Science Questions with Surprising Answers
Answers provided by
Dr. Christopher S. Baird

Why will a delivery truck filled with birds sitting on its floor be heavier than a truck with the same birds flying around inside.

Category: Physics      Published: December 9, 2012

delivery truck
A delivery truck filled with birds will weigh the same whether the birds are sitting or flying. Public Domain Image, source: NREL.

If the truck and the birds are the same, they will weigh the same no matter what the birds are doing inside. It seems that when in flight, the birds are not touching anything, so they cannot contribute any weight. But the key is that birds do not magically become weightless by flapping their wings. 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. 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 air therefore carries the weight of the birds to the truck, and it ends up the same. This is all in keeping with Newton's third law which states that every force has an equal and opposite force. The birds can't go up unless the air goes down. We don't typically notice the air going down because a bird is so small. But a bigger bird gives a better picture. Imagine a helicopter hovering a few feet over the ocean. The ocean surface below dips and ripples visibly because of the air hitting it. The ocean is feeling the weight of the helicopter.

Topics: action and reaction, birds in a truck, force, forces, weight