Simple Machines

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Simple Machines
Physics & Study Event
Forum Threads 2015 2014
Tests 2015 2014
Test Exchange Archive
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Question Marathons 2014
Division B Champion Piedmont Middle School
This event was not held last year in Division C

Simple Machines is an event that is currently being held in the 2015 season.

Contents

Event Overview

Simple Machines is an event in which you take a written test and use a homemade device (it must be a first class lever) to determine an unknown mass. The event allows you to take in any resources you wish to take in.

The included simple machines are levers, pulleys, wheels and axles, inclined planes, and wedges. Screws are supposed to be left out, but they may be included.

A simple machine is a mechanical device for applying force. They are useful because they can make physical jobs easier, by changing the magnitude or direction of the force, or the distance that the force is applied over.

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The Written Test

The written test will include topics such as IMA, AMA, efficiency, work, torque, power, and history. A free response answer will be marked as wrong if significant figures are not taken into account, although some graders may give partial credit. Units should always be included.

Conservation of Energy

Conservation of Energy states that in a closed system (no outside influences), energy is neither lost nor gained.

Energy can be changed. Work can be converted into heat through friction. Work can be converted into sound. Heat can be converted into work, through engines. But energy is never lost nor gained. This is an important concept to keep in mind.

IMA

IMA stands for Ideal Mechanical Advantage.

Ideal Mechanical Advantage is the number of times a machine would multiply an effort force if there were no friction. For example, if a machine has an IMA of 2, that means that the force applied was doubled by the machine (once again assuming no friction). If the IMA of a machine is 1/2, that means that the force applied was halved by the machine. If the IMA is 1, that means the force applied stayed the same.

However, machines with a high IMA are not always desirable. The higher IMA a machine has, the less distance it moves the output based on the input force. If a machine has an IMA of greater than 1, then it is moving the object less of a distance than it would have. A machine with an IMA less than one will move an object a further distance, at the sacrifice of force.

The IMA is equal to the input distance over the output distance. IMA had no units.

This balance between force and distance can be written in the following formula:

W = f x d

Where Work equals the force on the output multiplied by the distance the output moved.

AMA

AMA stands for Actual Mechanical Advantage.

AMA is nearly exactly the same as the IMA, but it takes into account things like friction and drag.

The AMA is equal to the output force over the input force. AMA has no units.

Efficiency

Efficiency tells you how friction affects the output work.

Conservation of energy says that energy is constant. However, some of your work is converted into heat.

Efficiency is equal to work out over work in, normally expressed as a percent. Efficiency is always less then 100%. Another way to express efficiency is AMA over IMA, which amounts to the same thing.

Work

Work represents how much mechanical energy is being transferred from one object to another.

Work can be negative. For example, if object 2 is transferring mechanical energy to object 1, then the work done by object 1 is negative. Emphasis should be put on the difference between work done on and work done by. The work done on an object refers to the mechanical energy transferred to that object, whereas work done by an object refers to the mechanical energy transferred from that object to another.

Work is equal to force times the distance that force is applied over. The SI unit of work is the Joule. A Joule is equal to a Newton times a meter.

Torque

Torque is basically the rotational form of force.

Torque is equal to the force times the distance the force is from the fulcrum (moment arm). The fulcrum is what the body rotates about.

If the force is expressed in Newtons and the distance is expressed in meters, then the units of torque would appear to be Joules. However, in order to put emphasis on the fact that torque is not work, the units would actually be Newton meters.

The net torque on a body whose rotational velocity is not increasing is 0.

Power

Power represent how fast energy is being transferred from one object to another.

Power is equal to energy over time to transfer that energy. The SI unit of power is a Watt, which is equal to a Joule divided by a second.

History

For this section of the test the internet is your best friend. Do some research, make some notes.

Significant Figures

See Significant Figures for info on significant figures.

The Device

In this part of the test you use a homemade first class lever to determine the ratios of three masses. The masses will be given to you at the event; you are not allowed to bring your own mass.

The goal is to determine the ratios of mass A to B and B to C as quickly and accurately as possible.

Materials

What is probably the strongest material to build your lever out of is metal. Unfortunately it is hard to work with, and you may end up with a more wobbly structure then if you had made it out of wood. It is also more expensive. So, if you have never worked with metal before, it may be a better idea to use wood.

If you use wood make sure not to use a wood that is too light (otherwise it might bend), and make sure that your lever is straight. You can always tape a ruler onto your lever, so it might be a good idea to cut the wood yourself just to make sure it's straight.

PVC pipe will bend and may break if the masses are too heavy. However, you might want to take your chances with this material so that you can use a sliding fulcrum design.

Designing your device

Practically all possible designs involve a stand to support your lever. Try and build a sturdy stand.

Here is a list of some possible designs:

1. Get a straight bar as your lever and put its center on an object acting as the fulcrum. This is far by the simplest, but it is not suggested. The lever may slide and get unbalanced, an it will probably be hard to work with. Plus, you will have to put the masses on top of the lever, meaning that it will be hard to tell what mark the objects are on.

