Motion and Its Causes

Force and Newton’s Laws of Motion

What is the difference between force and pressure?

Why does the stiletto heel of a woman’s shoe sink deep into the ground while if she wears a running shoe the heel does not? The force on the two heels is the same. What is different is the force divided by the area of the heel. The quotient of force divided by area is much larger on the stiletto heel. That quantity is called the pressure.

As an example, consider the case of a 110-lb (50-kg or 490-N) woman who first wears shoes with heels 1.5 cm by 1.5 cm. The area of each heel is about 2.2 cm2. Assuming that she puts all her weight on the two heels the pressure will be 500 N / 4.4 cm2 or 4.5 N/cm2. Pressure is usually expressed in pascals (Pa) or N/m2. In this case the pressure would be 1.1 million pascals or 1100 kPa. If she were to wear running shoes with heels about 7.5 by 7.5 cm then the pressure would be only 45 kPa.

The pressure of air is about 101 kPa. Why don’t we feel this atmospheric pressure? First, it is exerted uniformly on all parts of our bodies. Second, it is balanced by pressure from the insides of our bodies. We can feel the pressure if you try to breathe in but close your mouth and pinch your nose closed. You can tell that the external pressure on your lungs exceeds the internal pressure.


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