0
$\begingroup$

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.

$\endgroup$
1
$\begingroup$

"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.

$\endgroup$
  • $\begingroup$ 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. $\endgroup$ – Grumpy Sep 1 '17 at 0:11
  • $\begingroup$ 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. $\endgroup$ – Johnathan Gross Sep 1 '17 at 0:20
0
$\begingroup$

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.

$\endgroup$
  • $\begingroup$ 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...] $\endgroup$ – Grumpy Sep 1 '17 at 2:08
  • $\begingroup$ 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. $\endgroup$ – Grumpy Sep 1 '17 at 2:08
  • $\begingroup$ @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. $\endgroup$ – CR Drost Sep 1 '17 at 3:40

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.