# How Heavy is a Flying Fly?

Q:
If a fly is in an airborne airplane, does the weight of the plane change when the fly lands on a surface (say it sits on a seat) instead of it flying around inside the plane? Why?
- Mary Larson (age 11)
Diamond Elementary, Gaithersburg, MD, USA
A:
The Force of gravity is pulling down on the fly. For the fly to be suspended in air, the air must push up on the fly. The fly does this by pushing the air down. When the air hits the floor of the plane, it pushes down on the plane.

When the fly lands, gravity pulls the fly’s mass down on the plane directly. The force on the plane from the fly is the same either way.

You can figure out the total weight of the plane by adding up the weight of each atom within the plane. It doesn’t matter if the atom is from a fly or dust, it all pushes downward.

This also means that the total mass of the planet does not change when the plane takes off. The plane pushes on the air which pushes on the ground. Even though the plane is flying, it is still attached (through air) to the ground.

EJ

(published on 10/22/2007)

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