Relative Velocity: How Does a Housefly Move?

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Discussion Overview

The discussion revolves around the mechanics of a housefly's movement within a moving car, particularly focusing on concepts of relative velocity, the role of air, and the implications of a vacuum environment on the fly's ability to fly. The scope includes theoretical reasoning and conceptual clarification regarding motion and forces.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants propose that a housefly can move at the same speed as a car due to being carried by the air inside the car, requiring no additional energy compared to flying in a stationary environment.
  • Others argue that the fly's initial state affects its movement; if it starts from rest on a seat and then flies upward, it retains the car's horizontal velocity unless acted upon by a force.
  • A later reply questions the scenario of a vacuum inside the car, suggesting that if the fly is not in contact with the car, it would eventually move towards the rear if the car accelerates, as no force would act on it to maintain its speed.
  • Participants discuss the implications of a vacuum, noting that a fly cannot fly in such an environment because it relies on air to generate lift by pushing down against it.
  • One participant confirms that the fly requires air for lift, emphasizing that it must push down on the air to remain aloft against gravity.

Areas of Agreement / Disagreement

Participants generally agree on the necessity of air for the fly's flight and the effects of relative motion within the car. However, there are competing views regarding the implications of a vacuum and the initial conditions of the fly's movement, leaving some aspects unresolved.

Contextual Notes

The discussion does not resolve the complexities of how forces interact in different scenarios, such as the transition from a normal environment to a vacuum, and the assumptions regarding the fly's initial state are not fully explored.

abhiroop_k
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consider a housefly in a car moving with a certain vel...how does it manage to move with the same speed as that of the car even though it isn't in contact with the car itself??...as in does it spend its own energy to move at quite high speeds (considering its small size and energy)??...
 
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It's being carried along by the air inside the car. It requires no more energy to fly around inside the moving car than it would flying around inside your home.
 
It depends on what the fly was initially doing.

If the fly was sitting on a seat in the car, and then suddenly flies upward, it will still have the same horizontal velocity of the car (assuming no air friction) since there is no force in the horizontal direction slowing it down.
 
Doc Al said:
It's being carried along by the air inside the car. It requires no more energy to fly around inside the moving car than it would flying around inside your home.

what if there were a vacuum inside the car?...would the fly move towards the rear of the car eventually? (if it isn't in contact with the car initially)
 
abhiroop_k said:
what if there were a vacuum inside the car?...
How is it flying then?
would the fly move towards the rear of the car eventually? (if it isn't in contact with the car initially)
Assuming the fly is moving at the same speed as the car, it will keep moving at that speed unless something exerts a force on it to change.

Perhaps this is what you have in mind. Imagine that the fly is stuck inside an airless box whose walls are frictionless. The box is in the car, with the fly sitting on the bottom (slowly suffocating). If the car speeds up, the fly will start to slide to the rear of the car since there's no force acting on it to speed it up. (Eventually it hits the wall of the box and is pushed forward.)
 
okay...thanks...and also could you please clear this for me...the fly cannot fly in a vacuum because it needs an upthrust for flight which is provided by the air moving downwards??
 
abhiroop_k said:
the fly cannot fly in a vacuum because it needs an upthrust for flight which is provided by the air moving downwards??
That's right. To keep himself aloft (against gravity) he must continually push down on the air (which pushes him up).
 

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