eli5, Why do people refer to bite strength in PSI ?


So I was looking at another answer and it indicated that polar bears have a bite of 1200PSI.
This seems crazy, even a small force crushing something between the edges of two teeth would generate a huge PSI value.

Why do they use PSI, shouldn’t it be lbs-force (well OK it should be newtons, but that’s another issue)?

In: 3

PSI is just a unit of measure for pressure. The SI version would be Newtons/meter^2, or Pascal (From P=F/A). I think that would be kg-force/meters^2. It’s just not common to say kg-force/meters^2. Plus the values aren’t understandable to humans as easy as PSI.

That’s why we use “atm” or PSI instead of Pascal. 1atm is 10325 pa. Your news anchor ain’t gonna say “lmao, we have 9064 pascal’s today.”

Why would you care about only the force? The pressure absolutely matters. If you have 1000 lbs of force spread out over 1000 people, then you’re not going to do much damage.

It is a pet peeve of mine when they describe it as “bite force” in units of pressure, but pressure is a relevant unit.

You’re right, a small force crushing something between 2 sharp teeth would generate a high PSI value. And that’s kind of the point. The ability to concentrate force in a small area is more relevant to how dangerous or effective a bite is than the total force. For example, a piranha has a tiny mouth and not that much overall muscle to move it with, but the amount of force compared to its small size lets it easily bite off chunks of food. A blue whale certainly has a large overall amount of force available to close its *huge* mouth, but because the force is already o it across such a large area, it is not talked about as an especially powerful bite.

Indeed it should be lb force. It’s referred to as PSI incorrectly for similar reasons that people use lbf and lbm interchangeably. Almost no one giving these numbers out cites their sources either.

If you see PSI for bite force probably just assume it means lbf that they can apply to an actual single square inch of material.

You’re right that it’s a really sloppy use of terminology/units, but there’s valid reasons to care about both the pressure (psi or whatever) *and* the force (lbf or whatever).

Your ability to cut through materials, which is what we usually do with teeth, is almost entirely related to pressure. Whether you’re crushing (compressive strength…molars) or shearing (shear strength…incisors) or warping (yield strength…some of each), what matters is that the teeth apply more pressure than the relevant material property. So you can directly compare bite “force” in pressure to the compressive/shear/yield strength of the material you’re biting and have a reasonable estimate for whether your teeth will do anything useful to the material.

But teeth have another important function for *some* animals…they’re a way of grabbing and tearing things. In that case, pressure isn’t really that relevant, you don’t need to cut the material with your teeth, you just need to hold on to it. But the *force* you can apply is really important. A mouse with a bite “force” of 20,000 psi can totally cut my skin but isn’t going to pull my finger off because their bite force is probably only a few lbf. On the other hand, something with big blunt teeth (moose?) might only be able to put down 500 psi bite “force” but they can easily tear my arm off because their total bite force is probably up in the thousands of pounds (I am speculating on values here, don’t overthink the numbers, the point is that pressure and force are good for different things when biting).