Your swimming buddy Mark jumped into a pool parallel to the water surface. When he stood up, he yelled "ouch" and you noticed that the skin on his chest and belly looked red and irritated. How would you describe the properties of water to explain to Mark why this happened? Why doesn't it hurt when pool water is penetrated perpendicular to the surface, as with a hands-first or feet-first dive?
What will be an ideal response?
Because Mark was parallel to the water surface, the force of his mass was spread out over a relatively large area of the water, making the force per unit water lower than in a typical dive. The surface tension of water, while not strong enough to keep Mark from penetrating the water surface, was strong enough to momentarily resist him. The force of the water pushing back on Mark, however briefly, was enough to cause pain. In a typical dive position, the force of Mark's entire mass is spread only over a tiny area of the water, and thus the force per unit water surface is greater. This higher force is sufficient to immediately break the hydrogen bonds and overcome the surface tension.
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