Relative Velocity: How Does a Housefly Move?

[abhiroop_k]

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)??...

 

[Doc Al]

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.

 

[Salamon]

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.

 

[abhiroop_k]

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)

 

[Doc Al]

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.)

 

[abhiroop_k]

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??

 

[Doc Al]

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).