What is Air Resistance? + Terminal Velocity

Reading Level: 5th Grade

Reading Time: 2 min 35 sec

We’ve talked about gravity a few times now. One of the things I’ve brought up is the gravity of the earth and how its acceleration is equal to 9.8 m/s2.

No matter where you are on earth, gravity is basically exactly the same. So why then does a bowling ball hit the ground before a feather?

It all comes down to air resistance.

Think of it like I kind of friction.

Better yet, if you’ve read the post about how much the sky weighs you might remember how the sky can be thought of as a less dense version of the ocean. The air is denser towards the bottom (the ground) and less dense near the top (space, I guess)

When you sit in a pool and wave your hands side to side there is resistance, or friction.

Drop a heavy object in the pool, like a bowling ball, and it’s able to break through that resistance much quicker than, say, a feather.

Also just like in a pool, air resistance acts in the opposite direction compared to the movement of the object, like always walking directly into the wind.

So, what exactly causes changes in the amount of air resistance?

Size, shape, surface, and speed. The 4 S’s.

This is where things get weird (if you don’t already know)

Since gravity is constant, meaning the acceleration is fixed, the mass of the object should have absolutely nothing to do with how fast it falls.

If that makes sense to you, then congratulations because that’s exactly right.

But what does effect the speed of the fall? Well, let’s look at the surface area.

Ever use a plastic grocery bag as a parachute for an action figure? No? Just me? Okay, well, anyway, let’s imagine we have to of those bags in our hands right now.

Better yet, if you have two at home go and grab them. I’ll wait.

One is all squished up into a ball-ish thing. The other one is just all normal and baggy. Drop them both. Which drops faster? The ball-ish one, right?

Sure, the gravity acting on both of them, or the downward force, is the same, but the friction force of air resistance is different for both. It’s less for the ball and more for the one that is all spread out.

As happens, this actually brings up another term-topic-thing.

That thing is Terminal Velocity.

When you stick your hand out the window of the car you can move it around pretty easily. Keep it out the window as the car accelerates and suddenly it’s more difficult to push against the incoming air.

As things speed up, so does the amount of air resistance.

I just dropped an imaginary bowling ball from a very tall building because, I don’t know, science. Gravity is causing it to accelerate down to the ground. As it falls faster and faster, the air resistance increases too. The weird thing is that eventually that upward force of air resistance will start to balance out the downward force of gravity on the bowling ball. So much so that, yes, it reaches a net force of zero. Remember the whole moving objects sliding friction thing?

When the bowling ball reaches this point, it’s no longer accelerating, or changing speed. Its velocity is constant. That, my science friends, is what we call terminal velocity.

Think of it as an objects falling speed limit that depends entirely on those 4 S’s I mentioned.

Like what I’m doing here with Science People!? Support the project at www.patreon.com/devonbowker

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