# Bird in Plane

#### FSF17

If a plane is flying along and inside the plane there is a little bird flapping away flying from front to back in the plane. Does the plane weigh anymore because the bird is inside it ?

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#### Integral

Staff Emeritus
Gold Member
Yes. When the bird is flying it is exerting a downward force at least equal to its weight.

#### russ_watters

Mentor
Ie, whether the bird is just sitting there or flying has no impact whatsoever on the flight of the plane.

#### KingNothing

However, when the bird is moving it's wings 'up', if we assume that for at least an instant it is not pushing air downward, and it is freefalling you might say that for that instant it did not add weight to the plane.

#### pallidin

I agree with everyone, but I also agree with Decker. For example, if a jumbo-jet laden with 300 standing people(150 pounds average weight each) suddenly lifted their legs off the floor, a sudden weight reduction of 45,000 lbs. would occur. Of course, when they hit the floor, an increase would be in effect.
With respect to the bird scenario, weight cycling variations would be minimal and trivial, but would nonetheless occur given Decker's free-falling addition.
Lesson to be learned: Be certain the bird has a seat belt and it's tray is in an upright and locked position. :)

#### Chen

pallidin said:
I agree with everyone, but I also agree with Decker. For example, if a jumbo-jet laden with 300 standing people(150 pounds average weight each) suddenly lifted their legs off the floor, a sudden weight reduction of 45,000 lbs. would occur. Of course, when they hit the floor, an increase would be in effect.
Except that for a 150 lbs person to jump, he has to exert a force on the floor of the plane, thereby "adding" to its weight. See:

#### quantum mechanic

If the people suddenly lift (not jump) then a force would not be exerted, but the force with which they lift must be greater than the pull of gravity

#### Chen

quantum mechanic said:
If the people suddenly lift (not jump) then a force would not be exerted, but the force with which they lift must be greater than the pull of gravity
What is the difference between "lift" and "jump"?

#### quantum mechanic

Chen said:
What is the difference between "lift" and "jump"?
A jump implies the application of a force to the ground or in this case the plane other than the force due to gravity.

Newton's 3rd law explains why a person is able to leave the floor by jumping: The interaction of the pushing of the leg against the floor of the plane.

A lift implies keeping the body stationary and simply, yet awkwardly, simultaneously making both legs not make contact with the plane. This also follows newton's 3rd law of a force equal in magnitude oppositely directed; however, the upper body, not the plane is what pushes back. No force is initially applied to the plane other than the force due to gravity and the body begins to freefall, and as Decker stated, "if we assume that for at least an instant it is not pushing air downward, and it is freefalling you might say that for that instant it did not add weight to the plane."

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