# Homework Help: Rotational dynamics and static equilibrium question

1. Nov 7, 2007

### hkstyles

a crossing guard holds a STOP sign at arm's length, as shown. Her arm is horizontal, and we assume that the deltoid muscle is the only muscle supporting her arm. The wright of her upper arm is Wu = 18 N, the weight of her lower arm is Wl = 11 N, the weight of her hand is Wh = 4.0N, and the weight of the sign is Ws = 8.9 N. The location where each of these forces acts on the arm is indicated in the figure. A force of magnitude fd is exerted on the humerus by the deltoid, and the shoulder joint exerts a foce on the humerus with horizontal and vertical components given by fx and fy , respectively. (a) is the magnitude of fd greater than, less than, or equal to the magnitude of fx ? why?. Find (b) fd (c) fx and (d) fy

Last edited by a moderator: May 3, 2017
2. Nov 7, 2007

### learningphysics

Think about the 3 equations involved...

sum of torques about any point = 0 (1)

sum of forces in x-direction = 0 (2)

sum of forces in y-direction = 0 (3)

equation (2) should let you answer a).

equation (1) gives you part b). try to go on from there to get the other parts...