Physics of Fitness Fridays - Biking and Running, What a...Drag?

Today, let’s talk about what happens as you move through the air. Sometimes it’s not with the greatest of ease!

We live in an ocean of air. As a fluid, air moves around us and impacts how much effort we need to move.

When we run (or bike, etc), air flows past us and exerts a force on us as it does so. This force is called the drag force (D). It is dependent on our cross-sectional area A (think of a 2-dimensional area of your body that’s perpendicular to your motion), and our speed v.

The drag force is proportional to the square of your speed. So if you try to run twice as fast, the drag force you feel will be a factor of four greater. (If there’s ever a reason for running slowly, you just found one.)

The drag force is proportional to your area. That’s why a skier will crouch down on their skis to go faster, or, in my case, I’ll crouch lower on my bike to reduce drag.

If you want to maintain a constant speed, you need to counteract the drag force that is slowing you down. Thus, you need to exert an equal and opposite force to the drag force to cancel it out. Exerting this force requires power, defined as P = Fv. If we plug in for the drag force, we find that the power needed is dependent on the CUBE of the speed. That means that if you want to double your speed, you need to increase your power by a factor of eight! Holy smokes!

If this sounds exhausting, well, it beats the alternative. You could run in a vacuum to escape the drag force, but then, well, you’d be dead. So look on the bright side! The drag force just makes you stronger!

Everything I talked about above is just moving through STILL air, that is, with no wind. But what happens if you have a headwind? Or a tailwind? I’ll talk about THAT next week…