Stuntman Physics Question

[widebody911]

Originally Posted by VINMAN

Is that a laden or unladen stuntman?.... About three fitty.....

African or European?

[masraum]

Originally Posted by jyl

"lb" is not a unit of force .....
"lb" is a unit of mass. Force is mass x acceleration ( F = m *a ) and the unit of force is "lb * ft / sec^2". When we use "pounds" as a shorthand expression for force, we are implicitly leaving out the relevant acceleration..

Actually, you're both correct.

a slug is the Imperial unit of mass.

The slug is a unit of mass associated with Imperial units. It is a mass that accelerates by 1 ft/sec2 when a force of one pound-force (lbf) is exerted on it. Therefore a slug has a mass of 32.17405 pound-mass or

Also, in certain cases there is "Pound-mass" lbm and "Pound-force" lbf.

A pound-force is the amount of force required to accelerate a slug at a rate of 1 ft/s2

[javadog]

[QUOTE=jyl;5670003My copy of Halliday & Resnick is falling apart so badly that I need to buy a new one.[/QUOTE]

I can sell you mine. 1981 edition. Dog hasn't chewed it yet.

JR

[jyl]

Interesting, I never heard of "slugs" as a unit of mass. I'm guessing that is more of an engineer's term, and also that engineers are more likely to refer to force in "pounds".

I am more literal-minded. force = mass x acceleration, so for me the clearest unit to use for force is kg * meter / sec^2 (Newton) or lb * ft / sec^2.

[javadog]

Originally Posted by jyl

Interesting, I never heard of "slugs" as a unit of mass.

Page 64 in Halliday & Resnick...

It's a British thing. I never could figure out why they couldn't use "our" system, or the much better metric one. They always have to do something different....

JR

[Pazuzu]

A pound is not mass.

Period, end of story.

A kilogram is mass. A kilogram times a meter is a Newton, which is force. A Newton of force applied to wrench one meter long produces a newton-meter of torque.

That same torque is 0.738 pound-foots, which is the same as 0.738 pounds of force applied to a wrench a foot long.

Pounds are force, as are newtons. When you're in space, your "weight" is zero pounds. Your MASS does not change.

[M.D. Holloway]

Just use Mexican stuntmen and you won't have to worry worry about OSHA or anything else for that matter...

[jyl]

You're referring to pound-force (448 Newtons) rather than pound-mass (0.434 kg).

Pound -- from Eric Weisstein's World of Physics

To avoid the confusion between pound-mass and pound-force, maybe we should redo the calculation in metric units. I get the force on the pulley is 1986.85 Newtons, and the minimum rated load for the platform 202.53 kg, which is 467 lb (pound-mass) w/ rounding. The combined mass of the three stuntmen is 227.85 kg.

[masraum]

I much prefer metric units for this sort of thing.

[onlycafe]

Originally Posted by javadog

Page 64 in Halliday & Resnick...

It's a British thing. I never could figure out why they couldn't use "our" system, or the much better metric one. They always have to do something different....

JR

whitworth.

[David]

It seems to me the bigger issue with OSHA is life lines and their connection points have to be able to handle 5000 lbs. I've looked this up on the OSHA site a few times trying to find out their exact definition of a 5000 lb rating and haven't found it. So I've taken it to mean 5000 lbs to bring the support to yield stress although I try to have safety factor of at least 3 if possible.