[Hugh R]

In the motion picture and television industry we used to let stuntmen do a stunt of attaching a line to one stuntman the line would go over a pulley attached to the underside of an aerial boom lift and then the other end of the line would go to a larger piece of rope with two stuntmen about the same weight would be on a ten foot A-frame ladder. The two stuntmen would grab the rope and jump off the ladder, and the other guys flies up in the air. In the Industry this is called a "Hong Kong Pull".

We no longer all the use of aerial boom lifts for two reasons; 1) OSHA doesn't like them because they aren't rated for lifting people and 2) we think they're underrated for this purpose. A stuntman wanted to know if I had any "math" to prove their underrated, let alone illegal. So here is what I came up with from a few websites that seem to be in agreement with each other, but intuitively I think differently, and at the end I'd like the PPOT Brain Trust to weigh in.

Stunt man 2 (mass 2) is 175 lbs and is the one going up. The two stuntmen on the ladder each weigh 175 lbs. [350 lbs for mass 1]

Force=mass x acceleration, lets assume a frictionless pulley, rope and air drag.

So, (M1 +M2)G = M1*G -M2*G

Rearranging: M1*G-M2*G/M1+M2

350*g-175*G/350+175

11,200-5,200/525=11.42 which is actually acceleration.

mass of M1 and M2 is 525 and multiplying that by actual acceleration of 11.42 we get 525 x 11.42= 6,000 lbs. The boom lift is rated at a distributed load of 500 lbs. In this scenario the load that the Pulley, and hence the boom lift would see is 6,000 pounds; 12 times the rated load. This is why we require the use of a rated crane.

This whole thing can therefore be summarized by the following:

Force = M2*G-M1*G; or 350*32 ft/sec/sec-175 * 32 ft/sec/sec or

11,200-5,200= 6,000 lbs.

Here is what I don't get, you subtract the mass that is rising from the mass that is descending. By my logic, they are both loads that the pulley "sees". One is rising against gravity and one is being pulled down by gravity. Wouldn't you want to add the two loads times G; rather than subtract one from the other?

Hope this makes sense.