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Energy , Work & Power

Energy , Work & Power. 1. How does our body get energy?. YUMMY!!!. From Food. 2. Where do cars get energy?. From Petrol. Txbk pg 5. What is Energy???. Definition: Energy is the capacity to do work. Without Energy = no light no electricity no water from tap

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Energy , Work & Power

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  1. Energy,Work & Power

  2. 1. How does our body get energy? YUMMY!!! From Food. 2. Where do cars get energy? From Petrol

  3. Txbk pg 5 What is Energy??? Definition: Energy is the capacity to do work. Without Energy = no light no electricity no water from tap no bus, no train!!! Without Energy = CLASS WILL BE VERY QUIET!!!!!!  can’t talk, can’t walk, can’t breath!!! Without energy, no living, non-living things would work. SO!!!!!!!!!!! Without energy: Everything dies  no life.   

  4. Any body or system that can do work possess Energy. Energy D = distance moved in the same direction as force applied Defined as: capacity to do WORK F x D 1 Joule = 1 Newton meter 1 J = 1 Nm

  5. Quiz • Which of the following are forms of energy? • Sound • Nuclear • Elastic Potential • Chemical Potential • Joules Is Energy MATTER? NO! Energy does not occupy space and has no mass.

  6. Forms of Energy kinetic Energies in Action Potential (stored) thermal ENERGY Gravitational light Elastic sound chemical nuclear electrical

  7. Kinetic Energy Energy a body possess due to its motion: Ek = KE = mv2 Ek = KE (J) m = mass (kg) v = speed of the body (ms-1)

  8. 30-second Quiz 1 Usain Boltholds the Olympic record of 9.69s for his 100-m race. Assuming his mass is 70kg. What is the kinetic energy KE that he possess? Velocity= = 10.32 m/s KE = ½ mv2 = ½ (70) (10.32)2 = 3727 J = 3730 J (3 s.f.)

  9. 30-second Quiz 2 A car with mass of 2000 kg is travelling with a speed of 5 km/h on PIE in a jam. What is its kinetic energy KE? Velocity= 5 km/h = = 1.389 m/s KE = ½ mv2 = ½ (2000) (1.389)2 = 1929 J = 1930 J (3 s.f.)

  10. Gravitational Potential Energy Energy a body has due to its position. Ep = PE = mgh Ep = GPE (J) m = mass (kg) g = gravitational field strength (N/m) h = height (m)

  11. 5 m 20kg 30- second Quiz 3 • A box of mass 20 kg is being pushed up a slope of 15m long with constant speed of 30 m/s as shown in Figure. • What is the gain in gravitational potential energy? m = 20 g = 10 N/kg h = 5m 15 m PE = mgh = 20(10)(5) = 1000J

  12. Other Types of Energy • Substances that can be burnt contain chemical potential energy. • Thermal energyof an object = total kinetic energy of the atoms or molecules in the object. • *** Heat is the transfer ofthermal energy from one body to another. • Molecular kinetic energy is known as internal energy.

  13. COE

  14. 7.2 Conservation of Energy Energy cannot be created or destroyed in any process, but only changes from one form to another or transferred from one body to another Total amount of energy CONSTANT

  15. Pile-driver for constructing buildings Conservation of Energy BEFORE AFTER Kinetic energy Amount of energy no change Gravitational energy Thermal energy and Sound

  16. Conservation of Energy • Conversion between PE and KE • For example, in the roller coaster, • As carriages move downwards, • PE  KE. • As carriages move upwards, • KE PE Conservation_of_Energy_demolition_ball.wmv

  17. Conservation of Energy KE = 0, PE = max KE = 0, PE = max Both KE + PE Both KE + PE KE = max, PE = 0 Assume negligible air resistance

  18. 15 –second Quiz 4 Conversion of energy Which one of the following correctly describes the energy conversion that occurs after a bungee jumper jumps from the bridge to the instant when the chord is extended to the maximum? A. EPE  KE  GPE B. GPE  KE  EPE C. GPE  EPE  KE D. KE  GPE  EPE ans B EPE = elastic PE KE = kinetic energy GPE = gravitational PE

  19. Electricity for Singapore 2) water turned into steam under intense pressure. steam high pressure steam 3) Turbine turned by steam 4) Generator produces electricity air for combustion turbine exhaust gases water for cooling Oil or gas condenser water 1) Fuel (oil or natural gas) is burnt Water runs through pipes to boiler

  20. Worksheet 7 A Q3 Text book Pg 127 Try 7B Q 3 (3mins) Try 7B Q 4 (3mins) Conservation of Energy

  21. 1 –min Quiz 5i 3 m An acrobat of mass 70 kg jumps down on to the seesaw and lift his partner upward. (Assume negligible air resistance and frictions Take g=10 N kg-1)(i) Calculate the loss of gravitational potential energy when the acrobat touches the see saw. Loss of GPE = mgh = 70 x 10 x 3 = 2100 J

  22. 1 –min Quiz 5ii 3 m An acrobat of mass 70 kg jumps down on to the seesaw and lift his partner upward. (Assume negligible air resistance and frictions Take g=10 N kg-1)(ii) What is the speed of the acrobat just before touching the see saw? loss in PE = 2100 Gain in KE = loss in PE ½ mv2 = 2100 ½ x 70 x v2 = 2100 v = 7.75 m/s

  23. 1 –min Quiz 5iii 3 m An acrobat of mass 70 kg jumps down on to the seesaw and lift his partner upward. (Assume negligible air resistance and frictions. Take g=10 N kg-1) (iii) Given that his partner has a mass of 60 kg, how high would he reach? By Conservation of Energy, Amount of energy transferred to partner = 2100 J Gain in PEpartner = Loss in KE mgh = 2100 60 x 10 x h= 2100 h = 3.5 m

  24. Green arrow acceleration g (no change) Blue arrow Velocity (highest at bottom, lowest at top) Brown arrow Spring’s force when stretched (greatest at bottom)

  25. GPE GPE + KE GPE + KE + Elastic PE KE + Elastic PE (just before max stretch) Elastic PE (max stretch) Green arrow acceleration g Blue arrow Velocity Brown arrow Tension

  26. Give examples of the various form of energy state the principle of the conservation of energy Give the formula for calculating KE and GPE? apply the relationships for KE and GPE to new situations or to solve related problems Next: Apply relationship Work Done = force x distance moved in direction of force to new situations or to solve related problems What have we covered so far??

