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        riddles >> easy >> Weighing helicopter in a tank
        
(Message started by: Kozo Morimoto on May 26^th, 2004, 2:21am)

Title: Weighing helicopter in a tank
Post by Kozo Morimoto on May 26^th, 2004, 2:21am

You have a large tank made out of perspex/glass etc which is transparent and air sealed.

You have a toy remote control helicopter (electric) inside the tank.

The whole setup is put on a scale and the weight is measured.

You switch on the chopper by remote and make it take off and hover inside the tank and the chopper is not touching any of the sides/top/bottom of the tank.

What is the weight reading on the scale now?

Title: Re: Weighing helicopter in a tank
Post by rmsgrey on May 26^th, 2004, 2:39am

Guess I'll hide this so other people can have a go. I think I'm right, but this looks like one of those questions where there are multiple plausible explanations, most of which are wrong...

::[hide]
While stationary, the helicopter is operated upon by two equal and opposite vertical forces - one downwards from gravity, and the other upwards from the air around it. The air around it therefore has a downwards force exerted upon it, which, once things stabilise, translates to a downwards force on the floor of the box. So, once the situation stabilises, the weight should be what it was initially.
[/hide]::

Title: Re: Weighing helicopter in a tank
Post by THUDandBLUNDER on May 26^th, 2004, 8:30am

As it is a closed system, the weight cannot change.

Title: Re: Weighing helicopter in a tank
Post by grimbal on May 26^th, 2004, 9:37am

Except maybe some heat and light. But it wouldn't be measurable.

Title: Re: Weighing helicopter in a tank
Post by Noke Lieu on May 26^th, 2004, 9:56pm

so there's this guy driving a truck, right... full of birds. Gets out and hits the side with a big stick. Drives a bit further, jumps out and hits the side of the truck with his stick, jumps back in.......

Actually, reminds me of the old experiment to determine the metabolic rate of hummingbirds.
Put a humming bird in a bell jar, with a current of air so it has to hover. Then reduce the partial pressure of oxygen unitl it can't flap its wings well enough. And falls to the bottom.

Title: Re: Weighing helicopter in a tank
Post by rmsgrey on May 27^th, 2004, 3:12am

on 05/26/04 at 08:30:11, THUDandBLUNDER wrote:

As it is a closed system, the weight cannot change.

Except if portions of the system accelerate with respect to each other. If the helicopter were in free-fall, the perspex container would not be supporting it in any way, so the weight would be decreased by the weight of the helicopter (until it hit the floor at which point the weight would be increased by the helicopter's weight plus the braking force)

Similarly, if you turned on a powerful electromagnet above the case, you'd expect the weight reading to change, despite not changing anything inside the "closed" system.

Title: Re: Weighing helicopter in a tank
Post by Cathos on May 27^th, 2004, 8:18pm

Forgive my ignorance, but doesn't a closed system mean it isn't acted upon by outside force?

Title: Re: Weighing helicopter in a tank
Post by Icarus on May 28^th, 2004, 5:42pm

The reason why rmsgrey quoted "closed" is because the system is already not closed in this sense. The force of gravity, attracting to the earth, which is exterior to the system, is acting on everything within the system. Thus his comment about external magnets is equally within bounds.

However, it is clear that Kozo intends a system in a steady state, which is under the influence of no external forces other than those related to Earth's gravitational attraction. So everything else is really only nitpicking.

Title: Re: Weighing helicopter in a tank
Post by rmsgrey on May 30^th, 2004, 6:19pm

If I really wanted to pick nits, I'd mention the energy input into the system by the radio signal that controls the helicopter. Though heat loss (the chopper is converting chemical (I assume) energy to heat (some of which via motion) which will create a temperature gradient across the glass/perspex) will exceed that once the system steadies.

Mind you, the mass lost as heat energy is going to be negligible compared to the fluctuations due to turbulence within the box.

Title: Re: Weighing helicopter in a tank
Post by Grimbal on May 31^st, 2004, 9:38am

And there might be some heat or light coming out of it, and the helix is radiating gravitational waves...