Helicopter in a box: Third law

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Discussion Overview

The discussion revolves around a thought experiment involving a radio-controlled helicopter placed in a closed clear-plastic box on a sensitive scale. Participants explore the implications of the helicopter hovering inside the box on the scale's reading, considering concepts from physics such as internal forces and impulses.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests that if considering internal forces, the scale reading should remain unchanged at 50N since all internal forces balance.
  • Another participant proposes that thinking in terms of impulses leads to the expectation that the scale reading would change at lift-off due to the upward acceleration of the helicopter, but would return to 50N once the helicopter hovers.
  • A later reply confirms the expectation of a change in scale reading at lift-off, attributing this to the helicopter's upward acceleration affecting the overall center of mass, but asserts that the reading would revert to its original value upon reaching equilibrium.
  • One participant references a related thought experiment, "pigeons in a truck," suggesting it has been discussed in other contexts, including a mention of a Mythbusters episode.

Areas of Agreement / Disagreement

Participants express differing views on the effects of the helicopter's lift-off on the scale reading, with some supporting the idea of a temporary change and others emphasizing the balance of internal forces. The discussion remains unresolved regarding the implications of these perspectives.

Contextual Notes

Participants do not clarify the assumptions underlying their reasoning, and the discussion does not resolve the mathematical implications of the forces involved during the helicopter's lift-off and hovering phases.

PeterPumpkin
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A thought-experiment. (This is not homework!) Supposing someone puts a radio-controlled helicopter in a CLOSED clear-plastic box. They then put the box plus chopper on a sensitive scale and recorded the weight. Say 50N for the sake of argument. Now suppose they started up the chopper and instructed it to hover in the center of the box. How would the scale reading change?

1) If I think in terms of internal forces, there should be no changes --- all internal forces balance. It should remain at 50N.

2) But if I think in terms of impulses I would expect the scale reading to change at lift off and then return to its original reading of 50N.

What am I missing? I wonder if anyone has actually done this.
 
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I believe you are right that it would change at liftoff - the helicopter is accelerating upwards, causing an upwards acceleration in the overall center of mass of the system, increasing the force against the scale at liftoff. However, once the helicopter reaches an equilibrium (hover), the force against the scale will go back to precisely what it had been prior to the liftoff.
 
PeterPumpkin said:
A thought-experiment. (This is not homework!) Supposing someone puts a radio-controlled helicopter in a CLOSED clear-plastic box. They then put the box plus chopper on a sensitive scale and recorded the weight. Say 50N for the sake of argument. Now suppose they started up the chopper and instructed it to hover in the center of the box. How would the scale reading change?

1) If I think in terms of internal forces, there should be no changes --- all internal forces balance. It should remain at 50N.

2) But if I think in terms of impulses I would expect the scale reading to change at lift off and then return to its original reading of 50N.

What am I missing? I wonder if anyone has actually done this.

It's been done:

http://mythbustersresults.com/episode77

CS
 

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