Say you have a truckful of birds, happily sitting on the floor of your truck (or lorry, if you prefer). The doors and windows are closed for convenience, and to prevent bird escape. If these birds were to begin flying around in the back of your truck, would the overall weight of the truck go down?
You may intuitively think that the weight of the truck should go down, as the birds are not sitting their masses on the truck’s floor, but that is not quite right. If you came here to confirm that the weight stays the same, you are closer to the correct answer, though studies have shown it’s a little more complicated than that.
To start with, why doesn’t the overall weight of the truck go down? This is because when birds flap their wings, they are not excluded from Newton’s third law of motion. For every action there is an equal and opposite reaction, and that includes any action performed by e.g. a blue tit.
“Gravity still acts on them and they still have weight, but they do not fall because their lift cancels their weight. In order to gain lift, which is an upwards force, the bird must impart an equal downwards force on the air,” Dr Christopher S. Baird, Associate Professor of Physics at West Texas A&M University, explained in a university blog post.
“As a result, the air accelerates downwards until it hits the floor of the truck. At that point the air transfers its downward momentum to the truck. The total downward force that the truck experiences is its own weight, plus the force due to the air beaten down by the wings, which equals the weight of the birds.”
The Mythbusters tested this in 2007, finding that the birds (and then a model helicopter) being in flight made no difference to the weight of the helicopter. So, all sorted? Not quite.
While if you average over time the weight remains the same, studies have shown that this weight fluctuates as the birds beat their wings. Using a sensitive device to measure the force produced by birds as they flapped their wings, a team from Stanford University were able to demonstrate that there was “double the lift during the downstroke [of the wings] so that the birds did not have to lift their weight during the upstroke”.
As an average over time, however, the truck would remain the same weight.
“The weight of a truck containing just a few flying birds will fluctuate in time,” Professor David Lentink told the BBC. “Only the lift of an incoherent flock of birds could cancel out [this change].”
With many birds beating their wings at random, the weight would remain roughly the same, or at least random enough that the weight fluctuations are no longer noticeable.
All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.
