Doing a lot of driving this week, and since the interstates I’m driving (I-49 and I-10) are deathly boring, I have a lot of time to think. Being a physics nerd, I do a lot of thinking about physics. This trip the physics thing I’m thinking about is tire pressure.
Tire pressure made a brief blip during the news a while back when it as a contributor to engine efficiency made a side appearance in the presidential election in regard to global warming. Climate gets done to death around here though, so let’s talk about pressure as what it really is fundamentally: force per area. Well, in thermodynamics it’s more useful and more interesting to think of it as the derivative of energy with respect to volume, but we’ll save that for later.
Stand bare-footed on a piece of styrofoam and it may well hold you up if it’s reasonably strong or if you’re thin. Put on some high heels (guy readers may wish to get some privacy first) and do the same thing, and you’ll punch a hole right through it. In both cases you’ve applied the same quantity of force. The difference is that instead of being spread over the whole area of the sole of your foot, it’s concentrated into the much smaller area of the spike. That what pressure is – force divided by area.
Your car tires have to hold up your car, and they’re typically inflated to something like 30 pounds per square inch? A typical car might weigh, I dunno, a thousand pounds. Probably considerably less if it’s a small car, but that’s way out of the area of my expertise. Now we’d like to recover the area of tire required to hold up the car. Pressure is force per area, so to get area we need force divided by pressure. Pretending the car is a thousand pounds, that’s a total of around 33 square inches worth of rubber in contact with the ground holding up your car, providing every but of the acceleration and steering you rely on. 33 square inches is a square around 6 inches, so it’s not much.
And that’s why tire care is important. Because there’s not a whole lot of it between you and sliding into a tree.