Figure 9.1 On a short time scale, rocks like these in Australia’s Kalbarri National Park are static, or motionless relative to the Earth. (credit: Richard Liblanc/Flickr)
Chapter Outline
9.1 The First Condition for Equilibrium
9.2 The Second Condition for Equilibrium
9.3 Stability
9.4 Applications of Statics, Including Problem-Solving Strategies
9.5 Simple Machines
9.6 Forces and Torques in Muscles and Joints
What might desks, bridges, buildings, trees, and mountains have in common—at least in the eyes of a physicist? The answer is that they are ordinarily motionless relative to the Earth. Furthermore, their acceleration is zero because they remain motionless. That means they also have something in common with a car moving at a constant velocity, because anything with a constant velocity also has an acceleration of zero. Now, the important part—Newton’s second law states that net F=ma𝐹=m𝑎, and so the net external force is zero for all stationary objects and for all objects moving at constant velocity. There are forces acting, but they are balanced. That is, they are in equilibrium.
STATICS
Statics is the study of forces in equilibrium, a large group of situations that makes up a special case of Newton’s second law. We have already considered a few such situations; in this chapter, we cover the topic more thoroughly, including consideration of such possible effects as the rotation and deformation of an object by the forces acting on it.
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