Drag

Find out about two different types of drag: pressure and friction.

Transcript

Hi, my name is Ashley, and I'm an Explainer at the National Air and Space Museum's "How Things Fly" gallery, and today we're going to be experimenting with drag. So, what is drag, and how does it affect how things fly? There's a lot of different types of drag. One of them is friction drag.

When air molecules come into contact with an object, it creates a lot of resistance. The air resistance that opposes an object's forward motion, is called friction drag. Friction drag is independent of mass, weight, and size, but it does depend on the amount of surface area that's exposed.

Over here I have a sheet of paper and a golf ball. I'm gonna drop them at the same time, from the same height, so they should fall to the table at the same time, right? Because gravitational acceleration is the same on all objects, 9.8 meters per second squared. So, let's see what happens.

Did you see that? The golf ball hit the table first. Now, why was that? The golf ball is very aerodynamic in shape. So, it's going to slice through all those air molecules much faster than the sheet of paper. The paper on the other hand is very flattened shape and it has a greater surface area. Because of this, as the paper is falling through the air, there's gonna be a lot of air molecules pushing up against the sheet of paper, creating a lot of drag, and preventing it from falling to the table as fast as my golf ball.

So what can I do to make my piece of paper fall to the table just as fast as my golf ball. Well, we're gonna reduce the surface area and make it more aerodynamic and shape. So now, let's see what happens this time. Did you see that now the piece of paper and the golf ball fell to the table at the same time, and that's friction drag.

Another type of drag is pressure drag. In order to understand pressure drag a little bit more, let's look at the shapes of these three setups. First, we have a sphere shape. Next, we have a flat disc shape. And last, we have a teardrop shape. All three of these shapes have the same cross-sectional diameter, the same mass, and I'm gonna send them at the same speed. So, which object do you think is going to stop first? Let's wait and find out.

As you can probably tell, our flat disk is already slowing down to a stop. So what's happening here? As air is moving across this shape, there's going to be an early separation of the airflow, which is going to cause a large turbulent wake.

The sphere is the next one to slow down to a stop. The sphere is slightly more aerodynamic in shape than our flat disk. So, as air is moving across its going to stay attached through the shape longer.

So as you can see, the teardrop shape is the last of the three shapes to stop moving. That's because, as the air is moving across the teardrop shape, it's going to stay attached even longer than it did for the sphere. And because of that it's gonna have the smallest turbulent wake. The teardrop shape is the most aerodynamic of all three of these shapes.

Now why do we want a small turbulent wake. The reason for that is that the smaller the turbulent wake this smaller the pressure difference between the front side of the object and the back side of the object. And now you're familiar with two types of drag that affect how things fly.