4 potential & kinetic energy


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4 potential & kinetic energy

  1. 1. Different Forms of EnergyChapter 3
  2. 2. Kinds of Energy• Gravitational Potential Energy: due to position• Kinetic Energy: due to motion• Heat Energy: due to movement of heat energyfrom regions of high energy to areas of lowenergy.• Radiant Energy: due to light• Chemical Potential Energy: due to bonds• Elastic Energy: stressed objects that return totheir original shape.• Electrical Energy: due to movement of electrons• Nuclear Energy: due to atomic fission/fusion
  3. 3. Kinetic Energy• Kinetic Energy is the energy an object hasdue to its motion.• The KE depends on the mass and thespeed.• Ek or KE= ½ mv2,• E is Energy in Joules, J• m is mass in kg, v is velocity in m/s
  4. 4. Example• What is the KE of a 6 kg curling stonemoving at 4 m/s?• KE = ½ mv2• = ½ x 6kg x (4 m/s)2• = ½ x 6 x 16• = 48 J
  5. 5. ActivityWhat is the Kinetic Energy of a5 kg ball travelling horizontally ata speed of 10 m/s?
  6. 6. Exam QuestionA golf ball is dropped out of a window which is 10 m above the ground. The ball has amass of 50 g. Disregard the effects of air resistance.10 mWhat is the kinetic energy of the ball just before it hits the ground?A) 10 JB) 7.5 JC) 5.0 JD) 2.5 J
  7. 7. Potential Energy• If we lift up an object against gravity, it nowhas the ability to move; it has the potentialto fall down and use up the energy we putinto it.• Ep , PE = mgh, unit is Joules, J• m is the mass in kg• g is the acceleration due to gravity, 9.81m/s2• h is the height above the Earth’s surface, m.
  8. 8. Activity• What is the PE of a 10 kg weight, 8 mabove the ground?• PE = mgh• = 10 kg x 9.81m/s2x 8m• = 784.8 J
  9. 9. ActivityA weather balloon with a mass of 4.0 kg, including the weather instruments, risesvertically in the air. It passes an altitude of 200 metres at a velocity of 2.0 m/s.2.0 m/s200 mAt this point what is its potential energy with respect to the ground?A) 8.0 ×103JB) 8.0 ×102JC) 8.0 ×101JD) 8.0 J
  10. 10. Total Mechanical Energy• The energy of a system transfers betweenPotential Energy and Kinetic Energy.• Total Energy = PE + KE• The PE of an object getstransferred to KE as itspeeds up.• As the PE decreases, theKE increases.
  11. 11. Exam QuestionA small airplane with a mass of 1000 kg, is flying at 60 m/s at an altitude of 250 m.250 m60 m/sWhat is the total mechanical energy of this airplane with respect to the ground?A) 1.8 × 106JB) 2.5 × 106JC) 4.3 × 106JD) 6.1 × 106J
  12. 12. Exam QuestionA stone with a mass of 100 g is thrown horizontally from the top of a cliff overlookingthe ocean with a velocity of 20 m/s. Disregard the effects of air resistance.15 m20 m/sWhat is the kinetic energy of the stone just before it hits the water?A) 15 JB) 20 JC) 30 JD) 35 J
  13. 13. Total Mechanical Energy• What is the speed of a 500g rock thatdrops from a height of 78.4 m, just beforeit hits the ground?• ET = KE + PE, at first, v = 0 m/s• = ½ mv2+ mgh, since v = 0, KE = 0• = 0.5kgx9.81m/s2x78.4m, ET = PE only• = 384.6 J• As the rock approaches the ground all itsPE is transferred to KE, so PE = 0. So…
  14. 14. Total Energy, Part Deux• ET = PE + KE• 384.6 J = KE• 384.6 = ½ mv2• 384.6 = 1/2x 0.5kg x v2• 1538.4 J = v2• v = 39.2 m/s• So just before it hits the ground, the rockhas a speed of 39.2 m/s
  15. 15. Exam QuestionA 100 g ball is thrown vertically upward from the ground with a velocity of 20 m/s.Disregard the effects of air resistance.What is the kinetic energy of this ball after it has risen 5.0 metres?A) 20 JB) 15 JC) 10 JD) 5.0 J
  16. 16. Measuring Work• Work is defined as the energy that comesfrom applying a force in the same directionover a certain distance• W = F Δd = mad (horizontal)• = magd (against gravity)• Work is in Joules, J• Force is in Newtons, N• Distance is in metres, m
  17. 17. Activity• E.g How much work is done by a boypushing a car with a force of 800 N over adistance of 200m?• W = F d• = 800 N x 200 m• = 160 000 J = 160 kJ•
  18. 18. Exam QuestionA 200 g brick falls from a wall 4.0 metres above the ground. It hits the ground with avelocity of 8.5 m/s.4.0 mHow much work did gravity do on the brick?A) 8.0 JB) 7.2 JC) 3.4 JD) 1.7 J
  19. 19. Effective Force• The Effective Force is the forcecomponent that alters the motion of anobject.• It is the component that is parallel to themovement of the object.
  20. 20. Components• The horizontal component of a vector Athat is at an angle of θ from the horizontalis A cosθ.• The vertical component of a vector A thatis at an angle of θ from the horizontalis A sinθ.
  21. 21. Activity• If a boy pulls a cart at an angle of 30· tothe horizontal with a force of 100 N over adistance of 75 m, how much work has hedone?
  22. 22. Exam QuestionA sled has a mass of 10 kg.A child pulls the sled a distance of 20 metres with a force of 10.0 N at an angle of 35° withrespect to the horizontal. During this motion, a force of friction of 4.0 N acts in the oppositedirection of the motion.How much work is done on the sled by the child over the distance of 20 metres?A) 1.6 × 102JB) 1.1 × 102JC) 8.4 × 101JD) 3.5 × 101J
  23. 23. Efficiency• % Efficiency = Work output x 100• Work input• The maximum efficiency is 100%.• It is a measure of what energy is lost tofriction, vibration, and other factors.
  24. 24. Activity• Page 100, Q. 10-14• Page 101, Q. 25-29• Test• Review FOR FINAL EXAM!!!!• Review Sheet HW for next class.
  25. 25. Summary• The Law of Conservation of Energy states that inany transfer or transformation of energy, thetotal amount of energy remains the same.• The form of the energy may be changed, e.g.noise, heat, vibration, friction.• In situations where friction and air resistance aresmall enough to be ignored, and where no otherenergy is added to the system, the totalmechanical energy is conserved.
  26. 26. Summary• Energy is the ability to do work.• Work is the transfer of energy. (W=ΔE)• Friction often does negative work on anobject because it removes energy from it.• Gravitational Potential Energy is theenergy of an object due to its height abovethe Earth’s surface. PE = mgh• Kinetic Energy is the energy of a movingobject. KE = ½ mv2
  27. 27. Summary• E total = KE + PE (before) = KE + PE (after)• Heat is the measure of the amount ofthermal energy that flows from one bodyto another because of a difference intemperature.• Work done on an object can cause anincrease in the temperature of an object.• Effective work = Fd if the object moves inthe direction of the force.