Pressure

Pressure is the amount of force applied to a given area. Acrobats and cheerleaders sometimes stand on each other's shoulders to form a human tower. Even with perfect balance, there is a limit to how high such a tower can be built. Ultimately, the ability of the bottom person to bear the pressure, caused by the weight of all the people stacked above, is the limiting factor. Pressure, then, is the amount of force applied on a given area.

In this example, increasing the number of people in the tower increases the amount of force applied to the shoulder area, which in turn causes the bottom person to be under greater pressure. But pressure can also be increased without changing the amount of applied force. If the person standing directly above were to stand on one foot, thereby shifting all the weight onto a smaller area, the bottom person would feel increased pressure on that burdened shoulder.

Turning a nail upside down and driving its large, flat head through the wood by hammering its point, is a more difficult task than conventional nailing. Even if you were able to hammer the point with the same force, the flat head of the nail would spread this force over a relatively large surface area. As a result, there might not be enough pressure on the surface of the wood to cause penetration.

A force exerted over a small area causes more pressure than the same force applied over a large area. This principle explains why karate experts use the side of the hand when breaking a board, instead of the full palm which has more surface and would apply less pressure to the wood.

Similarly, a force exerted over a large area causes less pressure than the same force applied over a small area. This explains why it's possible to walk on top of deep snow with large, flat snowshoes when ordinary rubber boots would cause you to sink.