# Why is unit of pressure (psi) used to determine things like bite force of an animal?

Whether in tv documentaries or journals, whenever they talk about an animal's bite force, it's measured in PSI anytime imperial units are used (ex: National Geographic, NIH Journal). Many even seem to highlight the fact that it's pounds per square inch.

But when it's used in SI unit context (ex: BBC Earth), it's written in Newtons, a measurement of force.

It seems to me that a force should be measured in a unit of force and not pressure. Additionally, pressure alone would seem to tell us nothing since area is never specified, thus unknown force.

So why is a unit of pressure so dominantly used to indicate an animal's force?

For example, this science daily shows the croc having a bite force of 3700lbs. But then other sources (along with every other web search) will say 3700psi. This National Geographic article even writes psi side by side with newtons as if it was pound-force.

"Bite force" is just one of many anachronism that pop up in science when you look for them. Physics and the other sciences haven't always been such sticklers for accurately naming phenomena, both because they didn't care about rigor as much and because they didn't know better. In this case, the problem isn't bad units, it's a bad name. It really should be called "bite pressure" since it is the pressure that the teeth exert on whatever it is the animal is biting.

Strong jaw muscles and well shaped jaw bones will increase the amount of force that can be exerted, while sharp teeth decrease the biting surface, creating large pressures. When trying to pierce, cut, or grind, pressure is the important factor, not force.

• The part that convinces me that it is bad unit and not bad name is that PSI and Newtons are used side by side and converted like pound-force as if they're the same type of unit. On the flip side, there are very few source using kPa in context of bite force and they're all in Newtons. Sep 1, 2017 at 0:11
• You may be on to something, but I don't think it contradicts my answer. It comes back to the rigor argument that I started out with. While kg-force and lb-mass may be usable units, there is no way to convert force to pressure the way it is being when talking about psi of bite force. Sep 1, 2017 at 0:20

Suppose I hang a weight from a string, and the weight is just heavy enough that any heavier a weight would break the string.

What would you expect from two strings side-by-side? Well, we could hang another weight, exactly as heavy as the first, from that other string: but again, any heavier on either string and you'd expect it to break. So two strings hold twice the weight, three strings hold three times the weight, and so on.

This is not in general true if you make the string half as long. Half as long, it will hold about the same weight -- a little more due to the mass of the string that's now missing -- before breaking. What changes is that if we measure the distance it stretches, between its unladen and loaded length, generally half the string will only stretch half as far.

So to get actual "material properties" we have to divide the stretch by the rest length -- this is called the "strain" of the material -- and we have to divide the force by the cross-section area -- this is called the "stress" of the material. And then we find that materials actually have a stress-strain graph which we can plot, and this result will hold for other lengths and other cross-sectional areas. Also we find out that the material breaks at certain stresses.

Now if we are talking about whether a predator's teeth can crunch your bones, it turns out that a bunch of similar physics is in play and your bones will crunch at a particular stress -- just where the strings were in tension the bones being crunched are in compression. These material properties are often different! A good example is concrete; when you're pushing it together you're usually trying to push rocks into other rocks, and that is very difficult; when you're pulling it apart you're usually trying to pull apart the cement that binds those rocks, which is much easier. So it's much more breakable under tension than under compression. (Meanwhile a string almost can't be compressed; it squirms out of the way when you try. Maybe with a hydraulic press...)

So pressures are more useful to know how much crunch something can provide, and they are the bite-force proper divided by this area of teeth or mouth or so that it acts over.

