Mechanics

# Fluid resistance

The body inside the liquid moves in the opposite direction to the movement of the liquid

In the study of Physics, fluids correspond to substances whose internal cohesion forces are very small. Therefore, they usually have no shape of their own and need a container to contain them.

In the figure above, suppose there is a moving object inside a liquid or gas, that is, inside a fluid. Experience shows that the body is acted upon by a force that opposes the motion of the object, which we call the fluid resistance force .

Although this force has a complex behavior, we can, in an approximate way, determine it through the following mathematical equation:

r = kv n

In the above equation we have:

v – modulus of speed
n – constant that is 1 for low speeds; and for high speeds it is 2
k – constant that depends on the nature of the fluid, the shape of the body and the area of ​​the largest cross section perpendicular to the motion

This resistance force can appear at different times. In many cases, it is interesting to decrease the value of ka in order to decrease the value of the resistance force. For example, for a snow skier, it is interesting that he always performs his route crouching, because, in this way, the area A is reduced and, thus, the air resistance is reduced.

On the other hand, aerial skiers, also known as parachutists with boards, lie down with open arms in order to increase area A and, thus, increase air resistance.