In a machine or mechanical device, there is a concept of work done by the human on the machine (input) and work done by the machine (output).  For example the work done by a cyclist in pedaling is input work to the bicycle while the work done by the bicycle in overcoming friction to move forward is output.

So, input work is the work that we do as humans.  We abbreviate input work with a Wi.  

Output work is the work that the machine does.  We abbreviate output work with a Wo.

Mechanical advantage is the advantage gained by the use of a mechanism in transmitting force.  More specifically it is the ratio of the force that performs the useful work of a machine to the force that is applied to the machine.  

The equation for mechanical advantage (abbreviated MA) is to the right.  There are no units for mechanical advantage because it is a ratio.  The units for force, Newtons, cancel each other out.

The ideal mechanical advantage represents the change in input force that would be achieved by the machine if there were no friction to overcome.  The ideal mechanical advantage is always greater than the actual mechanical advantage because all machines have to overcome friction.

The equation for ideal mechanical advantage (abbreviated IMA) is to the right.  There are no units for ideal mechanical advantage because it is a ratio.  The units for distance cancel each other out.

The term ideal machine refers to a hypothetical mechanical system in which energy and power are not lost or dissipated through friction, deformation, wear, or other inefficiencies.  This is a machine that only exists in our factious perfect physics world.  As you can see in the equation to the right, for an ideal machine, the input work equals the output work.  You can only use this equation if the physics problem states, "for an ideal machine."

In the real world, the work that we input into a machine will always be less than the work that the machine puts out.

The efficiency of a machine is defined as the ratio of output work to input work, or the mechanical advantage to the ideal mechanical advantage.  The efficiency of an ideal machine is 1 or 100%.

Depending on what information you have in the problem will determine what equation you will use.  One equation is not better than the other.

Click here for the paper that I use in the video.  As you watch this video, pause it and try the math on your own.  When you have your answer, start the video and see if you are right.  It is important to know where YOU make your mistakes so you can correct them for next time!

Remember there are more practice math problems under Chapter 10 Resources.  Practice the math and see if your answer matches my answer.

Click on the picture to take you to the practice problems that I will be going over the next time we have class.  Remember, you need to try the problems on your own to see where you make your mistakes if you want to learn how to do the math problems on your own.

Be sure to head over to google classroom and fill out the exit pass.