Viewing 7 posts - 61 through 67 (of 67 total)
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  • #4040
    Aeronaut
    Participant

    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.

    #4041
    Duke Leto
    Participant

    Ugh.

    Technically, the terms of the agreement would be me allowing you to be happily ignorant. If Eric was talking about a ton of mass in his slide he was being colloquial. A ton is a measure of weight, never of mass, and a ton is 2000 lbs not 64000 lbs. I cannot make that any clearer and if you persist in asserting that a ton is 64000 lbs, then you are a fool.

    #4046
    Brian H
    Participant

    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

    #4048
    Lerner
    Participant

    Duke, no name-calling is allowed on this forum.

    Also, you are wrong. All units of weight–kg, metric tons, pounds, grams, etc. are also used as units of mass. Objects have mass and thus inertia, no matter where they are. An object that weighs one ton on Earth has a mass of one ton wherever it is, including in space.

    #4049
    Brian H
    Participant

    Lerner wrote: Duke, no name-calling is allowed on this forum.

    Also, you are wrong. All units of weight–kg, metric tons, pounds, grams, etc. are also used as units of mass. Objects have mass and thus inertia, no matter where they are. An object that weighs one ton on Earth has a mass of one ton wherever it is, including in space.

    Exactly. 2000 lbs. weight on Earth is the result of having 2000 lbs mass (colloquial/usual use of “lbs.”) The point of this discussion is disputing the remarkable statement that 1 ton of mass represents a “weight” of 32,000 lbs. in, e.g., the hold of a 747-400F air freighter.

    #4050
    Duke Leto
    Participant

    Lerner wrote: Duke, no name-calling is allowed on this forum.

    Also, you are wrong. All units of weight–kg, metric tons, pounds, grams, etc. are also used as units of mass. Objects have mass and thus inertia, no matter where they are. An object that weighs one ton on Earth has a mass of one ton wherever it is, including in space.

    Well, I’m sorry for being a prick, but it’s pretty justified in this case. Look at the whole of the thread.

    I certainly understand that colloquial usage recognizes the mass/weight equivalence at sea level. Aeronaut is arguing, as best as we can understand him, that since you have to multiply Kgs by ~ 10m/s^2 to get the weight (force) in newtons, you have to multiply tons (mass) by 32 ft/s^2 to get the weight in tons (weight). And thus that since you gave the weight of the DPF as “2 tons of mass”, it actually has a “weight” of 64000 lbs. And since this is in total contravention of basic HS Physics and common sense, we’re naturally a bit consternated.

    #4053
    Aeronaut
    Participant

    Lerner wrote: Duke, no name-calling is allowed on this forum.

    Also, you are wrong. All units of weight–kg, metric tons, pounds, grams, etc. are also used as units of mass. Objects have mass and thus inertia, no matter where they are. An object that weighs one ton on Earth has a mass of one ton wherever it is, including in space.

    Eric, Brian, and Duke, thank you for clearing this up. After I reread the last 3 posts several times I finally saw how that works. As you’ve probably guessed, I passed on high school physics. I also passed on trig and calculus, while we’re at it. A 4,000 pound (more or less) FF power block makes a lot of my calculations a lot brighter. Thanx for persisting.

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