Lol, nice one.
I still hear kips every once in a while from an old timer, it’s 1000 pounds or pounds-force.
Useful for whom?
I have a little digital scale which internally measures who-knows-what, but the readout is in grams. In fact, I cannot think of a common item or specification that uses that unit. (And some common items use weird units, like strength specified in kilopounds per square inch.)
ETA
I don’t know if they are that old. I was thinking of how you grab a welding electrode and there is a specification on it like E7018, where “70” means 70,000 pounds per square inch.
For thrust-to-weight ratios for rockets and aircraft engines, for instance. For such applications, it’s neat to have a unit of force that can easily be put into relation to a unit of mass.
I suppose one has to massage the units somehow. I read a book where the author decries the practice of specifying specific impulse in seconds. Explicitly using “kiloponds” relies on a magic number in the same way.
You are assuming circular orbits, but I believe that for most speeds between 0.333 mm/s (0.013 inch/second) and 0.47mm/s (0.019 inch/s) at ten cm separation the orbits will be elliptic and the speed and the separation will not be constant.
Well, for each speed in that range you can calculate a circular orbit that has that exact speed as orbital velocity. But for sure it’d also be possible to have elliptical orbits whose orbital speeds vary within that range.
Assuming you can get underneath it and exert a constant force of 890 N (200 lb) with your legs for a full second while braced against something like the ISS floor, F=ma says you’d be able to accelerate a 1000kg floating sphere to .89 m/s (2.9 ft/s) over this distance (~1.5 ft).
I built a neat device that measures mass directly. Of course there’s no way to intrinsically measure mass, but the relationship F=ma is reliable at low speed. My first revision applied a certain force to an object, and I then measured the acceleration, which together gave me the mass. For the second revision I decided to hold acceleration constant and measure the force instead, which worked just as well. But the third revision was the best, where I decided to make use of an ambient constant acceleration field, meaning I only had to measure force, which dramatically simplified the device. I call it a “scale”.
A 1000 kg object will weigh 1000 kg on Earth or the Moon or anywhere else. The scales have to be calibrated to the local acceleration, but that’s no different than calibrating them for any other reason, like how many newtons it takes to deflect a spring by so many millimeters, etc. And that calibration will be wrong in another environment, like one at a different temperature.
It should also be mentioned that, according to some models, gravity does behave differently at sufficiently short distances (where “sufficiently short” might plausibly be as much as a few millimeters). But this is very difficult to test with Cavendish-like experiments, because the closest together you can get two spheres is the sum of their radii, and for the force to be detectable, the spheres have to be fairly big.
Of course, “difficult to test” is not the same thing as “impossible”, and so some researchers have come up with very clever ways of arranging the masses, and detecting very small forces, and reducing all of the many sources of noise, and so on, in order to test gravity at shorter and shorter length scales. So far, they haven’t detected any deviation from Newton’s Law, but there’s always the possibility that the next experiment, at shorter length scales and with more precise measurements, will.
We really, really need stable white dwarf matter for this and a number of other applications.
The question is whether gravitational mass (the m in the formula for gravitational force) and inertial mass (the m in the formula for momentum or kinetic energy) are exactly the same, and if so, why. Within the limits of experimental error, these two properties appear to have exactly the same value, but do we know of any underlying physical reason that they should?
It’s necessary to preserve the equivalence of gravity to acceleration in General Relativity.
Well, yes, it’s one of the postulates of General Relativity. That’s why we care that it’s true. But is there any reason why it should be true?
Of course, that’s partly a meaningless question, because even if we did say that some particular thing were the reason, we could just ask why that thing is what it is.
It would be quite easy to create a small horizontal displacement. If we assume the rope is 8’ long, a horizontal force of 10 pounds would displace the ball about half an inch. And it only stops there due to gravity.
A better thought experiment would be to consider the way a massive object moves when a small force is applied and neither gravity nor static friction resist the motion. An example would be adjusting the position of a large boat that’s alongside a dock. Provided wind and current are not factors and you are (as you should be) content with slow motion, the force you must apply is surprisingly small - 20 lbs will easily suffice for a 20-ton boat.
A neat real-live example:
The tungsten weight is “only” 373 kg, but that’s still quite substantial. A few posters correctly estimated Don Pettit’s mass as 75 kg based on the relative displacement.
Why the heck does the ISS have that much dead weight aboard; what’s it for?
Well, they call it a “counterweight”. I imagine that they move it around to various places to make sure the station’s center of mass is where they want it to be.
I could see that they’d want microgravity experiments to be as close to the center of mass as possible to minimize the Earth’s tidal pull.
When they boost ISS to a higher orbit, they need to have the center of mass in a particular place.
Somewhat disappointingly, it’s actually a counterweight for their exercise machine. You can’t tell from the vid, but it’s actually a stack of plates that’s been shipped up over a few flights.
- Tungsten Plates – The metal plates will serve as the counter mass, a weight that balances another weight, for the space station’s exercise device technology. The plates are designed to integrate with the E4D (European Enhanced Exploration Exercise Devices). A total of 14 plates will launch in two additional upcoming flights.
- Tungsten Tray Lower Counter Mass – Each tray will house two tungsten plates and will be assembled in orbit. The trays will stack together to become the lower counter mass for the station’s exercise device technology.
Tungsten Plates – A total of 14 tungsten plates will serve as the counter mass of the Vibration Isolation & Stabilization System designed to integrate with the European Enhanced Exercise Device.
According to the design whitepaper:
The purpose of a VIS system is to protect the on-orbit spacecraft from crew-imparted exercise loads through attenuation, and to provide sufficient platform stability for a crewmember to exercise safely and effectively.
Kinda surprising that “crew-imparted exercise loads” could meaningfully affect the station, but I guess they’ve done the math and they need some attenuation system.