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Simple Energy-Force-Distance Challenge

  1. Aug 1, 2007 #1

    I am not a physics student, and I have a simple question/challenge.

    I want to lift 100kg from the floor by 5 meters. I have 2,943 Watts of power to do this. Is it possible?

    As i understand this is not possible. The force that 2,943 Watts provides is 588,6 newtons using the formula: Energy[2,943]=Force[588,6]/Distance[5m].

    Is this correct? Is there any way to use the 2,943 Watts of power to lift 100Kg?

    Thanks for your help!
  2. jcsd
  3. Aug 1, 2007 #2

    Doc Al

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    Staff: Mentor

    Watts is a unit of power (energy per unit time) not energy; energy is measured in Joules.

    Your equation doesn't make sense. Perhaps you are thinking of Energy = Force*distance?

    You need about 980 Newtons of force to directly lift 100 kg. To lift it 5 meters will require 4900 Joules of energy.
  4. Aug 1, 2007 #3


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    Homework Helper

    Just gear your power source so that force is increased while velocity is decreased, for example a multi-loop pulley would work just fine.

    As mentioned, it will take 4900 watt x seconds to do the lift.

    Since you have 2943 watts as a power source, it needs to be setup so that it takes (4900/2943) 1.665 seconds or longer to lift the 100kg weight 5 meters.
  5. Aug 1, 2007 #4
    Sorry for clarity sake

    Well, maybe I set out the problem wrong. I simply wanted to know if I can lift 100kg with 2,943Watts.

    I know the units are of different type but I would like to know if I could in theory or even in practice lift the weight in any way with the energy provided in Watts.
  6. Aug 1, 2007 #5
    (in your problem) power says only how much time takes to lift smt. i.e. you can lift anything you want , but it takes a specific amount of time as Jeff Reid calculated. So the answer is yes theoritically(but still watt is not unit of energy;)
  7. Aug 1, 2007 #6
    Are we suggesting that a small amount of electricity power can lift a large weight? Time is not an issue so if it takes a second, a minute or an hour it does not matter.
  8. Aug 1, 2007 #7


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    Again, watts is power and lifting a weight requires energy. So since energy is power times time, you can essentially take an arbitrarily small amount of power for an arbitrarily long time and lift any weight any distance. There are, of course, practical limiting factors such as friction and other efficiency losses and the universe ending before you lift your weight to the specified height if the power is too low, but you get the idea.

    For example, if you want to lift 100 kg 5m, that's 4900 n-m or joules (energy is force times distance). So since watts is joules per second, you can do that amount of work in 1 second with 4900 watts or 5 seconds with 980 watts or 4900 seconds with 1 watt, etc.
  9. Aug 1, 2007 #8
    I can see my limitation that I only have 2,943 Watt of electricity in total. Sorry if I was not clear. Know nothing of physics and you are all helping me a great deal.

    I don't have an infinite electricity capability. 2,943 Watt is my limit. This is the equivalent of 2,943 joules that I have in total, which will not get my 100kg weight up 5m.

    So I would be correct to say it is impossible.
  10. Aug 1, 2007 #9
    No, I think you're not quite clear on what the previous posters have said. Maybe this will help. What is the answer to this question:

    Say I can only drive my car 10 miles an hour. Is it possible for me to drive 1000 miles?
    Last edited: Aug 1, 2007
  11. Aug 1, 2007 #10
    not if the fuel runs out, I am limited on that. So if my electricity runs out...
  12. Aug 1, 2007 #11

    Hint: look up the definitions of "joules" and "watts". Pay attention to the answers you've been given above.
  13. Aug 1, 2007 #12
    I still think it is I who has formulated the question all wrong due to my ignorance. Once again thank you for your patience.

    One Watt is a measuremente equal to one joule per second.

    The TOTAL amount of electricity available to me is 2,943W for ONE second only. So that means I have potentially 2,943 joules to spend to lift my weight. Thus i will not be able to lift my weight of 100kg up 5 meters.
  14. Aug 1, 2007 #13
    But I didn't say anything about fuel! I only said that my velocity is limited to 10 miles an hour. Given only that information, is it possible for me to drive 1000 miles?
  15. Aug 1, 2007 #14
    Of course, it just takes longer. I appreciate what you are saying. I do believe I solved my problem. Unless somebody objects to

    I think I cracked it. I don't do physics, but its sure uselfull! In fact... necessary :)
  16. Aug 1, 2007 #15


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    Staff: Mentor

    You never said before that you only had that power available for one second. I still think you are confused about the difference between energy and power. If that is truly what you are looking for then yes, you don't have enough energy available - but besides the inconsistencies, you have also shown some pretty specific misunderstandings of the concepts of energy and power. Make sure you undersand what this thread has taught before just discarding it...
    Last edited: Aug 1, 2007
  17. Aug 2, 2007 #16
    Thank you for your help. I am totally ignorant in the subject of physics. I am very impressed with the help you have provided and although I am not a student of any kind of physics I will review my ideas on this matter.
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