# Effect on the weight of a bird in a closed cage with a spring attached to it

I have read the question in which the difference in the weight of a bird in a closed and a open system is described.

But what happens if there is a closed cage with wires and also a spring attached to that system?

In my notebook, it is written that:

in this system, the weight of the bird will be same as that of the bird in an open cage.

• Hi! Do you understand the concept of weight? – Gert May 22 '19 at 20:49
• @Gert yeah I do – Ishi May 23 '19 at 2:47
• @ArchismanPanigrahi Spring is attached on the cage and yeah its a wire cage i'll mention it in the question. – Ishi May 25 '19 at 6:57
• A sketch of the setup would be great. – Jasper May 25 '19 at 7:11
• @Ishi There was a typo. Does the closed cage have air inside? – Archisman Panigrahi May 25 '19 at 7:58

The weight of the bird is given by $$\vec W = m \cdot \vec g$$, with m being its mass and $$\vec g$$ the acceleration of gravity. The only visible dependency here is on gravity. It does not depend on any other force!

You can interpret the weight of an object simply as a force acting on it.

1. The bird can be standing in a cage on the ground, in which case its weight would be counteracted by the force exerted upwards by the cage floor.

2. The bird can be standing in a free-falling cage, in which case its weight would not be counteracted by any force. It would feel weightless, but it is in fact its weight that is accelerating it down.

3. The bird can be standing in a cage attached by a spring to a roof, in which case its weight would still be counteracted by the force exerted upwards by the cage floor. The spring can stretch due to the mass it is suspending, but that does not change this fact. Even if you make the cage bounce with the spring, what changes is the force exerted by the cage floor on the bird, not the bird's weight.

In a given system with a defined acceleration constant, the weight of an object with constant mass is constant.

• @PM2Ring The weight is still the same. I fail to see how that would make the concept easier to understand. – João Bravo Jun 1 '19 at 5:48

The weight of an object is the force with wihch the earth pulls it down, that is $$W=mg$$ where $$m$$ is its mass and $$g$$ the acceleration of gravity. So, the weight of a bird does not depend on it's enclosure.