2. Build a stand and then hang the lever off of it. This is a good design because it has minimal friction and thus is accurate. Because of it's low friction it may be time consuming, however, because you will spend a long time getting it adjusted just right so that it is balanced.

3. Build a stand and put a bar on top. Drill a hole through your lever and slide the bar through that hole. Effectively it is just like design 1, however it is easier to work with and it is higher off the ground. It has more friction and thus is not as accurate, but it is faster to use then design 2.

4. Build a stand and hang a ring from it that the lever can slide through. This design is the fastest. However, because you are moving the fulcrum and getting more torque on one side then the other, it is also the least accurate. If you use light wood or PVC pipe this error can be minimized.

Some tips

  • Keep in mind that a lever is not balanced when it is level, it is balanced when it is not rotating.
  • Make your lever as long as is allowed. The longer it is, the higher of a mass ratio you can deal with, and the more ready you are.
  • Remember that you don't know how large the masses are going to be. Its probably a good idea to keep your lever high off the ground so that you can be ready for any situation.
  • Practice.

Types of Simple Machines

There are six types of simple machines.

Pulleys

A simple pulley is a string looped around a fixed axle. If a load is attached to one end of the string, you could pull it up, but would require the same amount of force to pull the load over the pulley as it would if you had no pulley. You could balance it by putting an equal load on the other side. Take a look at this diagram:

Simple pulley.png

Pulleys can be more useful than that when there is more than one pulley wheel to a pulley system. If the load's weight is spread between two strings, there is an IMA of 2. Here is an example of a pulley system using two pulley wheels, with an IMA of 2:

Pulleyima2.png

Imagine that you are pulling on the string with the little arrow. If you pulled it 2 feet downwards, the hook will rise 1 foot. This is because there are two strings that lift the hook and only one string that is being pulled. This means that you can lift a heavy load as if it were only half as heavy. Now, imagine if three strings were attached to the hook. The IMA would now be 3. But remember only count the supporting lines, or the lines that are being pulled up. In the diagram, the line with an arrow is being pulled down, so it is not counted in the IMA.

In conclusion, the easy way to tell what the IMA of your pulley system is to count the number of pulley wheels.

Pulleys are easily acquirable at your local hardware store. To make the circular part of the pulley, you could also use wheels from Lego sets (without the tires, of course).

Inclined Planes

"Inclined plane" is just a fancy word for a ramp. To find the IMA of an inclined plane, divide the diagonal length of the ramp by the vertical length of the ramp.

Ramp.png

The IMA of this ramp is 60/5=12. Since the circular weight (the one being lifted) is less than 12 times heavier than the square weight, the square weight is able to lift the circular weight. (The picture is not to scale)

Wheel and Axles

A wheel and axle system can be used in many ways: to transport something, to turn something else on the axle, or to turn another wheel and axle.

Wheelaxle.png

This diagram shows how to find the IMA of a wheel and axle simple machine. Basically, the IMA of a wheel and axle is the radius of the wheel divided by the radius of the axle.

Levers

A lever is, in basic terms, a rigid bar resting on a pivot point, or the fulcrum. To work the lever, effort must be applied to move the load, which is usually opposite the effort, across the fulcrum. There are three types of levers.

  • First Class
    • The fulcrum is in the middle, the effort is on one side, and the load is on the other. An example of a first class lever would be a seesaw or a crowbar.
  • Second Class
    • The fulcrum is to one side, the load is in the middle, and the effort is on the other side. An example of a second class lever would be a wheelbarrow or a nut cracker
  • Third Class
    • The fulcrum is to one side, the load is on the other side, and the effort is in the middle. An example of a third class lever would be tweezers or your elbow.

1st, 2nd, and 3rd class levers respectively

To find the IMA of a lever, divide the distance between the fulcrum and the effort by the distance between the fulcrum and the load. Therefore, if the fulcrum is moved closer to the load, the easier it is going to be to lift a load, and the higher the IMA is.

Wedges

A wedge is a simple machine that separates two objects by converting downward force to sideways force. Imagine a triangular block of wood and two adjacent bouncy balls. If you put the triangular block between the two bouncy balls and pushed down, it would separate the two bouncy balls. The IMA of a wedge is how far the wedge went down divided by the distance it separated the two objects.

Screws

Screws convert rotational force to vertical force. A screw is essentially inclined plane wrapped around a central axis. An example would be a drill or a screw for construction. The formula for IMA is 2(pi)L/p, where L is the length of the handle and p is the distance between adjacent screw threads.

Other Events that Involve Simple Machines

External Links

How Simple Machines Work

Calculating IMA for lever, wheel and axle, and pulley

Calculating IMA for incline, wedge, and screw

Simple Machine Sample Exercises

Trial Event Rules

Franklin Institute page on simple machines

Wisc-Oline Mechanical Advantage Presentation