How fast is a bird flying inside a moving bus?

12 September 2010

Question

If a bus is travelling at 100 miles per hour [mph], and you are at the back of the bus holding a bird, and then it is let go and it flies towards the front of the bus, how fast is the bird flying?

Answer

Chris - George says, "If a bus is travelling at 100 miles per hour [mph], and you are at the back of the bus holding a bird, and then it is let go and it flies towards the front of the bus, how fast is the bird flying?" Dave - Okay, this is essentially a question about relative speeds. Compared to someone standing on the ground, the bird is flying at 120mph or something depending on how fast it's flying - so compared to someone on the ground, 120mph. Compared to someone on the bus it's doing say 20mph. The bird, because it's moving through air at 20mph, the important thing for the bird is the fact that it's going at 20mph because the speed the bird is moving through the air will affect how its wings behave and whether it can fly and things like that. So basically, all answers are true depending on where you're looking from. The most important thing is how fast the bird is moving through the air, so 20mph. Chris - So as far as the bird is concerned, the bird is going along at whatever speed it flies at. As far as an observer in the bus is concerned, the bird is flying at however fast the bird thinks it's flying. But a person outside the bus thinks the bird is flying at the bird speed plus the bus speed. And this is where is breaks down with light, isn't it? Because if you did the experiment with light, that is not what you'd see. Dave - No, the bizarre thing about the way the universe seems to work is that if you did the same experiment with light, everyone would see that it was going at about 300 000 km per second and everything else changes to make that work. So time and space will warp so that appears to work.

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