So we have treis, who can explain why the scale’s reading should change, and we have Mythbusters, who, what, had an actual helicopter in an actual airtight box on some actual scales?
If the helicopter is rising or descending at a constant speed, then the net force on the helicopter is zero, and the apparent weight of the box is unchanged. If the helicopter is accelerating, regardless of whether or in what direction it’s moving, there will be a net force on it, and the apparent weight of the box will be changed. Of course, in order for the helicopter to go from just sitting on the floor to rising at a constant speed, it has to have accelerated somewhere in between, but that acceleration could be arbitrarily small. It’s possible that, in the Mythbusters experiment, the acceleration was just too small to be noticeable on their scale.
This is what I was thinking, only didn’t state anywhere near as clearly.
It seems to me that as the helicopter accelerates upward or downward, there will be a temporary change in the weight on the scale.
You can do this yourself just by squatting on a bathroom scale, then watching as you straighten your legs. Unless you go pretty slowly (so the scale doesn’t register the extremely low acceleration) you will see the weight go above the original weight, then below the original weight as you stop moving upward, then restabilize at the original weight. The faster you straighten your legs, the more the deviation from the original weight.
The average of all that will come out to the original weight, but the weight has to vary if the ‘copter accelerates, doesn’t it?
I want to know what happens when the bowling ball hits the rotors.
Frankly, you won’t catch ME heaving bowling balls upward in a box with an operating helicopter, I can tell you that! Especially when I’m apparently trying to manage a barbell at the same time. Give it a rest, son!
Exactly. The key is that weight is a force. Your mass on the scale will not change (unless you’re nearing the speed of light, which I assume is not part of the quiz). Whenever speed or direction changes, acceleration is non-zero, so a la Newton - force = mass * acceleration - force (and weight) has to change. The questions about throwing the ball and lifting the barbell follow another Newtonian law, that for every force there is an equal force in the opposite direction. So when the ball is accelerated (a force), then that force has to be transmitted downward through the scale and is measured as increased weight.
We get confused because we usually equate weight and mass, and the two are fundamentally different.
I think that doofus **Princhester ** has missed one of the subtleties of question 1b, in assuming steady state. I agree with those that say for question 1, if the helicopter is accelerating or decelerating the scales will not read X but something more.
They had a toy helicopter - RC - in a closed box roughly the size of a shipping container that sat all four corners on scales. The scales were electronic, accurate to fractions of an ounce (I don’t remember the details but I was impressed at their range and resolution) and they had one on each corner of the container. I think they said their sampling rate was 10 samples/sec.
All this was run back to a laptop that had the recording software.
The first toy helicopter impacted the side of the box and made an expensive mess. The second took off, very slowly ramping up the throttle. Hovered for maybe 5 or 10 seconds, and dropped back. Except for some noise in the signal, the weight of the container was constant.
This was all an extension of the birds in the truck legend. The first iteration of this was about 36 pigeons in the container. The weight of the birds was visible on the graph when they were perched. When released, the weight of the box fluctuated as the birds flapped around the container but, as the mythbusters pointed out, there was as much noise above the baseline as below. The box never got anywhere near as light as the actual weight of the birds.
The helicopter was tried afterward since it’s accelleration and velocity was nice and steady. It was an attempt to prove the noise in the signal with the birds was about the birds changing direction and bumping into stuff.
This all confirms what is said above. The changes in velocity of the object inside the box, the acceleration and decelerations, are visible exterior to the box but the steady state activity in the box are not observable outside it.
I didn’t see the MythBusters experiment, but as an “industrial scale guy,” I can say that most scales capable (size-and-weight wise) of supporting a helicopter would most likely be calibrated in increments too large to detect small changes in force.
Some large(ish) scales (and scale indicators) are available that can detect smaller changes, say around .1 pounds. The prices on these suckers are pretty hefty, and the scale system has to be carefully built to isolate extraneous outside forces, making them very difficult (but not impossible) to use in modern industrial and laboratory applications.
I routinely get called out on service calls to companies that have scales calibrated so fine that even moving around them (within a few feet, more-or-less) causes radical fluctuations in indicated weight, just from the air displacement of the moving person.
So, just to make sure: we’re all agreeing with treis?
We better be.
Mythbusters did the the helicopter one, with a modle helicopter, and a very sensitive scale. There was no change on the scale.