[Todd Freed]

Todd Freed  asked the Naked Scientists:

Chris,
 
Here's my science question:
 
Imagine a big airliner flying through the air. Say we have a fruit fly buzzing around the cab not touching any surface. Does it add to the aeroplane's weight, and if so, does it have anything to do with the fact that the aeroplane is sealed and airtight? What if you crack open a window, does it still add to the weight?

Todd
Idaho Falls, Idaho

What do you think?

[lightarrow]

Todd Freed  asked the Naked Scientists:

Chris,
 
Here's my science question:
 
Imagine a big airliner flying through the air. Say we have a fruit fly buzzing around the cab not touching any surface. Does it add to the aeroplane's weight, and if so, does it have anything to do with the fact that the aeroplane is sealed and airtight? What if you crack open a window, does it still add to the weight?

Todd
Idaho Falls, Idaho

What do you think?

1. The fly has a total density lower than that of air inside the plane. Then it subtracts weight to the plane, because it has the space that were previously occupied by the denser air.
2. The fly has a total density greater than air, then it add weight, even if it flies inside: when the plain accelerates (in any direction), the fly's inertia push it in the opposite direction, so the fly must use its wings in order not to crash against the wall; the force made by the wings to the inside air, in turn, will be applied to the plain that so accelerates less, exactly has if it was heavier.

[lyner]

When the fly took off, the weight of the plane actually increased temporarily by a small amount because the fly was accelerating upwards.

[chris]

Exactly; it's exactly the same question as "if all the passengers jumped up in the air during a flight would the plane weigh less?"

The answer is yes - transiently, but at the moment they all jumped it would weigh more because, as Newton's 3'rd Law tells us "for every action there is an equal and opposite reaction" - so the force accelerating the jumping passengers upwards would accelerate the plane downwards making it momentarily heavier. While the passengers were airbourne it would become lighter again.

Therefore the average weight of the plane remains the same, but at any given moment it might change.

Regarding the fly, this is flapping it's wings to remain aloft. This means that air molecules are being accelerated downwards (because the fly needs to produce lift) and these air molecules will be hitting the bottom of the plane and pushing it downwards. Hence the fly is still contributing to the weight of the plane.

Chris