Aerodynamic Drag – The Physics Hypertextbook
When taking into account other factors, this relationship becomes However, as the person's velocity increases, the magnitude of the drag force increases until. Drag depends on the density of the air, the square of the velocity, the air's viscosity Since we are dealing with aerodynamic forces, the dependence can be. In fluid dynamics, drag is a force acting opposite to the relative motion of any [ edit]. Educational materials on air resistance · Aerodynamic Drag and its effect on the acceleration and top speed of a vehicle.
He weighed less but had a smaller frontal area and so a smaller drag due to the air.
Drag Force and Terminal Speed - Physics LibreTexts
The size of the object that is falling through air presents another interesting application of air drag. If you fall from a 5-m-high branch of a tree, you will likely get hurt—possibly fracturing a bone. However, a small squirrel does this all the time, without getting hurt. You do not reach a terminal velocity in such a short distance, but the squirrel does. The following interesting quote on animal size and terminal velocity is from a essay by a British biologist, J.
You can drop a mouse down a thousand-yard mine shaft; and, on arriving at the bottom, it gets a slight shock and walks away, provided that the ground is fairly soft. A rat is killed, a man is broken, and a horse splashes. For the resistance presented to movement by the air is proportional to the surface of the moving object. So the resistance to falling in the case of the small animal is relatively ten times greater than the driving force.
Then we find that the drag force is proportional just to the velocity. Because each of these objects is so small, we find that many of these objects travel unaided only at a constant terminal velocity.
To move at a greater speed, many bacteria swim using flagella organelles shaped like little tails that are powered by little motors embedded in the cell. If we compare animals living on land with those in water, you can see how drag has influenced evolution. Fish, dolphins, and even massive whales are streamlined in shape to reduce drag forces. Birds are streamlined and migratory species that fly large distances often have particular features such as long necks.
In humans, one important example of streamlining is the shape of sperm, which need to be efficient in their use of energy. Geese fly in a V formation during their long migratory travels. This shape reduces drag and energy consumption for individual birds, and also allows them a better way to communicate.
- Aerodynamic Drag
The objects are placed in a uniform airstream created by a fan. Calculate the Reynolds number and the drag coefficient.
6.4: Drag Force and Terminal Speed
Unfortunately, the frictional force on a body moving through a liquid or a gas does not behave so simply. Where'd that extra symbol come from?
Who put that C in there and why? Let's run through all the symbols one at a time, explain their meaning and how they relate to pressure drag.
In essence, let's take the equation apart and put it back together again.
More density means more mass, which means more inertia, which means more resistance to getting out of the way. The two quantities are directly proportional. Exactly what we mean by this is subject to debate. To me, and in the context of this model, area is the cross sectional area projected in the direction of motion. I would further simplify this by calling it the projected area.
Take the cross section of the object in the direction of its motion. This is the area of the tube of fluid that must be cast aside to let the object pass. This is the most logical thing to call the area, but not everyone agrees with me. To some, the word "area" refers to the area of contact between the object and the fluid.
This also makes sense, but not in the context I've described above. Surface area is not important when one is dealing with pressure drag, but it is important when dealing with viscous drag — drag caused by layers of the fluid sticking to the object and to one another.
More surface area means more of the object is in contact with the fluid, which means more drag. Viscous drag is just as real as pressure drag, but I don't want to deal with it right now. I hope that this is self-evident. An object that is stationary with respect to the fluid will certainly not experience any drag force.
Start moving and a resistive force will arise.
Get moving faster and surely the resistive force will be greater. The hard part of this relationship lies in the detailed way speed affects drag. According to our sensible model derived from Bernoulli's sensible equation, drag should sensibly be proportional to the square of speed.