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Homework Help: Could you check these for me - please

  1. Mar 27, 2006 #1
    Could someone check I have these setup right and check my answers - if not where have I gone wrong.

    How long will it take a 50 hp motor to do 1 MJ of work?

    1hp=746W = 50*746=37300W

    1 MJ = 1000000J / 37300W = 26.8 secs


    A 650 W microwave oven is used to reheat food for one minute. How much energy is supplied?

    W=p*t = 39000=650*60

    Answer: 39000J


    How much energy is required to heat 600 g of copper from 22 oC to 50 oC?

    Specific Heat Copper = 385J/Kg Kelvin





    You push as hard as you can for 6 seconds on a crate of mass 250 kg, but are unable to move it. What is your rate of doing work (power)?


    But in this case nothing is moving so I'm not doing any work - is that right?


    Finally what is the best way of setting this one up?

    The immersion heater in an electric hot water system is rated at 9 kW. If the tank is initially filled with 40 gallons (about 150 litres, with a mass of 150 kg) of cold water at 15 oC, how long will it take the heater to raise the temperature to 40 oC?

  2. jcsd
  3. Mar 28, 2006 #2
    They look fine. Except: Pet peeve warning! Danger! Danger!

    In the first line are you really trying to say that 1 hp = 37300 W? And saying that 746 W = 37300 W is just silly.

    In the second line you are equating 1 MJ with 26.8 s. The units are wrong.

    (Reality check) Yes, I DO understand what you are trying to say here, but the statements you are explicitly making are incorrect. People have confused themselves with this kind of shorthand and, perhaps more importantly for you, lost points on exams. Try to make sure you are writing on separate lines or using commas to separate these statements.

    (Pet peeve rant over! :biggrin: )

    For the last problem, you know how much heat you will need (using the Q equation) and you know the rate that energy is being pumped in by the heater. So you should be able to use this to find the time to heat the water.

  4. Mar 28, 2006 #3
    Ok so I need some help here..this is how I see it and what you say makes sense however my boots are stuck in the mud and I need help pulling them out. What should I be doing to get unstuck?


    deltaT=40-15 = 25



    How do I go about getting this into time?
    Last edited: Mar 28, 2006
  5. Mar 28, 2006 #4


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    Power is the rate at which work is done, hence -

    [tex]P = \frac{\Delta W}{\Delta t} = \frac{\Delta Q}{\Delta t}[/tex]

    -Hoot :smile:
  6. Mar 29, 2006 #5
    How are the units wrong?
  7. Mar 29, 2006 #6
    So t=p*q ?
  8. Mar 29, 2006 #7
    You originally wrote the line:
    You have an equality all the way through so the first entry is equal to the last...ie. you are saying that 1 MJ is equal to 26.8 s.

    What you "ought to do" is write the equation you are using. As an example:
    1 MJ = 1000000 J
    P = W/t
    t = W/P = 1000000 J/37300 W = 26.8 s

    It's a lot more to write, but I have found that doing so not only organizes things in your own mind better, but also (and here's the key) makes it MUCH easier for the grader to give you partial credit if something is wrong.

    I readily admit to being a pain in the butt when I teach, but I like to think it's good advice anyway. :smile:

  9. Mar 30, 2006 #8
    Yes and your advice is not wasted on me (Likewise for other 'helpers').

    Thank you for your support.
  10. Apr 3, 2006 #9
    I think you're in my class

    Hey this is so funny. We have to be in the same class. I'm the annoying chick with medium brown hair and a black shoulder backpack. Anyway, say hi, and thanks for posting this, I got stuck on the first question you posted too.
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