = 0, Fapp = mg W = (mg) dparallel  Since F and d are in the same   direction ...and dparallel = h W = (mg) h W = mgh mg

24. Gravitational Potential Energy But we know that in general, Eo + W = Ef. If our barbell had no energy to 
begin with, Eo = 0, then W = Ef But we just showed that we did W=mgh to lift the barbell... so mgh=Ef The energy of a mass is increased by an amount mgh when it is raised by a height "h".

25. Gravitational Potential Energy The name for this form of energy is Gravitational Potential Energy (GPE). GPE = mgh One important thing to note is that while changes in gravitational 
potential energy are important, their absolute value is not.

26. Gravitational Potential Energy You can define any height to 
be the zero for height...and 
therefore the zero for GPE. But whichever height you 
choose to call zero, changes 
in heights will result in 
changes of GPE. For 
example, the floor level can 
be considered zero energy or 
the ladder level can be zero. 0.5 m 0 m 0.5 m 0 m

27. Gravitational PE Gravitational PE (GPE): Energy stored by objects that are above the earth’s surface (objects that can fall) Depends on mass, acceleration and height GPE increases with height

28. Gravitational PE GPE = mass  gravity  height GPE = m g h = weight  height   GPE = m (kg)  9.8 m/s2 h (m) j = 1 Nm

29. 9 What is the change of GPE for a 5.0 kg object 
which is raised from the floor to a final height of 
2.0m above the floor? GPE=mgh GPE= (5kg)(9.8)(2m) GPE=98 J answer

30. 10 As an object falls, its GPE always _____. A increases B decreases C stays the same B answer

31. 11 What is the change of GPE for a 8.0 kg object which 
is lowered from an initial height of 2.0 m above the 
floor to a final height of 1.5m above the floor? GPE=mgh GPE= (8)(9.8)(-0.5) GPE= -39.2 J answer

32. 12 What is the change in height of a 2.0 kg object 
which gained 16 J of GPE? GPE=mgh h = GPE/mg h = 16/(2)(9.8) h = 0.82m answer

33. Kinetic Energy Kinetic Energy (KE):   Energy in the form of motion Depends on mass and velocity of moving object.   Object in motion has ability to do work http://www.youtube.com/watch?feature=player_detailpage&v=0ASLLiuejAo

34. Kinetic Energy The energy an object has by 
virtue of its motion is called its 
kinetic energy. The symbol we will 
be using for kinetic energy is KE. Like all forms of energy, it is 
measured in Joules (J). The amount of KE an object has is given by: KE = 1/2 mv2

35. Kinetic Energy KE = ½ mass  velocity2 KE = m  V2 2 (j) = (kg)  (m/s)  1 j = 1 kg m/s

36. 13 As an object falls, its KE always _____. A decreases B increases C stays the same. B answer

38. 14 A ball falls from the top of a building to the ground 
below. How does the kinetic energy (KE) compare to the potential energy (PE) at the top of the building? A KE = PE B KE > PE C KE < PE D It is impossible to tell. C answer

39. 15 What is the kinetic energy of a 12 kg object 
with a velocity of 10 m/s? KE = 0.5 mv2 KE = (0.5)(12)(100) KE = 600 J answer

40. 16 What is the mass of an object which has 2400 J 
of KE when traveling at 6.0 m/s? KE = 0.5 mv2 m = KE / (0.5)(v2) m = 2400 / (0.5)(62) m = 133.33kg answer

41. 17 A 3 kg object has 45 J of kinetic energy. What is 
its velocity? KE = 0.5 mv2 v2 = KE/0.5m v2 = 45 / 0.5 (3) v = 5.48 m/s answer

42. 18 If the speed of a car is doubled, the KE of the car is: A quadrupled B quartered C halved D doubled A answer

43. 19 Which graph best represents the relationship between the KE and the velocity of an object accelerating in a straight line? C A KE KE B D v answer D v KE KE v v

44. 20 The data table below lists mass and speed for 4 objects. Which 2 have the same KE? A A and D B B and D C A and C D B and C D answer

45. Elastic Potential Energy Energy can be stored in a spring, this 
energy is called Elastic Potential Energy. Robert Hooke first observed the 
relationship between the force necessary 
to compress a spring and how much the 
spring was compressed.

46. Hooke's Law Fspring = -kx k represents the spring constant and is measured in N/m. x represents how much the spring is compressed and is 
measured as you would expect, in meters. The - sign tells us that this is a restorative force. (if you let the spring go once it is compressed, it will go back to its original position)

47. Elastic Potential Energy The work needed to compress a spring is equal to the area 
under its force vs. distance curve. Area of a triangle = 1/2 b h (F= -kx) W = 1/2 (x)(F) W = 1/2 (x)(kx) W = 1/2kx2

48. Elastic Potential Energy The energy imparted to the spring by this work must be stored 
in the Elastic Potential Energy (EPE) of the spring: EPE = 1/2 k x2 Like all forms of energy, it is measured in Joules (J).

49. 21 Determine the elastic potential energy stored in a 
spring whose spring constant is 250 N/m and 
which is compressed 8 cm. EPE = 0.5 kx2 EPE = 0.5 (250)(0.082) EPE = 0.8 J answer

50. 22 What is the spring constant of a spring that is 
compressed 5 cm and has 0.65 J of elastic 
potential energy stored in it? EPE = 0.5 kx2 k = EPE/0.5x2 k = 0.65 / 0.5 (0.052) k = 520 N/m answer