Force = Pressure x Area

Which one is Goofy?

It’s true: MOST is a big Disney fan.  While visiting the original Disneyland in Anaheim last weekend, we saw the “Floating Ball” in Tomorrowland.  You can see it — without paying admission! — on this YouTube site:

http://www.youtube.com/watch?v=YVtbuWqmukg

The stone “ball” is said to weigh six tons.  We can’t verify that, but we can tell you that it may be so — at least from a hydraulics standpoint.  Let’s look at the basic equation, Force = Pressure x Area to see what we mean.

For the ball to spin as freely as it does, it must be completely lifted off its pedestal by the force of the water flowing up beneath it.  If the force downward — the weight of the ball — exceeds the force of the water flowing upward, then the ball would never get off the ground.  If the ball really weighs 6 tons, then the force of the water must be at least 6 tons, or 12,000 pounds.  No way!

But let’s do the math.  We didn’t measure the ball, but it looks like about four or five feet in diameter, and not all of that is in contact with the water.  Let’s say that we have a diameter of 30 inches that is in contact with the water.  The area that is in contact is about 700 square inches.  Now let’s guess that the water pressure here is typical for any household, about 35 psi.

Force = Pressure x Area, so 35 pounds per square inch x 700 square inches means that the upward force of the water is about 24,000 pounds — twice what we would need to lift our six-ton ball!

We hoping that this exhibit from Tomorrowland will make your Adventureland of the state exam become your Fantasyland of passing the exam.  Remember that MOST is the Main Street to get you there!  Or are we putting too much pressure on you?

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