Day 21 - Shambles Advent

Blog in a Teacup by Dr Helen Czerski

Day 21 – @karlrwebb

Every day until Christmas Dr Helen Czerski will be finding some cool, hidden science in pics of people’s every day lives. To get involved, tweet your pic to @helenczerski and @cosmicshambles with the hashtag #ShamblesAdvent

Thank you to @Karlrwebb for today’s picture, showing a very tidy car boot.  It’s a familiar sight to most of us, so familiar that there are some things on it that we almost never consciously see.  There are at least two of those in this picture, in the top left and right hand corners. They’re poking out from each side of the car below the hinges and connecting the car body to the hatch.  Those slim black rods are called gas springs, and you might not notice them very often, but any car user would certainly notice if they weren’t there. 

The hatch at the back of a car like this is made of metal and plastic and glass and it’s extremely heavy.  But you could drive a car for years and never have to face up to that, because the gas springs act like an anti-gravity device that makes that weight imperceptible.  Inside each one is a sealed tube of nitrogen gas, with a piston that can push inwards on the gas.  The gas can flow up the sides of the piston, but when the piston is pushed inwards, it takes up space and leaves less room for the gas.  So the gas is compressed.   

If you take a balloon and squeeze it, the balloon gets smaller but you can feel a force pushing back on your hands because the gas inside is now at higher pressure.  You have to keep pushing for the balloon to stay in its squeezed shape.     

The clever bit about the gas spring on a car is that it uses this mechanism to counteract gravity (both when the hatch is on the way down and also when it’s on the way back up).  Imagine the hatch starting in its top position, with gravity pulling it downwards.  If you let it go, it’ll fall at a considerable speed.  But now put a gas spring in the way.  The hatch can only fall if it moves the piston inwards and compresses the gas, and the force needed to do that has to be provided by gravity. That means the hatch closes nice and slowly. All you have to do is provide enough force to control the speed of the downward movement.  When the car hatch is closed, the pistons are squeezed all the way in, and the trapped gas is at enormous pressure – perhaps 50-100 times atmospheric pressure – and so the gas is pushing outwards on the pistons with enormous force.  When you release the car hatch to open it, that push helps you lift the hatch up, giving back all the stored energy it had inside.   You hardly have to put any effort in to lift the hatch, because the pistons are doing all the work for you.

A gas spring is a tiny ingenious piece of technology, but worth appreciating next time you are opening or closing a car boot.

Read all of Helen’s other #ShamblesAdvent entries here

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Dr Helen Czerski is a physicist, first and foremost, but she’s acquired a few other labels along the way: oceanographer, presenter, author and bubble enthusiast. A regular on The Cosmic Shambles Network, she has also presented a number of acclaimed documentaries for the BBC and her first book, Storm in a Teacup, which looked at the physics of every day things, was a bestseller. Recently she was awarded the prestigious William Thomson, Lord Kelvin Medal and Prize from the Institute of Physics.

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