What is pressure f

Edition print

The contact pressure

Have you ever wondered why there are snowshoes? Or why doesn't a camel sink into the desert sand? Both have something to do with the so-called Edition print to do. What this is all about, we want to clarify in the following.

What is the bearing pressure?

Imagine the following experiment (if you have an exercise mat at home, you may even be able to do it yourself): A rectangular object, such as a brick, lies on a soft surface such as an exercise mat. At first it lies on its largest side surface. Due to the weight of the stone, the mat is slightly dented. Now the stone is placed on its face, i.e. the smallest of the side faces. In this constellation, the mat is pressed in more, although the weight has remained the same.

The reason for the impression of the mat is that Edition print. As the experiment shows, it is from the surface depending on how an object rests on a surface - if the area becomes smaller, the contact pressure increases.

If we put the stone back on the larger side surface and weigh it down with a second stone, the mat will be more depressed. The contact pressure is also of the Weight force of the property.

Overall, we get the following formula for the contact pressure:

$ p_A = \ frac {F_G} {A} $

Here $ p_A $ is the symbol for the contact pressure, $ F_G $ the weight and $ A $ the surface that is on the contact. The Edition print so is the force, the per area works. It is important that only the part of the force is considered that is perpendicular acts on the surface, i.e. $ F \ perp A $.

Contact pressure - examples

Let's get back to snowshoes. Imagine a person who has a mass of $ 80 ~ \ text {kg} $ wants to go hiking in the snow. We first calculate how great the pressure is with normal hiking boots. Normal hiking boots have an area of ​​approximately $ A_1 = 0.0113 ~ \ text {m} ^ {2} $. Let us set the mass, the area and also the acceleration due to gravity $ g \ approx 9.81 ~ \ frac {\ text {m}} {\ text {s} ^ {2}} $ for the weight force $ F_G = m \ cdot g $ a, we can calculate the contact pressure:

$ p_ {A1} = \ frac {80 ~ \ text {kg} \ cdot 9.81 ~ \ frac {\ text {m}} {\ text {s} ^ {2}}} {0.0113 ~ \ text {m} ^ {2}} = 69451.33 ~ \ frac {\ text {kg}} {\ text {m} \ cdot \ text {s} ^ {2}} = 69451.33 ~ \ text {Pa} $

In the last step we still have the unit of the contact pressure, the Pascal, with the help of its definition $ 1 ~ \ text {Pa} = 1 ~ \ frac {\ text {kg}} {\ text {m} \ cdot \ text {s} ^ {2}} $.

Now we calculate the contact pressure that the same person with snowshoes would create. Snowshoes are flat shoes that increase the surface area. We assume a tread area of ​​$ A_2 = 0.1000 ~ \ text {m} ^ {2} $ and insert the values ​​as before:

$ p_ {A2} = \ frac {80 ~ \ text {kg} \ cdot 9.81 ~ \ frac {\ text {m}} {\ text {s} ^ {2}}} {0.1000 ~ \ text {m} ^ {2}} = 7848 ~ \ frac {\ text {kg}} {\ text {m} \ cdot \ text {s} ^ {2}} = 7848 ~ \ text {Pa} $

That is only a tenth of the contact pressure with normal hiking boots. With snowshoes it is possible to run even in very deep fresh snow without sinking in.

By the way: camels use a similar effect to avoid sinking into the desert sand. Your feet are designed in such a way that they become wider when you step on them, thus increasing the surface area you step on.

This video

In this video you will learn what the contact pressure is and how to calculate it. You will also learn why it plays an important role in snowshoes and camels. In addition to text and video, you will find on the topic Edition print Tasks with which you can test your new knowledge right away.