Investigating the effect of forces through the application of simple machines, such as the bow and arrows used by Torres Strait Islander Peoples or the spearthrowers used by First Peoples of Australia
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This elaboration provides an opportunity for students to explore how an understanding of the effects of unbalanced forces acting on an object is the basis for the development of hunting technologies used by Torres Strait Islander Peoples and First Peoples of Australia.
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Across mainland Australia and islands of the Torres Strait, a range of tools to increase the velocity and accuracy of projectiles was developed or produced by Aboriginal and Torres Strait Islander peoples. On much of the mainland, various styles of spear-throwers were developed and refined. Spear-throwers were effective as they provided an extension to the human thrower’s arm. In the Torres Strait and the nearby parts of northern Queensland bows and arrows were used alongside spear-throwers. In this teacher background information, the physics of the spear-thrower, as well as its cultural significance to Australia’s First Nations’ peoples are explored in detail.
The spear-throwers used by First Nations’ Australians were crafted out of hardwood, such as mulga, and ranged in length from approximately 50cm to 100cm. Spear-thrower design, including its dimensions, was carefully refined and optimised for each individual user and carefully matched to the projectile type used. They have a variety of shapes and widths, but typically taper towards the gripping end and have notches coated in resin cut on either side at one end to form a hand grip. The other end is narrow and has a ‘peg’ that is designed to fit into a socket or notch at the base of the spear, more technically called a dart. The ‘peg’ can be either integrated into the structure or made from an animal’s tooth or claw and attached by sinew and resin. Spears up to three metres in length rest on a spear-thrower and are gripped by the spear-throwing hand. The ‘peg’ is crucial in transmitting force to the spear allowing it to be propelled at a higher velocity than can be achieved by hand alone. The ‘peg’ also focuses the propelling force enhancing the accuracy of the throw.
A spear-thrower is an example of a lever, which is one of the six mechanisms classified as simple machines that are designed to alter the size or direction of a force on an object. The others are wheel and axle, pulley, inclined plane, wedge and screw. The lever is basically a rigid bar to which a load force and effort force are applied, and which rotates around a fulcrum or pivot point. There are three orders or classes of levers, differing in the relative positions of their load, effort and fulcrum.
Throwing a spear using a spear-thrower involves a complex interaction and co-ordination of the thrower’s skeleton and muscles. Since the spear-throwing process involves a series of levers, defining which class of lever applies is often debated. When viewing the system of spear-thrower and spear in isolation, it can be classified as a second-class lever, where the fulcrum as well as the load are situated at the peg. When including the arm and wrist action, a spear-thrower can be classified as a first-order lever which positions the fulcrum between the effort arm and the load arm. The effort force exerted by the thrower’s hand pivots around the wrist, acting as the fulcrum to move the spear positioned at the longer end of the thrower. The distance from the wrist to the shorter (proximal) end of the spear-thrower is very much smaller than the distance from the wrist to the other (distal) end of the thrower. Consequently, as the farther end of the spear-thrower moves through a much greater distance than the nearer end but in the same amount of time, it moves at a much greater speed. The spear-thrower acts as a speed multiplier, which comes at a cost to the mass of the projectile that can be propelled in this way. Spears thrown with the aid of longer spear-throwers are therefore typically lighter than those that are designed to be thrown by relatively shorter spear-throwers. This also means that spears thrown by spear-throwers need to be lighter than spears thrown by hand. The development of light-weight, balanced projectiles exploited the full potential of the spear-thrower and established a new method of casting spears at greatly increased velocity.
When forces on an object are balanced, the object’s motion stays constant. The unbalanced force applied to a spear by the spear-thrower causes it to accelerate from rest. Once launched, the unbalanced forces acting on the spear are air resistance, causing it to slow down, and the Earth’s gravitational attraction, causing it to fall.
Many examples of spear-throwers are elaborately decorated indicating their cultural significance. Spear-throwers from central Australia were light-weight and easily portable and designed to be used as a multipurpose tool. The spear-thrower could be useful as a shield, and could also be used as a fire saw, a digging stick, a receptacle for mixing ochre, a carrying dish, or as a percussion device during ceremonies. Some examples from central Australia have a sharp piece of rock embedded in the handle and held in place with spinifex resin. This enabled the spear-thrower to be also used as a tool for cutting, chiselling, shaping, and sharpening.
By studying the physics involved in the use of spear-throwers, students are given an opportunity to appreciate First Nations’ peoples’ knowledge and exploitation of levers and to develop and consolidate their own understanding of the effects of unbalanced forces acting on an object.