Hammers are interesting tools. Much of their shape derives from simple physics: kinetic energy is a function of the mass of an object and its squared velocity. (To be precise, kinetic energy is one half of the mass times the squared velocity.) A corollary of this principle is that even small objects travelling at supersonic speeds, like a bullet weighing only a few grams, possess a lethal quantity of kinetic energy.

For a hammer to do its job in pounding in a nail, it needs sufficient energy to force the nail into wood. That energy can come from a large-but-slow-moving mass or a smaller and faster mass. Because energy is a function of the square of the velocity, faster is better. That suggests that hammers should be relatively light weight but move fast.

To increase the speed of the hammer head, hammers use a lever arm: the handle. Holding a handle near its end and twisting your wrist, the long handle increases the velocity of the hammer head, allowing you to hammer home the point — er — hammer home the point of the nail.

So why don’t hammers have long handles, say 2 to 3 meters (about 6 to 9 feet)? Several reasons. First of all, a tool that long would be difficult to use in tight quarters. But such a long lever arm would have another consequence as well: it would be difficult to hit a small target, like a nail, when any imperfection in our aim is magnified over such a large distance. So hammers represent a compromise between a long handle (for velocity) and a short handle (for accuracy and compactness).

Now the best way to use a hammer is to hold it near the end of the handle and use both your arm and wrist to speed the hammer toward its target. People can bend their wrist backwards, but chimpanzees can’t. So it is harder for them to make the precise corrections needed to make the hammer squarely hit the nail. Viki does not even attempt this method of hammering. Instead she holds the hammer near the head and pounds her nails in with low velocity. Watch carefully and you will see Viki use different parts of the hammer to strike the nail: the head of the hammer, the claw, and the cheek of the hammer where the head attaches to the handle. She is most successful when using the cheek, probably because it is larger and thus requires less precision in the strike. Also note that, in spite of her variety of grasps and contact parts, Viki always strikes the nail with the iron head, and doesn’t try to use the wooden handle as a hammer.