1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Help solving for energy please?

  1. Dec 7, 2015 #1

    A screenshot of the question can be found at the bottom of this post.


    1. The problem statement, all variables and given/known data

    D's mass is 50kgs
    He does full squats over 100 kgs
    His displacement is 1.2 m
    acceleration=10

    2. Relevant equations
    F=ma
    W=(force)(displacement)


    3. The attempt at a solution
    First I need to transform his weight from kg's to newtons.
    His weight in newtons is 490.3

    Now I can solve for work
    W=(490.3)(1.2)
    W=588.36

    This is not a possible answer choice on the sheet,may I please have some help understanding what I have done wrong?
     

    Attached Files:

    Last edited by a moderator: Dec 7, 2015
  2. jcsd
  3. Dec 7, 2015 #2

    Merlin3189

    User Avatar
    Gold Member

    You've accounted for his weight, but what about the 100 kg?

    And they suggest g=10 m/sec/sec, so you don't need to be so precise and you could do it with mental arith.
     
  4. Dec 7, 2015 #3
    I wasn't sure if I should convert his squatting weight to Newtons,but in that case it would be 980.6 newtons
     
  5. Dec 7, 2015 #4

    Merlin3189

    User Avatar
    Gold Member

    To me the language is strange, because I thought squatting was intransitive. But the question only seems to make sense if they mean he is squatting while holding 100kg.
    So if he is 50kg and is holding 100kg, when he squats and stands again, what is the total weight he lifts?

    Your conversions are correct, though as I said, they don't ask you to use g= 9.806, so you could just use g=10 msec-2
     
  6. Dec 7, 2015 #5
    So would I add the two masses together 50kg + 100kg=150 kg which translates to 330.7 lbs. From there should I multiple by the gravity and then divide by the displacement?
     
  7. Dec 7, 2015 #6

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Yes.
    Why ever would you want to do that? What are the units in the answer options?
    Yes.
    What standard equation or physical law leads to that?
     
  8. Dec 7, 2015 #7
    Sorry on the dividing by the displacement theory I was just being stupid,forgive me. I think I've figured out what to do though. Notice on the picture I posted of the equation it is mentioned that F=ma therefore I multiplied his mass by 10. F=500N and then from there I multiplied by 1.2 and got an answer of 600 then I had to multiple 600 by 2 and in result I got 1200. In this question I am trying to find out how much energy the gentlemen burned doing a full squat.
     
  9. Dec 7, 2015 #8

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    So what is your final answer (include units)?
     
  10. Dec 7, 2015 #9
    My final answer is 1200 (J), can I have your help with a different question if you don't mind?
     
  11. Dec 7, 2015 #10

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    That's not right. You've forgotten something. if you cannot see your error, please post all your steps in detail.
    Please start a new thread for a new question.
     
  12. Dec 7, 2015 #11
    Here are my steps as follows

    F=ma
    F=(50)(10)
    F=500N

    W=(force)(displacement)

    W= 500*1.2=600

    To find joules burned I multiply by 2 I believe

    600*2=1200

    Final answer 1200(j)
     
  13. Dec 7, 2015 #12

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Why?
    What about the 100kg burden?
     
  14. Dec 7, 2015 #13
    Well if I were to add that into the equation,I would get an answer that is not at all one of the answer choices. The answer choices for this particular problem are

    600,1200,1800,and 1000 (all in Joules)
     
  15. Dec 7, 2015 #14

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    I agree with one of those. Trying to match one of the answers does not justify leaving out the 100kg. You should be looking for some other error.

    You did not answer my other question. Why do you multiply the work done by 2?
     
  16. Dec 7, 2015 #15
    Well when you do a squat you go from the first position and down into the second one, so when I multiplied 500 by 1.2 and got 600 I then multiplied it by 2 and got 1200.
     
  17. Dec 7, 2015 #16

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Do you do the same work going down as coming up?
     
  18. Dec 7, 2015 #17
    In this case I'd assume so.
     
  19. Dec 7, 2015 #18

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    I can't think why. I would find it much easier to lower 100kg than to raise 100kg.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Help solving for energy please?
  1. Please help solve (Replies: 7)

Loading...