A cook holds a 2.59-kg carton of milk at arm's length (see the figure below). What force vector F B must be exerted by the biceps muscle? (Ignore the weight of the forearm. State the magnitude of the force vector F B.)
N

The torques created by the weights are clockwise relative to the pivot, while the torque created by the biceps is counterclockwise; thus, the second condition for equilibrium (netτ=0) becomes

r2wa+r3wb=r1FB.

Note that sinθ=1 for all forces, since θ=90º for all forces. This equation can easily be solved for FB in terms of known quantities, yielding

Now, the combined weight of the arm and its load is (6.50 kg)(9.80m/s2)=63.7 N, so that the ratio of the force exerted by the biceps to the total weight is