Jaw Models
levers - torques - muscular forces
What it shows:
The biting force of an animal depends upon the magnitude, direction and point of application of
forces exerted by the jaw muscles. A mammalian jaw exerts a greater force than does a reptilian
jaw despite a more delicate joint structure, because evolution has improved the physics of eating.
How it works:
The demonstration consists of two dimensional cardboard models of reptilian and mammalian
lower jaws (see figure 1). Both are about 30cm in length. They are pivoted at the correct
jaw point (the origin of force R), with muscular forces provided by a rubber band, attached
where the muscle should be, at the origin of force M.
The reptile jaw is simply a bar, with the muscle pulling upwards at a point close to the joint.
Because of this, the force applied to the food -B is less than the force M exerted by the
muscle (taking moments about the pivot). The limiting factor in biting power is the
strength of the joint as a downward force R is exerted on it; this will also determine
how large a muscle the animal can safely evolve.
In the mammalian jaw, the force M is applied further from the joint and a second muscular
force (the temporalis muscle) T acts from an additional piece of high bone called the
coronoid process. If the lines of action of forces T, B and M all intersect so that their
resultant torque is zero, no force is applied at the joint so R is zero (or close to zero if the
forces don't exactly align). A smaller joint mechanism suffices, and there is no limit to the
jaw muscle size. Enjoy that steak!
figure 1a. reptilian Jaw
figure 1b. (typical) mammalian jaw
Setting it up:
The jaw is pivoted on a lab clamp mount on a lecture bench. The lecturer applies the muscular
force using the rubber band. Food is supplied in the delicious form of a foam rubber cube.
Comments:
This demo is the realization of the differences between primitive and advanced jaw design,
as discussed in Kane & Sternheim, Physics p.76 (Wiley, 1980) Rating *