Question

A cook holds a 2.66-kg carton of milk at arm's length (see the figure below). What force B must be exerted by the biceps muscle? (Ignore the weight of the forearm. State the magnitude of the force B.)

Answer 1

The answer depends upon the arm's length and another dimension separating the bicep from the rotator cuff. These dimensions should have been on the "figure below" that you did not provide.

The weight of the arm itself should add to the stress force required to support the milk carton.

Answer 2

A figure with the missing dimensions that you need, and the method of solution, can be found on page 142 of this web site:
http://physics.ucsd.edu/students/courses/summer2009/session1/managed/session1/physics1a/documents/ch_8_solutions.pdf