Reply To: Richard Branson – Virgin Pledge

[Brian H]

Aeronaut wrote: Duke, I’ve had the distinct impression that nobody’s reading Eric’s slide, just responding. It clearly states 2 tons of mass. I wish Jimmy hadn’t linked to the slug- its purpose has something to do with continuity between units in the English system. Not needing the slug is another powerful reason for using Metric, in addition to round number gravity and continuity of units.

Here’s 2 familiar formulae that demonstrate mass as I understand it- Weight =mass * gravity, and Momentum=mass * velocity. Weight is not a constant, but mass is. That’s why Eric gave the “weight” in terms of mass. Aboard a spaceship, nothing would have any weight, but try to compute length of burn to achieve specified delta-vee without knowing the mass of everything from the contents of your propellant tanks on down to what’s in the galley and holding tank.

If that doesn’t make it clear, we had best agree to disagree, and simply move on, in my opinion.

Spaceships are irrelevant, though if a ship was accelerating at 1G, each mass of 1 ton would “weigh” 2,000 lbs. inside the ship, just as it does on the Earth’s surface under 1 G acceleration (gravity). Your factor of 32 is a complete mystery to all of us. Again I ask: where did you get it?

Repeat: mass and weight are identical in identical units under 1 G acceleration.

Sorry, no agreement to disagree about something this basic. I’d as soon agree to disagree about the heliocentric solar system being accurate.

P.S.
Inspecting your latest post again, I see you are using weight=mass*gravity. Since gravity is 32ft/sec², that would explain your factor of 32. But the units are force, in that case. 32ft-lb/sec² IS 1 lb. weight-force. On Mars 1 lb. mass would experience a downward force of (guesstimate) 12ft-lb/sec². Etc. On a balance scale it would still show as 1 lb., but on a sprung weight scale would show as about 6 oz.

Here’s a fun discussion of the differences between pounds and ‘poundals’, etc: http://www.physicsforums.com/showthread.php?t=259981