Even simpiler physics question

[Fritzo]

If a fly is in a car with the windows rolled up, and it hovers in the center of the car, then the car starts moving, will the fly hit the back windshield or move along with the car because the mass of air inside the cabin is moving along with the car?

 

[bonkers325]

42 planes take off

 

[sdifox]

frame of reference. The fly will be fine.

Quantifier: Yes, inertia plays a role, so the fly does go back a bit in terms of relative position, but accelerations in car is not great enough to splatter the fly on the back windshield.

 

[DisgruntledVirus]

Originally posted by: sdifox
frame of reference. The fly will be fine.

Yes.

The air inside the vehicle is sealed inside the vehicle. So it won't be moving, unlike the air outside the vehicle.

 

[Gibsons]

Originally posted by: DisgruntledVirus

Originally posted by: sdifox
frame of reference. The fly will be fine.

Yes.

The air inside the vehicle is sealed inside the vehicle. So it won't be moving, unlike the air outside the vehicle.

The fly still moves back relative to the car, the same way you get pushed back into your seat when the car accelerates.

 

[DisgruntledVirus]

Originally posted by: Gibsons

Originally posted by: DisgruntledVirus

Originally posted by: sdifox
frame of reference. The fly will be fine.

Yes.

The air inside the vehicle is sealed inside the vehicle. So it won't be moving, unlike the air outside the vehicle.

The fly still moves back relative to the car, the same way you get pushed back into your seat when the car accelerates.

No it doesn't, because the fly uses the air in the vehicle to be able to fly. Since the air in the vehicle won't be moving since it's a sealed vehicle, the fly will remain in the same position.

 

[Jeff7]

Originally posted by: DisgruntledVirus
No it doesn't, because the fly uses the air in the vehicle to be able to fly. Since the air in the vehicle won't be moving since it's a sealed vehicle, the fly will remain in the same position.

Put a human in a spaceship. Accelerate the spaceship to 1/2 light speed. Splat.

If we could solve that inertia problem simply by sticking wings on a person, well, one major obstacle to high-speed space travel would have been solved long ago.

 

[MotionMan]

Originally posted by: Jeff7

Originally posted by: DisgruntledVirus
No it doesn't, because the fly uses the air in the vehicle to be able to fly. Since the air in the vehicle won't be moving since it's a sealed vehicle, the fly will remain in the same position.

Put a human in a spaceship. Accelerate the spaceship to 1/2 light speed. Splat.

If we could solve that inertia problem simply by sticking wings on a person, well, one major obstacle to high-speed space travel would have been solved long ago.

I think there may be a flaw in your analogy since people in space float, not fly.

I have never had a fly splat against the inside of my windshield or rear window.

MotionMan

 

[Brainonska511]

Originally posted by: MotionMan

Originally posted by: Jeff7

Originally posted by: DisgruntledVirus
No it doesn't, because the fly uses the air in the vehicle to be able to fly. Since the air in the vehicle won't be moving since it's a sealed vehicle, the fly will remain in the same position.

Put a human in a spaceship. Accelerate the spaceship to 1/2 light speed. Splat.

If we could solve that inertia problem simply by sticking wings on a person, well, one major obstacle to high-speed space travel would have been solved long ago.

I think there may be a flaw in your analogy since people in space float, not fly.

I have never had a fly splat against my inside of my windshield or rear window.

MotionMan

The inside of the space ship is not a vacuum and gravity has nothing to do with your horizontal motion on the ground.

 

[DisgruntledVirus]

Originally posted by: Brainonska511

Originally posted by: MotionMan

Originally posted by: Jeff7

Originally posted by: DisgruntledVirus
No it doesn't, because the fly uses the air in the vehicle to be able to fly. Since the air in the vehicle won't be moving since it's a sealed vehicle, the fly will remain in the same position.

Put a human in a spaceship. Accelerate the spaceship to 1/2 light speed. Splat.

If we could solve that inertia problem simply by sticking wings on a person, well, one major obstacle to high-speed space travel would have been solved long ago.

I think there may be a flaw in your analogy since people in space float, not fly.

I have never had a fly splat against my inside of my windshield or rear window.

MotionMan

The inside of the space ship is not a vacuum.

:laugh:

Thanks, I think we understand it's not a vaccum. Motionman is talking about gravity, not vacuum.

