Cardboard animal jaws as examples of levers.
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.
This demo is the realization of the differences between primitive and advanced jaw design, as discussed in Kane & Sternheim, Physics p.76 (Wiley, 1980)