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Work & Energy

Work & Energy. Conceptual Physics. What is Energy??. The ability to do work If an object has Energy, then it is able to move or transform things What is work? Work occurs when a force makes an object move Work is a transfer of energy

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Work & Energy

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  1. Work & Energy Conceptual Physics

  2. What is Energy?? • The ability to do work • If an object has Energy, then it is able to move or transform things • What is work? • Work occurs when a force makes an object move • Work is a transfer of energy • When you do work on an object, you transfer energy from you to that object • This means W =∆E or the amount of work done on an object is equal to the gain or loss of E for that object

  3. Doin Work… • When something is sped up or slowed down • When something’s height above the ground is increased • When a force makes an object move… • W = Fd • W – Work (J) • F – Force (N) • d - displacement (m)

  4. Work Examples • How much work does it take to lift a 30 kg suitcase onto the table, 1 meter high? W =Fd • The Force “F” required to lift an object is equal to its weight -remember….Weight = mass x gravity • The distance, “d” is equal to the height • Gravity does 20 J of work on a 1 kg (10 N) book that it has pulled off a 2 meter shelf

  5. Units for Work/Energy • Unit of work (energy) is the N·m, or Joule (J) • What else can energy be measured in? • One Joule of energy is equal to 0.239 calories, or 0.000239 Calories (food) • What does it mean to say a piece of food has 1oo calories??

  6. Many types of Energy • Electrical • Chemical • Thermal • Solar • Mechanical • Sound • Nuclear

  7. Mechanical Energy • Gravitational Potential Energy • An object is able to do work by virtue of its position above the Earth • Stored Energy as a result of an objects position • Is equal to the work done against gravity in lifting it • PE = Weight x height • PE = (mg) x h • PE= mgh • h  always measured from some reference level, usually ground • Kinetic Energy • An object is able to do work by virtue of its motion • Energy of Motion • KE = ½ mv2 • Elastic Potential Energy • Will be discussed later

  8. Gravitational Potential Energy • Stored energy b/c of its raised position • b/c of gravity it has the potential to do work • Remember W = Fd • F is equal to ‘mg’ and the ‘d’ is the same as the ‘h’ So W = Fd = PE = mgh h

  9. Stairs vs. Ramp vs. direct lift • Raising which of these blocks requires the most work? • Ans- All the same, since they are all getting moved up to the same height they require the same amount of work done b/c they all gained the same amount of PE • Which requires the least force? • The ramp, because W = Fd since it has a longer distance to travel, the force is reduces. The other two since you are lifting it straight upwards require that you lift with a force equal to the object weight • In this manner, a ramp can be very useful….. Even though same work….. Reduces force

  10. Kinetic Energy • The kinetic energy for a mass in motion is K.E. = ½mv2 • Example: 1 kg at 10 m/s has 50 J of kinetic energy • Ex. Tank shells • Ball dropped from rest at a height h (P.E. = mgh) hits the ground with speed v. After ball falls, no PE left, all energy is now KE. Expect mgh =½mv2 • In this case all of the PE converted into KE. So energy was conserved.

  11. Power • The rate at which work is done • Units  Watts (W)  1 W = 1 J/s • P = W/ t • OR since W = Fd we can say • P = (Fd)/t which since v = (d/t) we can say • P= Fv is an alternative form of the power equation, and can be used to express instantaneous power when velocity is not constant

  12. Work – Energy Theorem • Work done is equal to the change of Energy of that object • W=ΔKE • however much Kinetic Energy an object gains or loses is equal to the amount of work done by/on it • Work is a transfer of Energy from one object to another

  13. Conservation of Energy • Energy can never be created nor destroyed • Energy is never lost, only transferred • This holds true for all forms of energy • In any closed system the total amount of energy remains constant

  14. Conservation of Mechanical Energy • All mechanical energy must be conserved in any closed system • In other words, the sum of all forms of mechanical energy stays constant • MEi = MEf • Or PEi + KEi = PEf+ KEf

  15. PE at the top = KE at bottomCons. Of Mech. Energy

  16. Example problem • A 500 Kg cart starts from rest and accelerates to a speed of 10 m/s. • A) What is the carts initial kinetic energy? • 0 J ….. Starts from rest KE= ½ mv2 v=0 • B) What is the cart’s final kinetic energy? • KEf = ½ mv2 = ½ (500)(102) = 25,000 J • C) What is the carts change in energy? • ΔE = ΔKE = KEf – KEi = 25,000 J – 0= 25000 J • D) How much work was done on the car? • Work = ΔE = 25000 J

  17. Example Problem

  18. Elliptical Orbits • When faster?? When Slower?? • Why?? • Just like falling objects, when you lose ht. you lose PE and gain KE • So when close to sun we have converted most PE to KE and when we are far away vice versa

  19. Elastic Potential Energy • PEE = ½ kx2 • k = spring constant in N/m • x = amount of compression or stretch in an elastic object from its equilibrium position • This is the third type of mechanical energy • PEE can also be transferred into PEG and KE and Cons. Of Mech. E also applies to conversions between this and the other types of ME

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