 

[Brainonska511]

Originally posted by: DisgruntledVirus

Originally posted by: Brainonska511

Originally posted by: MotionMan

Originally posted by: Jeff7

Originally posted by: DisgruntledVirus
No it doesn't, because the fly uses the air in the vehicle to be able to fly. Since the air in the vehicle won't be moving since it's a sealed vehicle, the fly will remain in the same position.

Put a human in a spaceship. Accelerate the spaceship to 1/2 light speed. Splat.

If we could solve that inertia problem simply by sticking wings on a person, well, one major obstacle to high-speed space travel would have been solved long ago.

I think there may be a flaw in your analogy since people in space float, not fly.

I have never had a fly splat against my inside of my windshield or rear window.

MotionMan

The inside of the space ship is not a vacuum.

:laugh:

Thanks, I think we understand it's not a vaccum. Motionman is talking about gravity, not vacuum.

See my edited post.

It's too early in the morning. The coffee has yet to kick in.

 

[MotionMan]

Originally posted by: Brainonska511

Originally posted by: MotionMan

Originally posted by: Jeff7

Originally posted by: DisgruntledVirus
No it doesn't, because the fly uses the air in the vehicle to be able to fly. Since the air in the vehicle won't be moving since it's a sealed vehicle, the fly will remain in the same position.

Put a human in a spaceship. Accelerate the spaceship to 1/2 light speed. Splat.

If we could solve that inertia problem simply by sticking wings on a person, well, one major obstacle to high-speed space travel would have been solved long ago.

I think there may be a flaw in your analogy since people in space float, not fly.

I have never had a fly splat against my inside of my windshield or rear window.

MotionMan

The inside of the space ship is not a vacuum and gravity has nothing to do with your horizontal motion on the ground.

Vacuum or not, astronauts still cannot fly like a fly - they float due to a lack of gravity. Still, flies are not splatting against the inside of windshields or against rear windows.

MotionMan

 

[MotionMan]

I am no scientist, but can one say that the inertia of the air in the car is related to the car whereas the inertia of the fly is related to the air?

If so, that may be the answer.

MotionMan

 

[LordMorpheus]

Originally posted by: DisgruntledVirus

Originally posted by: Gibsons

Originally posted by: DisgruntledVirus

Originally posted by: sdifox
frame of reference. The fly will be fine.

Yes.

The air inside the vehicle is sealed inside the vehicle. So it won't be moving, unlike the air outside the vehicle.

The fly still moves back relative to the car, the same way you get pushed back into your seat when the car accelerates.

No it doesn't, because the fly uses the air in the vehicle to be able to fly. Since the air in the vehicle won't be moving since it's a sealed vehicle, the fly will remain in the same position.

The reason you get pushed backwards in your seat is that your body has inertia and requires a force to accelerate - the force is provided by the seat pushing on your ass and back.

A fly also has inertia, and will also require a force to keep it stationary inside the car - if the fly was hovering dead center in the car and creating only enough force to counteract gravity, then yes, it would move backwards. Speaking practically, though, a fly's flight path is too erratic for this to be noticeable.

Originally posted by: MotionMan
I am no scientist, but can one say that the inertia of the air in the car is related to the car whereas the inertia of the fly is related to the air?

If so, that may be the answer.

MotionMan

No. Both inertias are related to the non-accelerating reference frame (the road).

The air also has inertia. When you accelerate, there is a slightly higher pressure at the rear of the car than at the front of the car. Try it, this pressure differential is great enough to overcome the inertia of a helium balloon, so a helium balloon will move forward in the car when you stomp on the car and backwards in the car when you stomp on the brake.

 

[rivan]

Originally posted by: Brainonska511

Originally posted by: MotionMan

Originally posted by: Jeff7

Originally posted by: DisgruntledVirus
No it doesn't, because the fly uses the air in the vehicle to be able to fly. Since the air in the vehicle won't be moving since it's a sealed vehicle, the fly will remain in the same position.

Put a human in a spaceship. Accelerate the spaceship to 1/2 light speed. Splat.

If we could solve that inertia problem simply by sticking wings on a person, well, one major obstacle to high-speed space travel would have been solved long ago.

I think there may be a flaw in your analogy since people in space float, not fly.

I have never had a fly splat against my inside of my windshield or rear window.

MotionMan

The inside of the space ship is not a vacuum and gravity has nothing to do with your horizontal motion on the ground.