• Thank you, I understand why pressure would be a useful unit. But, I feel it doesn't answer the question being asked of why a unit of pressure is used for force. [cont...] Sep 1, 2017 at 2:08
• For example, this science daily shows the croc having a bite force of 3700lbs. But then other sources (along with every other google search) will say 3700psi. This NatGeo article even writes psi side by side with newtons as if it was pound-force. Sep 1, 2017 at 2:08
• @Grumpy Yeah, they measured both bite force and tooth pressure for that study; Nat Geo misquoted. The force was 3700 lb-f, the tooth pressure was 360,000 psi for that saltwater croc. Sep 1, 2017 at 3:40

Thank you for asking this question! You are absolutely correct. Bite force (any force) should be reported in units of pounds-force or Newtons. Better yet, I would model the jaw as a hinge and report the torque around the hinge since the force would likely be higher closer to the back of the jaw. I suspect when bite forces are measured, the analyst is reporting the result from a pressure gage directly without multiplying by the area of the sensor. I see this all the time in mechanical testing, especially if a hydraulic-based sensor is used, but it sure would be nice to understand this!

I absolutely agree with Jon Hilden. Bite strength is a torque. The limit of an animals bite must be a function of jaw muscle strength and jaw geometry, unless you're about to crush your teeth, which involves a whole different set of numbers than those given for bite strength (e.g., 30,000 psi vs. 150 psi). A couple of examples:

1. If you take an animal and dull it's teeth (e.g. give it 100x more tooth surface area), it wouldn't have 100x as strong a bit.
2. If you want to bite something really hard, you bite it with your back molars, because you can clamp down far harder (far more 'bite force') with the teeth closer to the hinge of your jaw.
3. You an chip your tooth on a rock. Dentin has a compressive strength in the 10,000s of psi, which you are therefore achieving because the surface area between tooth and rock is very close to zero. If it were actually psi rather than torque, that small area would mean you'd be applying almost no force.

I think the Newton force argument is specious because it doesn't account for area of impact. PSI does, so would seem more valid. The terms pressure and force are defined differently but may be defined as identical, if desired. In other words, why can't pressure and force be defined in similar or identical terms? If the sharp area of a tooth is 1/1000 of a square inch and the tool used measures psi, one must multiply by 1000 to get the actual pressure being applied that is gripping or piercing flesh and/or bone during a bite! Newtons does not seem to be adequate to measure or describe the differences of the pressure or force of a bite done with the varying sharpnesses of different teeth! How does it account for sharp edges? PSI is obviously the most valid measurement, if the area of the teeth doing the bite is taken into account because then all bites will be reduced to a the same quality of measurement, how many pounds of pressure (or force, current definitions of force in Newtons being negated as invalid for bite measurement if area of application isn't taken into account) per square inch being the singular rule. We now have a true standard that can be accomplished, assuming the working area of a sharp tooth or teeth, or of the point of a knife or sword can be accurately measured. I think it can. I am surprised that scientists versed in physics still have this problem that is related directly to having logical thought, the most important ability anyone called a scientist must have. I believe bite force results will be much higher in reality than is currently measured. And I saw a crocodile rear bite measured at @10,000 psi for a Nile Croc, not even a Salty! I can't imagine the posterior teeth pressure of Sperm whale or Great White during an active hunting bite have ever been accurately measured.

As an engineer, bite FORCE being reported in psi has ALWAYS grated on my nerves (like the proverbial fingernails on a chalk board).

That said, torque (admittedly a much more difficult quantity to measure than a raw force - particularly where an animal's ability to bite, esp. one of the 'wild' variety, is concerned) is IMO the ONLY truly accurate/repeatable means by which the ability of any given animal's bite effort (which, for lack of a better term, I'll refer to as "attainable bite effort" or 'ABE') can be quantified.

As such, the ABE (i.e torque generated) within the strength and structure of any animal's jaws is the ONLY reasonable means by which a definitive, reasonably repeatable, and comparative result makes any realistic sense.

Trying to utilize FORCE (whether lb-f or N) or PRESSURE (whether psi or kPa) leaves FAR too many unknown variables (e.g. area over which the force was measured; the distance from jaw 'pivot' to the point of neasurement, etc.) to be of any REAL value (definitive value at least). They are also far too easily manipulated (e.g. in order to minimize, exaggerate or otherwise 'skew' one or more results).