Taking a sip of coffee one day, you start thinking about the effort it takes to

Question

Taking a sip of coffee one day, you start thinking about the effort it takes to do so. Your forearm is a type of lever. As shown in the figure below, the biceps muscle is attached to the forearm bone about 50 mm in front of the elbow joint. The cup of coffee in your hand is 350 mm from the elbow, and you wonder about the work done by your biceps on your forearm as you raise the cup very slowly. You realize that you can use energy principles to estimate the forces involved. Ignore the inertia of the forearm. Inertial of the coffee cup with coffee is on average is 300 gm.

Explanation / Answer

since the system is in equlibrium

net torque acting on the system is zero

torque due to forearm= torque due to cup

r1*F1 = r2*F2

50*m = 350*(300)

m is the mass or inertia of the forearm

m = (350*300)/(50) = 2100 gm

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