# Easy physics argument.

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#### natoo24

##### Guest
I had this debate with my friend and i was wondering if someone could clear this up for me.

Imagine we get on a airplane and there's also a fly there too. Why doesn't the fly feel as much force as we do when the airplane takes off?

I told him that it was because the fly's mass compared to the plane is relatively smaller than our mass compared to the plane? Was i right?

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#### Mee_n_Mac

##### Guest
I would say you are correct, the fly experiences less force on it than you would because both of you will have the same acceleration imparted onto you by the plane and Mr Newton wasn't kidding when he said F = M*A. Same A, less M gets you less force. A fly ... errr ... flying about would experience even less (peak) force as the air would move around it and thus the fly would experience less peak A, as the drag from this moving air (which is the source of the force a flying fly would "feel") would stretch out the acceleration. Think of a service cart in the aisle with it's wheels partly locked. As the plane accelerates down the runway, the back of the plane moves towards the cart. The floor exerts a force on the wheels and some of this gets to the cart to bring it up to the same speed as the rest of the plane. Relatively speaking the cart appears to move towards the back of the plane and eventually will get up to the same speed as the floor, etc. You however get slammed into the seatback (relatively speaking) pretty immediately.

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#### SpeedFreek

##### Guest
Hi natoo24

What if the fly were flying around the cabin for the duration of the flight?

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#### emperor_of_localgroup

##### Guest
The reason we feel force when the plane is accelerating is because the seat is applying a strong reaction force on our body. Now, if the fly is sitting on something inside the plane, it would experience the acceleration (a smaller force as a previous poster pointed it out) but because of its 'sticky' legs, the fly will not slide on the seat.

But if the fly is flying inside the plane during take off, my guess is, it would not feel the acceleration or the force. A siimilar situation is a fish in a completely filled fish bowl inside a plane. Again, my guess is, the fish will not feel the acceleration or the force during take off.

Please correct me if my guess is wrong.

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#### Mee_n_Mac

##### Guest
The reason we feel force when the plane is accelerating is because the seat is applying a strong reaction force on our body. Now, if the fly is sitting on something inside the plane, it would experience the acceleration (a smaller force as a previous poster pointed it out) but because of its 'sticky' legs, the fly will not slide on the seat.

But if the fly is flying inside the plane during take off, my guess is, it would not feel the acceleration or the force. A siimilar situation is a fish in a completely filled fish bowl inside a plane. Again, my guess is, the fish will not feel the acceleration or the force during take off.

Please correct me if my guess is wrong.

As I tried to say above, a flying fly must (eventually) be accelerated up to the same speed as the aircraft. If it doesn't the back (or floor) of the plane will eventually catch up to the fly and hit it. Let's just concentrate on the takeoff roll since it's just one dimension and easier to explain. So the plane accelerates down the runway and the fly, having inertia, "wants" to remain in the same place (let's pretend he's hovering). But we know from experience that he doesn't get pinned to the back bulkhead, so what happened ? Well the air in the plane gets accelerated along with the plane and the fly gets dragged (pun intended) along with the air. Same would be true of a fish in a bowl of water. If the fly was stationary prior to the rollout and is moving with the plane afterwards, he must have undergone some acceleration. Whether this acceleration has the same exact value at all points in time as the plane is debateable, I'll opine it isn't quite a big at first but lasts a bit longer so the integrated value (the velocity) ends up the same. Make sense ?

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#### Jerromy

##### Guest
If the fly was sitting on your nose as the plane accelerated you would not only feel yourself being forced against the seat, but the fly being forced against your nose! The relative mass at 2g acceleration would be double for both of you and you and the fly would both "feel" twice as heavy as compared to sitting on the ground. I'm rather curious as to if the fly were flying towards the front of the plane while it was accelerating would the fly "feel" like it is accelerating faster even though it is moving at a steady speed relative to the plane? I would have to guess that the fly would still feel the acceleration EVEN if it is flying.

P.S. the mass of the plane is of *almost* no relevance!

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#### SpeedFreek

##### Guest
Mee_n_Mac":3pgzbwk3 said:
As I tried to say above, a flying fly {SNIP}

Still, I knew the flying fly would spice up the answers a bit!

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#### kg

##### Guest
I think the real question is what is happening to the air inside the plane because the fly is going to be pushed forward by it as if it were a tail wind. Is there a difference in air pressure from the cockpit to the tail end of the plane as it accelerates?

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#### origin

##### Guest
kg":3vqpfnfc said:
I think the real question is what is happening to the air inside the plane because the fly is going to be pushed forward by it as if it were a tail wind. Is there a difference in air pressure from the cockpit to the tail end of the plane as it accelerates?
Yes, the air 'piles up' in the back of the air craft so the air pressure is higher in the back. This difference in pressure can even been seen in a car accelerating. A helium balloon inside the car will move towards the front of a car as the car accelerates because of the pressure difference between the back and the front of the car.

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#### Mee_n_Mac

##### Guest
origin":308tlhcc said:
kg":308tlhcc said:
I think the real question is what is happening to the air inside the plane because the fly is going to be pushed forward by it as if it were a tail wind. Is there a difference in air pressure from the cockpit to the tail end of the plane as it accelerates?
Yes, the air 'piles up' in the back of the air craft so the air pressure is higher in the back. This difference in pressure can even been seen in a car accelerating. A helium balloon inside the car will move towards the front of a car as the car accelerates because of the pressure difference between the back and the front of the car.
Interesting ! I'd have thought that given the compressibility of air, the back of the car would move and the air, having inertia, would begin to "pile up" at the back while remaining stationary (relative to the ground) at the front ... at least initially. Therefore a balloon in the front would also remain stationary (at first) and appear, relative to the car, to move backwards. Then as the denser air caught up with the balloon it would start to be accelerated by said air and so stop (relative to the car). After that the pressure differential would begin to push the balloon to the front, though I wonder by how much. I could imagine some oscillations might occur before the pressure wave damped out ...

I feel an experiment is in order ! Time to borrow the van and make a trip to iParty.

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