The main muscles that hold your head upright attach to your spine in back of the point where your head pivots on your neck. Figure P8.66 shows typical numbers for the distance from the pivot to the muscle attachment point and the distance from the pivot to the center of gravity of the head. The muscles pull down to keep your head upright. If the muscle relaxes?if, for instance, you doze in one of your classes besides Physics?your head tips forward. In the questions that follow, assume that your head has a mass of 4.8 kg, and that you maintain the relative angle between your head and your spine.

A) With the head held level, as in Figure P8.66, what muscle force is needed to keep a 4.8 kg head upright?

B) If you tip your body forward so that your spine is level with the ground, what muscle force is needed to keep your head in the same orientation relative to the spine?

C) If you tip your body backward, you will reach a point where no muscle force is needed to keep your head upright. For the distances given in Figure P8.66, at what angle does this balance occur?

riy 4.2 cm 5.0 cm I 10 cm