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How much work can be realized? Simple physics I can't understand.

  1. Feb 10, 2014 #1
    1. The problem statement, all variables and given/known data
    To be able to break cement, the hammer of a machine needs to have a mass of 1.00 x 10² kg.
    The motor of the machine lifts it at a height of 5.00 meters every 10.0 seconds.

    a) How much work did the machine realize within 1.00 minute?


    2. Relevant equations

    W = Fd

    1 min = 60s/1min

    3. The attempt at a solution

    Work = 100 kg * 5.00 m = 500 J

    60 seconds * 500 J = 30000 J

    Is that how its done? I'm pretty confused.
     
  2. jcsd
  3. Feb 10, 2014 #2
    You have the right idea. The first part:
    is the work done in lifting the block 5.00m, and the problem tells it lifts the hammer 5.00m in 10.0s, so you can also say that's the work done in 10.0s. Then, since the answer needs to be the work done over a full minute, you just have to make the conversion from work done in 10.0s to work done in one minute.
     
  4. Feb 10, 2014 #3
    So, 500 J * 60 seconds = 30000 s

    then

    30000 s * 1 min /60 s?
     
  5. Feb 10, 2014 #4

    collinsmark

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    Gold Member

    'Couple of comments:

    1.00 × 102 kg is the hammer's mass, not its weight. Weight is the measure of force. :smile:

    The hammer is raised once every 10 seconds (not every second, but once every 10 seconds). So how many times is the hammer raised in a minute?
     
  6. Feb 10, 2014 #5
    No, you're mixing up your units. What you have is this:$$500J \cdot \frac{1}{10s}$$That's the work done per second, or the rate at which work is done. Then you have find the work done in a full minute. That means using this relationship:$$ amount = rate \cdot time$$
    Or, just make a proportion:$$\frac{500J}{10s} = \frac{?}{1min}$$, where [itex]?[/itex] is the amount of work you're looking for.

    EDIT: Yes, what collinsmark said. That 500J needs to be recalculated using weight instead of mass.
     
  7. Feb 10, 2014 #6
    Weight = 1.00 x 10 2 kg * 9.81 m/s = 981 N

    Work = 981 N * 5.00 m = 4905 J

    in 1 minute the hammer realizes 6 * 4905 J = 29430 J?
     
  8. Feb 10, 2014 #7

    collinsmark

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    Gold Member

    There ya go. :approve: That looks right to me. :smile:
     
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