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MA Question

  1. Jan 25, 2009 #1
    1. The problem statement, all variables and given/known data
    The figure shows scale drawing of a pair of pliers being
    used to crack a nut, with an appropriately reduced centimeter grid.
    Warning: do not attempt this at home; it is bad manners. If the
    force required to crack the nut is 300 N, estimate the force required
    of the person's hand.( The Handle is 10cm)

    2. Relevant equations
    MA=input force/output force

    3. The attempt at a solution
    Is this even possible to get the Input force with out knowing the MA?
  2. jcsd
  3. Jan 25, 2009 #2


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    You need the mechanical advantage - but this is just the ratio of the handle length to the jaw length (a nutcracker is just a see-saw)
  4. Jan 25, 2009 #3
    You don't need mass or acceleration in this case - you are already given the force. For this problem, you need to look at the moment (or torque) applied vs. output.
  5. Jan 25, 2009 #4


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    I think the OP is using MA to mean mechanical advantage (actually strictly speaking in this case - it's velocity ratio)
  6. Jan 25, 2009 #5
    Mechanical Advantage is what i need. So this problem doesnt have enough information?
  7. Jan 25, 2009 #6
    The pliers are not moving, so their angular momentum remains constant
    at zero, and the total torque on them must be zero. Not only that, but each half of the pliers
    must have zero total torque on it. This tells us that the magnitude of the torque at one end
    must be the same as that at the other end. The distance from the axis to the nut is about 2.5

    cm, and the distance from the axis to the centers of the palm and ngers are about 8 cm. The
    angles are close enough to 90  that we can pretend they're 90 degrees, considering the rough
    nature of the other assumptions and measurements. The result is (300 N)(2.5 cm) = (F)(8 cm),
    or F = 90 N.
    183 I found this at the back of the book.I stil dont get it 8*90 =720 and 300*2.5=750
  8. Jan 25, 2009 #7


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    You haven't included the drawing - but presumably you can estimate the distances pivot-nut and pivot-hand, the ratio of these is the mechanical advantage (assuming 100% efficency)
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