You are holding a cage containing a bird. Do you have to make less effort if the bird flies from its position in the cage and manages to stay in the middle without touching the walls of the cage? Does it make a difference whether the cage is completely closed or it has rods to let air pass?

Case 1- Completely closed cage:


In this case, our hand has to support the weight of the cage, the bird and the air inside the cage. When the bird is flying inside the cage, it flaps its wings up and down. With every down stroke of the wings, it pushes the air which further exerts a force against the wall of the cage. This force is nearly equal to the weight of the bird. So effectively, there is no reduction in our effort to hold the cage.


Case 2- Open cage with rods:


In this case, the air pushed downward by the wings of the bird is somewhat dispersed due to the incoming air from outside the cage. So effectively, we have to apply a little less effort in this case to hold the cage.


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