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Homework Help: A few simple work/energy questions~

  1. Aug 3, 2010 #1
    1. The problem statement, all variables and given/known data

    1. A 1000kg car has a 50kW engine and is traveling at 60 kilometers per hour-

    a) solved!
    b) calculate how much work is done by the engine during this time
    c) calculate what the force being exerted on the car during this time

    2. Determine the energy used to operate a 240V water heater continuously for four hours. The resistance of the element is 12 ohms.

    3. The attempt at a solution

    For b, I feel the only way to get the answer is by finding out how much energy is required to keep a car going at 60kmp/h (neglecting friction/wind)- but only f=ma comes to mind which is useless as there is no acceleration. To find the work (energy), I would need to find the force required right? Work= Fd

    For c, I'm quite stumped. I can't think of what equation I need to solve it.

    For 2, I've worked everything except the last.
    First I solved for I(amperes)
    I=V/R --> 240/12 = 20A
    P=IV --> 20.240 = 4800W

    Now, it's asking how much energy (power/Watts) is used in 4 hours. With the result of 4800W, do I just times it by 60x60x4?? Is 4800W used in a second, a minute or an hour? I'm not entirely sure.

    Cheers for any help! It will be very much appreciated!
  2. jcsd
  3. Aug 3, 2010 #2
    1. b) a 50kW engine is an engine that is doing 50kW of work, but since the car isn't accelerating you could say that no work is being done. This isn't exactly true, but for basic physics problems you can assume it to be true. If you want a more accurate idea about what the engine is doing, then that involves P-V diagrams and efficiencies of the particular engine. Since that information is lacking, that's probably not the direction your suppose to go in.

    c) You were on the right track when you said there was no acceleration. If there is no acceleration then there is no net force acting on the car. There is a friction force that is causing the car to move forward, and a drag force that acts in the opposite direction.

    2. Power = Volts2/ Resistance

    Power = 2402/12

    Power = 4,800 joules/second

    4,800 joules/second *60 sec/min *60 min/hour * 4 hours = 69,120,000 Joules or 19.2 kWh of energy
    Last edited: Aug 3, 2010
  4. Aug 3, 2010 #3
    What is the question for (a)? If it involves you finding time, then use the formula W = Pt to find the work done.

    For (c), use the formula power = force x velocity.

    For 2, 4800W is equivalent to 4800 J/s, so you need to time 60x60x4 to find the energy consumed in joules.
  5. Aug 3, 2010 #4
    Thanks very much Trex & ark, I appreciated your help very much.

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