  27. 7.3 Work • Work is done when a force produces motion. • Work = force × distance moved in the direction of the force • SI unit: joule (J). W = F × d Final position Initial position d F d must be in the direction that force F is applied

  28. 10-second Quiz 6 • In which of the following cases is work done? • A. A person pushing a wooden box forward. • B. A person pushing the wall of a building. • C. A farmer carrying a bag of rice. • D. Two opposing teams of people • pulling a tug-of-war rope which • is stationary. ans A F x D WORK =

  29. 7.3 Work • No work is done unless a force causes an object to move in direction of applied force. No work is done!! Wall did not move even though force is applied wall No work is done!!

  30. 7.3 Work

  31. d = 1.5 m Fr = 6N Initial position Final position F 7.3 Work On horizontal plane, Force required = Force to to move the block overcome friction = 6 N Hence, Work Done = F x D (D in direction of F) = 6 x 1.5 = 9.0 J

  32. 7.3 Work

  33. 15 m 5 m 20kg • An object of mass 20 kg is pulled up a slope of 15mlong with a constant speed. The height of the slope is 5m. The frictional force between the object and the slope is 30 N. • What is the work done to overcome friction? • What is the total work done in pulling the object up the slope? 2 min- Quiz 7 Rope Fr = 30N

  34. 15 m 5 m 20kg • What is the work done to overcome friction? • What is the total work done in pulling the object up the slope? 2 min- Quiz 7 Rope • WD to overcome friction • = 30 x 15 • = 450 J Fr = 30N b) WD to lift 20 kg up 5 m = mgh = 20 x 10 x 5 = 1000J Total WD = WD to overcome friction + WD to lift 20 kg up 5m = 450 + 1000 = 1450 J

  35. Try 7B Q 7 ans : a) 2400 J, b) 2400 J (explain) c) KE = 1200 J Those finished, do Q 9: ans a) 0N, b) 100N, c) 150J, e) 50W Q11: ans a) 2 ms-2 b) 60J Work

  36. 7.3 Work • Work is done when a force produces motion. • SI unit: joule (J). W = F × d WD against another force WD to change object speed eg, 1. Against gravity pull 2. Against elastic forces 3. Against friction etc

  37. 7.3 Work W = F × d WD against anotherforce 1. against gravitational force on object Final position Force to overcome gravity pull, F = W = mg h F WD against gravity pull = F x d = W x h = mgh Initial position W = mg

  38. 7.3 Work Fe W = F × d WD against anotherforce 2. against elastic forces WD to stretch spring = Fe x extension d

  39. 7.3 Work W = F × d WD against anotherforce 3. against friction WD to overcome friction = Fr x d Final position Initial position F Fr d

  40. 7.3 Work W = F × d WD to change speed of object Work done on object to change its speed. F Frictionless d v m/s u m/s KEf = ½ mv2 KEi = ½ mv2

  41. 7.3 Work W = F × d WD against anotherforce WD to change object speed eg, 1. Against gravity pull 2. Against elastic forces 3. Against friction etc

  42. 1- min Quiz 8 A bullet of mass 50g was travelling at a speed of 200ms-1 before striking a sandbag. It travelled through 20cm of the sandbag before stopping. What was the total resistive force produced by the sandbag? Sandbag Conservation Of Energy, Loss in KE = WD by bullet to move through 20 cm of sandbag ½ mv2 = F x d ½ (0.05) (200)2 = F x (0.20) F = 5000 N

  43. Go through Wksht 7B Q11: ans a) 2 ms-2 b) 60J Work

  44. 7.4 Power 70kg 70kg Height risen 10 m Walking leisurely Chased by dog Took 30 secs Took 1 sec Feel more tired running upstairs compared to walking upstairs.

  45. 7.4 Power Defined as: Rate of work done or Rate of energy conversion Refers to how fast work is done or how fast energy is converted

  46. 7.4 Power 70kg 70kg Height risen 10 m Walking leisurely Chased by dog Took 30 secs Took 2 secs SAME WD But running took less time then walking! WD = F x d = mg x h = 7000 J WD = F x d = mg x h = 7000 J Work is done slower Work is done faster

  47. 7.4 Power 70kg 70kg Height risen 10 m SAME WD But running took less time then walking! Walking leisurely Chased by dog Took 30 secs Took 2 secs WD = 7000 J WD = 7000 J Work is done slower Work is done faster Running guy has more power!

  48. 7.4 Power Defined as: Rate of work done or Rate of energy conversion Power = = = = • SI unit : watt (W) • Other units: Joule per second (J s−1)

  49. 1- min Quiz 9 A man of mass 60 kg takes 1 min to run up a flight of stairs from X to Y as shown. What is his working power? • 60 J • 80 J • 60 W • 80 W ANS: C

  50. 1- min Quiz 9 3600 Δ in Energy 60 Time A man of mass 60 kg takes 1 min to run up a flight of stairs from X to Y as shown. What is his working power? Height h = 6 m, m= 60 kg, t = 1 min = 60s Gain in GPE = mgh = 60 x 10 x 6 = 3600 J t = 60 s Power = = = 60 W

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