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Muscle tension required to lift a child at a constant speed

  1. Jan 2, 2017 #1
    1. The problem statement, all variables and given/known data
    When a mother lifts he child with her foot (as shown in figure) what force should the upper muscle exert to lift her child at a constant speed?
    24e163d.jpg

    2. Relevant equations
    W = F x D
    f = mg

    I think these might be the right equations to use, correct me if I'm wrong.

    3. The attempt at a solution
    I'm not sure if I need to add up the forces and distances shown in the figure? If I did W would be:
    W = (0.2m + 0.38m)(39N + 98N)
    W = 0.58 x 137 = 79.46Nm

    Do I need to convert kg into N?

    f = mg
    f = 137N x 9.80 m/s
    f = 1342.6

    Is this at all in the right tracks?
     
  2. jcsd
  3. Jan 2, 2017 #2

    russ_watters

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    Staff: Mentor

    Work is not needed here. This is a levers problem. You need to convert the forces to torques and sum them.
     
  4. Jan 2, 2017 #3
    I
     
  5. Jan 2, 2017 #4

    I gave it a shot and got an answer that is 45Nm, can the force of the muscle be written in Nm?
     
  6. Jan 2, 2017 #5
    No, the force has to be in N (Newtons).
    I think your answer of 45 Nm is the correct torque that the muscle has to provide, but it is not the force. You have to calculate the force from that torque.
     
  7. Jan 2, 2017 #6

    russ_watters

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    Staff: Mentor

    No, force is in Newtons. Newton-meters is probably a torque... By my math, you calculated the torque applied to lift the leg and the baby. So you aren't done yet: the question asked for the force applied by the muscle above the knee.

    You can take the problem in steps that way, but generally the physicists here (and probably your professor too) would prefer you write the torque-sum equation and then solve for the force you are looking for. Then plug in the numbers and compute in one step.
     
  8. Jan 2, 2017 #7
    So I can do F = t/D ?
    I tried to do 45Nm / 0.02m = 2250 N

    Is there any more work than that needed?
     
  9. Jan 2, 2017 #8
    I think that's right.
     
  10. Jan 2, 2017 #9

    russ_watters

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    Staff: Mentor

    That looks like the right answer to me!
     
  11. Jan 2, 2017 #10
    Thanks for the help, really appreciate it.
     
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