Simple Machines

A team’s score for the Simple Machines event is based entirely on their answers to a 100 question (150 point) written test. Students may need to examine and/or manipulate the object at each station in order to answer the questions.

The pizza cutter was used as an example of a tool where the fulcrum is considered to be the wrist or elbow of the user.  Therefore, it is a third class lever.

You are correct that the answer key is wrong.  The intent was to have two pulleys with the same Mechanical Advantage, but where one changes the direction of force but the other does not.  However, the example was not well thought out.

 The test you have from 2014 was the test used for the district competitions.  It was provided to coaches, for reference only.  It should have been destroyed after reviewing with the students in preparation for that year's regional competition.

In the case of screws, students may be asked to evaluate how a change in diameter, or pitch will change the mechanical advantage.

When determining which simple machine a tool is categorized as, we use the original design of the tool.  That means that a screwdriver, a tool designed to insert screws, is a wheel and axle.

 Many people also use a screwdriver as a lever, for instance to open a paint can (I’ve done it myself), but that is not the design intent of the tool.  So, it would not normally be considered a lever on our test.  If the screwdriver tip does not act as a wedge, i.e. moving two surfaces apart, it should not be considered a wedge.  All of that said, I understand that the average 3rd grader will not grasp the nuances so I generally avoid anything that could be misinterpreted this way.  If I use a screwdriver I will include a picture showing it being used for its original purpose.

Tools such as spatulas which are designed to separate something, say a pancake from the surface of a griddle, could be considered wedges because they are generally held at an angle.

The wheel and axle is a device that consists of a wheel attached to a smaller axle so that these two parts rotate together and force is transferred from one to the other.  The center of rotation is the same for both the wheel and the axle.

 When a rotating force is applied to the wheel, the axle also turns.  Because the wheel moves through a greater distance than the axle, the axle moves with more force.  The work performed, defined as the force multiplied by the distance, must be the same for both the input and the output.

 For a screwdriver, the handle is the wheel, and the shaft which fits into the screw head is the axle.  A screwdriver where the handle is the same diameter, or only slightly larger, than the screw’s diameter would require more effort to turn than one where the handle was much larger.  So, there is a mechanical advantage to using a screwdriver with a handle larger than the shaft.

 A light bulb can be considered a wheel and axle because the force used to screw it into a socket is applied to the globe, which is usually larger in diameter than the screw base.  The force on the globe is transferred to the screw base.  A light bulb with a large globe will be easier to tighten into the socket than one with a smaller globe.

 This is distinctly different from a wheel on a wagon, for instance, where the wheel turns but the axle does not.  In this case the wheel merely serves to reduce or eliminate friction.

Students will not use pi in calculations.  They only need to know when a formula for determining mechanical advantage includes Pi.  Students will only be asked to estimate or approximate mechanical advantage.  Scratch paper will be available.