Sir Isaac Newton’s most useful application of his own third law

Sir Isaac Newton (1642 - 1727) invented the first known cat flap.

Whilst Sir Isaac Newton is best known for discovering the law of gravity as he sat under an apple tree, he also has other discoveries and inventions to his credit. The humble cat flap originated out of one of his other experiments when he was studying colour and the order it came in a prism.

He went up to his attic where there was only one window and in order to control light, he needed to black out the room. His cat loved to be with him, and she constantly nosed the door open, which would then let the light in – thus spoiling his experiments. Not wanting to upset her, he decided to cut a small opening in the doorway which he then covered with felt attached to the top of the opening – and voila! The very first cat flap had arrived!! Now cat and genius were happy – the cat could come and go at will, and Isaac wasn’t disturbed by her comings and goings, and could continue his experiments in peace.

It is said that all geniuses sometimes have a blind spot and when his cat had kittens, so anxious was he to please them, that he cut several smaller holes alongside the original one so that they could come and go whenever they wanted to. It didn’t occur to him they could use the existing one!

Now for the scientific bit The cat flap is like a pendulum – it takes the same amount of time to swing from one side to the other side. Every time your cat comes in or out it demonstrates Newton’s third law of motion – for every reaction there is an equal and opposite reaction.