ABSTRACT
Start with a simple observation that all of us make, often in childhood. When wading in a stream of water, we can feel the water pushing against our legs. In a very rapid stream, the “push” of the water might be great enough that we have to be careful not to be knocked down by the rushing water. Wading on the coast, we can feel the push or pull of the tide on us. Sometimes we can get swept off our feet by a big wave coming in. Certainly if we were to stand under a waterfall (Figure 8.1), we would feel the rush of water over our bodies.
