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Curious Kids: Why it is that the things close to the train windows zoom by really fast, but things further away seem to go by much slower?

By David Paganin, Adjunct Professor (Research) in Physics, Monash University

When looking out of a train window, things close by seem to move past faster than things that are far away. Flickr/Larry W. Lo, CC BY

This is an article from Curious Kids, a series for children. The Conversation is asking kids to send in questions they’d like an expert to answer. All questions are welcome – serious, weird or wacky!


Why it is that the things close to the train windows zoom by really fast, but things further away seem to go by much slower? – Ada, age 7, Katoomba.


Superb question, Ada!

Things close to the train window seem to zoom by very fast because they appear much larger than things that are far away.

Imagine looking out of a window. Suppose you see a big tree, one that is 10 metres wide, far away in the distance. When you push your thumb against the window, you can cover the whole tree, even though the tree is much bigger than your thumb.

Now, imagine you have an ant on the nail of your thumb. As you have your thumb pressed against the window, the ant takes one second to walk from the left to the right of your thumbnail.

Now imagine at the same moment, a cat runs halfway across the 10 metre wide tree in the distance.




Things close to the train window seem to zoom by very fast because they appear much larger than things that are far away.
Flickr/Sean Hickin, CC BY

Even though the cat is faster than the ant (it has run five metres in one second), in your eyes, the cat seems to have only travelled half a thumb width in one second, while the ant has travelled one whole thumb width in one second. The ant seems to zoom by faster than the cat, even though in reality, it is much slower.

If you look at an aeroplane high in the sky, what you discover is even more fascinating. Imagine covering the image of the plane with your thumb as it zooms across the sky. The ant took one second to cross your thumb. The cat needed two seconds. The plane might need five seconds! So, the plane seems to be slower than both the ant and the cat, but we know that planes are in fact much faster.




Distant objects take longer to cross our line of vision than ones close by.
Flickr/Jeff Laitila, CC BY

Things appear to move slower when they are far away because they seem smaller, and take longer to cross our line of vision. Likewise, they appear to move faster when they are close by, because they seem bigger.

PS: You have the same first name as a very famous scientist! Ada Lovelace was one of the first people to think of how to design a computer. She was also one of the first to write computer programmes. This was super clever for someone born over 200 years ago, and her work has helped shape the world we live in today.


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The Conversation

David Paganin receives funding from the Australian Research Council.


Originally published in The Conversation.