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Homework Help: Torque problem involving bicep holding ball

  1. Dec 16, 2011 #1
    1. The problem statement, all variables and given/known data
    A schematic diagram of the human arm is shown holding a
    heavy iron ball (mass 5.0 kg). The triceps muscle connects to the
    forearm 3.0 cm to the left of the elbow joint, while the biceps
    muscle connects 5.0 cm to the right. Each of the muscles can
    exert an upward force on the forearm when needed. The
    forearm and hand together itself have a mass of 4.0 kg and a
    center of mass 15 cm to the right of the elbow joint. The distance from the triceps
    to the center of the ball is 35 cm

    a. When the ball is being held as shown, the force exerted by
    the triceps is approximately zero. What force must the
    biceps be exerting?

    2. Relevant equations
    For this problem I attempted to use sigma torque equation to find how much force is needed for the bicep to hold the ball up.

    3. The attempt at a solution
    I found the forces drawing a free body diagram to be
    force of bicep, force of ball, force of arm, and force of triceps.

    so force of biceps = (5.0 x 9.8N) - (15 cm x 9.8N) - (3.0 x 9.8)
    I feel like Im still missing a big chunk in my equation, but I seem to be having trouble with these torque problems any advice on how to solve them? =X
  2. jcsd
  3. Dec 16, 2011 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    The quantities in the equation appear to have inconsistent units.

    How is torque defined?
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