There's the operative word. The fly's exerting enough force against the air when sitting still to maintain his spot in the air.

The car will out-accelerate him and he'll have to overcome inertia (whatever's left after the acceleration of the mass of air around him has it's effect) to maintain his position in the car.

I'd think it would require some effort to maintain position, but it would probably be fairly trivial.

 

[Brainonska511]

Originally posted by: MotionMan

Originally posted by: Brainonska511

Originally posted by: MotionMan

Originally posted by: Jeff7

Originally posted by: DisgruntledVirus
No it doesn't, because the fly uses the air in the vehicle to be able to fly. Since the air in the vehicle won't be moving since it's a sealed vehicle, the fly will remain in the same position.

Put a human in a spaceship. Accelerate the spaceship to 1/2 light speed. Splat.

If we could solve that inertia problem simply by sticking wings on a person, well, one major obstacle to high-speed space travel would have been solved long ago.

I think there may be a flaw in your analogy since people in space float, not fly.

I have never had a fly splat against my inside of my windshield or rear window.

MotionMan

The inside of the space ship is not a vacuum and gravity has nothing to do with your horizontal motion on the ground.

Vacuum or not, astronauts still cannot fly like a fly - they float due to a lack of gravity. Still, flies are not splatting against the inside of windshields or against rear windows.

MotionMan

Then look at a land speed racer. That's on the ground, the driver is held down in his seat by gravity. Yet when the rockets on the back of the car turn on, the driver is pushed back into his seat. Same concept as the space ship, but now there is gravity.

If I had to take stab as to why your back windshield is not covered with flies, it would be because the acceleration of the car is not great enough and that the air in the car that would push back on the car is enough to keep it relatively motionless in regards to relative position in the vehicle, though the fly would still move towards the back a little bit.

 

[darkxshade]

Can't this be tested with a remote controlled miniature helicopter?

 

[Beanie46]

Originally posted by: Brainonska511

Then look at a land speed racer. That's on the ground, the driver is held down in his seat by gravity. Yet when the rockets on the back of the car turn on, the driver is pushed back into his seat.

Actually, the driver is NOT pushed back into his seat, but instead the seat is moving forward into the driver. The driver's inertia is trying to keep him stationary, at least until it's been overcome by the rocket car's forward momentum. The "pushed into the seat" feeling is really a "the seat moving into a stationary object that's not moving as fast" feeling.

 

[jemcam]

Dude, you can't even spell "simpler".

 

[911paramedic]

As Einstein would say, "Its all relative."

The fly will remain where it is.

 

[KMc]

Originally posted by: DisgruntledVirus

Originally posted by: Gibsons

Originally posted by: DisgruntledVirus

Originally posted by: sdifox
frame of reference. The fly will be fine.

Yes.

The air inside the vehicle is sealed inside the vehicle. So it won't be moving, unlike the air outside the vehicle.

The fly still moves back relative to the car, the same way you get pushed back into your seat when the car accelerates.

No it doesn't, because the fly uses the air in the vehicle to be able to fly. Since the air in the vehicle won't be moving since it's a sealed vehicle, the fly will remain in the same position.

The only way the fly can move forward (relative to ground) with the car is if it is accelerated forward by some force. Are you suggesting the air in the car will somehow accelerate (push) the fly forward to match the acceleration of the car?

The issue here is that the air inside the car does indeed have a certain mass to it - after all, air in an incompressible state can be thought of as a fluid. When the car accelerates, it also accelerates the air inside the car with it. Therefore, that mass of air can indeed impart a force onto an object. The dilemma is that we can easily picture that upon the initial acceleration something heavy in the car would want to move backwards relative to the car (or stay at rest, relative to the ground). The force acted on that object by the air is very very small and we think it irrelevant. But what if we make the object also very very small? How small does the object have to be before the force of the air in the car moving forward significantly counteracts the ability of the object to stay at rest? That's the root of the problem.

 

[speg]

The fly will not stay in the same place. Try to tie a ball on a string, and hang it from the ceiling of your car.

 

[Born2bwire]

These threads make baby Feynman cry. 🙁

 

[MotionMan]

But the ball is not flying.

MotionMan

 

[Born2bwire]

Originally posted by: MotionMan
But the ball is not flying.

MotionMan

But the fly is hovering. It will be subject to same fluid motions that the hanging ball or helium balloon would be.