Originally posted by Spl Eagles
I don't see why a speed of MPH isn't possible. You just move at x MPH along the circumference.
Technically you can do that, but it's very bad practice and adds to the work required.
Because revolutions are a measure of degrees or radians over time. MPH is a relative linear measure which a ferris wheel doesn't have. MPH is a linear measure of speed relative to a stationary object. The whole wheel on a ferris wheel turns, so relative to anything on the wheel, MPH is 0. The center is revolving at RPM, but the car isn't going away from the center, it's revolving around it.
If you are on a train, traveling in a circle, you can have MPH in relation to the stationary track. However, the track isn't stationary on a ferris wheel.
You want the whole wheel to turn with a certain period, and that is measured in Revolutions/Time. 1 RPM is more accurate and easier to work with than 1 MPH. 1 RPM = 360 degrees a minute or 6 degrees a second. Knowing that, you know how to make it turn at the right speed. Using MPH, in order to determine degrees per second/minute/hour you first have to determine the circumference of the wheel, using that and the speed given you determine how long it takes to make one rotation. Either way, making something rotate requires a rotation speed, so you can say 1 RPM and know right away how fast it's turning, or you can say 1 MPH and have to do math to determine RPM.
Stating a wheels rotational speed in a linear terms will also cause problems if you decide to change the wheels diameter. Your RPM's will go up or down depending on whether you made it larger or smaller.
TL;DR You technically can express it that way, but it's not the proper way to refer to a revolving object and almost always involves more work.
Moving on to the practical application. He is trying to fake a ferris wheel by using a coaster train, so in this case MPH is acceptable because the track is a stationary point of reference meaning the transports move at MPH. However, he is also asking for someone to animate a 3DS file for him, which will require RPM. Personally, I think the time spent trying to fake it would be better spent learning to model it in 3D yourself so you don't have to ask people to do it for you.