# Birds in a truck

Discussion in 'Physics & Math' started by BenTheMan, May 20, 2007.

1. ### SyzygysAs a mother, I am telling youValued Senior Member

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Here is a chart of an elephant jumping in a truck. While the elephant is accelerating and still touching the floor, the overall weight is MORE. While it is in the air, the weight goes down to the truck's weight, and when it is landing the overall weight is going to be MORE again for a split second, until the elephant's mass becomes still again:

Last edited: Mar 25, 2009

3. ### RJBeeryNatural PhilosopherValued Senior Member

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Syzygys, I feel you have summed up the thread nicely. There should be some sort of concensus indicator so that future visitors to the thread can just jump to the summary post.

5. ### PeteIt's not rocket surgeryRegistered Senior Member

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...Except that the area between the curve and the elephant+truck line should be the same above as below, and that the case of a flying bird (that does not touching the floor) is qualitatively different to a jumping elephant (that does).

But yes, it would certainly be a valuable component of a summary.

7. ### RJBeeryNatural PhilosopherValued Senior Member

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Thanks Pete. I was referring more to post 120 than 121 but it's still nice to have a generally-accepted resolution to the OP.

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My mistake!

9. ### geistkieselValued Senior Member

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2,471
Yes the truck weight will change. The weight will decrease as the helicopter gains altitude and the air movoing against the floor decreases. If the truck is big enough the down moving air will eventually be absorbed by the air underneath the helicopter and the helicopter weight will evaporate, as a measure.

Similarly, the direction of the air flow from the birds wings is only periodically in a down direction, hence there is no way for the supposed forces due to the wings to be measured equivalent to the weight of the bird. Bird flight is a relatively local event where the air pressure above the the wing surface wings is less than the air pressure below the wings. Maintaining altitude is met by forward motion as the air moves over the surface of the wings - the bird motion is equivalent to air motion. The wings are not connected by any physical substance to the truck analogous to elephants standings on the floor.

I fail to see the applicatioon of the 'closed system' scenario.
:shrug:​