The weight of a truck filled with flying chickens.


Some time ago a heard that a truck filled with chickens would weigh the same if the chickens either are on the floor of the truck or if they are hovering just under the roof. The reasoning is that the chickens are part of the same «system» as the truck, so the whole «system» would always weigh the same, even if the objects inside it is not in contact with it physically. Can someone explain what is meant by this «system» and if this is a true statement?

Thank you!

In: 1

The assumption here is that the truck is sealed and nothing can get inn or out. In that case it does not matter how things are arranged inside the truck, on the floor or flying, the entire sealed system have the same mass and therefore the same weight. In this case what would happen is that in order to fly the chickens flap their wings to push air downwards. The force pushing the air down is producing an equal force on the wings pushing the birds upwards and countering the force of gravity. However the air that gets pushed downwards will end up hitting the floor of the truck. So the floor of the truck gets pushed downwards by the same force that lifts the chicken up. It just goes through the air instead of through the chickens feet.

A lot to unpack here. First of all. Where are the chickens going? Who is driving the truck? And why is his weight not being calculated?

All lolz aside. I think this is a major oversimplification. Minimal physics knowledge aside. If the chickens are “hovering” there in theory has to be something countering their weight, equal to the weight of the chicken, which would scale according to the variable amount of chickens. Chicken up, means force down. This force down onto the truck means they are interacting and engaging in one system. I think…. 😅😅

The system in physics, it is a portion of the physical universe chosen for analysis. So it is the truck, the chicken, the air in it. There is not direct contact between the chickens and the truck but there is interaction via the air.

In that example, it is an enclosed compartment the chickens are in not just a wire cage exposed to the outside. If that was the case the weight would not be the same.

When the chickens stay up in the air there has to be an upward force equal to the force of gravity. Weight is the downward force of gravity on an object and to say still in the air you need to counteract it like by punching down air.

According to Newton’s third law “for every action (force) in nature, there is an equal and opposite reaction” so the upward force the chicken generates with the wings results in an equal downward force on the air. The air will push on more air and it will finally hit the floor of the truck and through its body of it to the tires and the ground.

So if you put a force sensor below wheels the force from gravity on the chicken will be translated to it. It does not matter if the chickens are flying or standing on the floor. The difference will just be if the air is involved in the force transfer or not.

In physics there are two types of systems: open systems and closed systems. The exact system is determined by the physicist working on the problem, though is usually pretty obvious for a given problem.

A closed system is one where nothing can enter or exit, such as your truck.

An open system allows energy and/or matter to enter or exit. A relevant example might be a railroad cattle car.

As for birds on a truck, that was [MythBusters Season 5, Episode 9](

I’m with you guys to a point, but it might not be that simple. err, momentum and time. Imagine a truck with enough bouncy balls to cover the floor. Truck tops a hill, all of the balls are momentarily thrust into the air, and there is not one fraction of a newton exerted on the floor at that instant. I’m not saying the energy changed, as it was converted to kinetic as the balls went up, but weight? Yep it changed.