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Homework Help: Conservation Laws Homework help

  1. Nov 20, 2004 #1
    Hi, I'm having a few problems with my Homework I'll post one question for now then if I find out where I got stuck I'll go on to my next question. Alright...

    "A 1500kg car accelerates up a hill in an attempt to pass a semi truck, it had an initial velocity of 12m/s if accelerated at a rate of 2m/s^2 for the 180m up the slope. If the hill had a vertical height of 15m and ther was 400j of work done by the friction how much work was done by the cars engine?"

    Alright, well I know the Mass, Vi, A, D, G, H, and Vf and the W done by the friction.

    But I'm a little unsure about how to go about figuring this question out. If someone could start me off that would be great.
     
  2. jcsd
  3. Nov 21, 2004 #2
    First of all, energy must be conserved (as you suggested). So, try to find an expression for the initial energy and the final energy.
     
  4. Nov 21, 2004 #3
    Hi,
    The total energy of the system must be conserved, now at the start the car has velocity 12m/s and hence kinetic energy: 1/2mv^2. Since the change in the potential energy depends on CHANGE in height, assume it has initial height of zero, and so the total initial energy is: 1/2 m(12^2).
    Now since acceleration is constant you can use
    vf^2=vi^2 +2ad
    Put your values in to get vf, then u can get the final kinetic energy. Then calculate the change in potential energy: mg(15)
    Now the final kintetic energy plus the potential energy, subtract 400J should equal the initial kinetic energy.
    Hope that helps
     
  5. Nov 21, 2004 #4
    yeah thanks you two, I had figured out the final kinetic energy, and potential energy but was a biy unsure as to what to do with the 400j. Thanks! I'll add my next question in an hour or so.
     
  6. Nov 21, 2004 #5
    Actually now that I relooked over my answer that doesnt make any sense to me I get a number like 8687250-400. Can someone verify if what rayveld said is correct. Also if someone could maybe rewrite what he said that would be great help.
     
  7. Nov 22, 2004 #6
    Hi sorry if my last post was confusing i was in a rush. My answer to it came out as:
    3,254,730 - 400=3,254,330 J
    I don't know if its right but there should be a lot of work done by the car's engine to accelerate it at 2m/s.
     
  8. Nov 22, 2004 #7
    can someone verirify if rayveld has the right asnwer I did it and keep getting something different
     
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