0 Comments


An apple weighs about 1 newton, the world record for jet engine thrust is 570,000 newtons. And the Saturn V rocket that launched people to the moon had a thrust of 33,360,000 newtons But how can we measure forces this big accurately? Well, we need to ask this guy. Hi my name is Rick Seifarth I’m a physical
scientist in the Mass and Force Group. Am I supposed to look at the camera? Sure. Okay. You can look at anything you like. Rick manages a dead weight machine
that can apply 4,448,222 newtons. Why such a strange number? Because that’s
exactly equal to 1,000,000 pounds of force. There are twenty fifty thousand
pound increments in this machine 20 times 50 equals 1,000,000 pounds. Cumulatively that’s 4.45 mega newton. Elsewhere in the world, are there larger
masses that people have calibrated? I ask that question to every visitor
we get and I’ve not gotten.. one response that says: yeah I know
where bigger stuff is calibrated. So if I really want to go out
on a limb and brag I’ll say these are the largest mass objects ever calibrated. Anywhere The machine works like this, below ground
are the 20 carefully calibrated masses. Their weight is used to calibrate
force sensors also called Force Transducers, in the lab upstairs. This is a one million pound capacity 4.45 mega newton, and this is 13.3 mega newtons,
three million pounds force. This is the biggest machine of it’s type
in the world, and obviously one of a kind. One of these, will be placed on the compression head right here. Then, a hydraulic ram in the attic starts to raise the green lifting frame And once the force transducer
contacts the red loading frame, well then it starts lifting the weights downstairs. And as the lifting frame continues
to rise, more and more of the 50,000 pound weights become
suspended by the force sensor. And since the weights create an
accurately-known force, the readouts from the force transducer
can be precisely calibrated. These devices are then sent out into the field. Well literally, there’s a test stand that’s
set up with one of these gizmos- a force transducers – embedded
in the test stand somewhere. And so the rocket is fired up, perhaps ramped up, ramped down and those forces are monitored. You know, in the old movies you’d hear: “go to 104% of power.” Well, how do they know it’s 104% of power? Because somebody’s measured it somewhere. When it comes to minimizing uncertainty, these guys aren’t messing around. This piece right here is approximately 50,036.27 pounds of mass. Approximately. Approximately, yes! [laughter] Think about that for a moment. Each of these pieces has a mass equivalent to ten minivans, and their exact value is known to within several American nickels. Just a few American nickels worth of difference? Yeah. That tiny uncertainty is measured with respect to the very definition of the kilogram using K20 – that’s the United States fundamental mass standard. This is done by comparing combinations of known weights, starting with K20, with larger unknown weights, gradually working up to larger and larger masses. For example, here a single 5-kilogram mass is compared to two 2s and a 1. At some point in this process, they convert and start working in pounds, going from 50-pound masses to 500 pounds, and then 2500 pounds. And then, using a scale in the floor, to reach 10,000 pounds and then 20,000, and finally 30,000 pounds. Combinations of these huge weights are then used to calibrate the 50,000-pound masses which form the weight stack. And we have to have that, because what we sell is the vertical force vector that’s generated by these weights hanging in a gravitational field. We sell that with an uncertainty attached to it of 0.0005%, five parts per million, so that means at full one million pounds of applied force, we guarantee that to be accurate – if you will – within five pounds. Not only does that mean the masses have to be accurately calibrated, it also means the gravitational acceleration at this location has to be taken into account. It’s actually slightly less than Earth’s standard gravity, so an additional 600 pounds are required. Plus, the buoyant force must be counteracted, since these masses displace 125 pounds of air, a further 125 pounds must be added to reach a million pounds of force. What strikes me as kind of amazing is that, like, this machine needs to exist, in a way. When I would have thought about it, like how would Boeing measure its forces, I would have thought OK, they just, you know, calibrate a device that works up to X but then they can, you know, generalize. Well, generalize – you know what I mean, like – Yes! There’s an axiom that says “one physical test is worth a thousand expert opinions.” And that has proven itself time and again, particularly in the world of physical testing. If you’re getting on an airplane that somebody has built, are you willing to accept a 10% uncertainty on these numbers or do you want it to be – the uncertainty on these measurements to be down in the mud?

Leave a Reply

Your email address will not be published. Required fields are marked *