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Homework Help: Is This Right?

  1. Jan 25, 2009 #1
    1. The problem statement, all variables and given/known data
    You are trying to loosen a stuck bolt on your RV using a big
    wrench that is 50 cm long. If you hang from the wrench, and your
    mass is 55 kg, what is the maximum torque you can exert on the

    2. Relevant equations

    T=Fr I did T=55kg*.5m

    3. The attempt at a solution
    I thought the answer would be 27 kg m^2
    It doesn't seem quit right though.
  2. jcsd
  3. Jan 25, 2009 #2
    Force is not the same thing as mass. In this case, what kind of force results from your mass?
  4. Jan 25, 2009 #3
    So you mean i have to turn the weight into GE or KE?How could it be GE if i dont know the height?
  5. Jan 25, 2009 #4
    No, weight IS the force you want. But mass is not the same as weight.
  6. Jan 25, 2009 #5
    But if it says 55kg or mass doesnt it mean weight?
  7. Jan 25, 2009 #6
    no, mass is not the same as weight.use w=mg of F=ma
  8. Jan 25, 2009 #7
    Weight is the force of gravity. The force of gravity equals mass * acceleration.

    Mass is NOTTTTTTTTT the same as weight.
  9. Jan 25, 2009 #8
    How do i know the velocity?
  10. Jan 25, 2009 #9
    Let me reiterate with more definition:

    Weight = mass * acceleration due to gravity (this will be, in most problems, a constant)

    Acceleration due to gravity is otherwise known as g = 9.8 m/s^2.
  11. Jan 25, 2009 #10
    Well, the acceleration is basically the pull of the earth, called acceleration due to gravity. It has a fixed value of 9.8 or approximately 10!
  12. Jan 25, 2009 #11
    so the answer is 269.5? and using w=ma i got that the guy is 539 kilograms.
  13. Jan 25, 2009 #12
    Oy.... Weight, when calculated by kg and m/s^2, is in newtons (N).

    It would be 539 N, not kg. How can you multiply kg * m/s^2 and get kg?

    Read your textbook or your notes more.

    And yes, that's the correct number in Newtons.
  14. Jan 25, 2009 #13
    So the right answer is 269.5? What units would it be in?
  15. Jan 25, 2009 #14
    Oh, sorry. Torque is force times the radius. You multiply the units of force and the units of the radius together to get the units of torque.

    Figure it out.
  16. Jan 25, 2009 #15
    Look at the equation to find units. Your equation is the most valuable thing you have to determine units.

    torque=F*r correct? So units of Force are Newtons and units of radius are meters...

    So wouldnt it make sense to having units of torque be N*m?
  17. Jan 25, 2009 #16
    Now I got 134. I did 539*.25m.
  18. Jan 25, 2009 #17
    You meant to put 0.5 m, not 0.25m.

    Example of finding your units:

    What are the units of velocity?

    We know velocity equals distance divided by time.

    v = d/t
    Distance is usually in meters, and time is usually in seconds.
    units of v = units of distance / units of time
    units of v = meters / seconds
    units of v = m/s

    Do the same for torque.
  19. Jan 25, 2009 #18
    so can any at least tell if its wrong?
  20. Jan 25, 2009 #19
    Read my entire posts....
  21. Jan 25, 2009 #20


    User Avatar
    Homework Helper

    Let me recommend that the OP read :

    Torque is based on the distance the Force acts.

    Hence m*g*Lhandle = Torque in units of N-m

    As for the answer 134 it looks like the Length of the handle is not used correctly.
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