Updated September 2026, 1 week ago
Stand-up Maths
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The Rug Puzzle: how many triangles?
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![How fast is a bullet? [featuring: pendulum calculation]](https://i.ytimg.com/vi/IoYm8lcJQ0o/mqdefault.jpg "How fast is a bullet? [featuring: pendulum calculation]
Thanks to the Cambridge Engineering Department for building the pendulum.
http://www.eng.cam.ac.uk/
Much appreciation to Maria Kettle and Cambridge Engineering Outreach for organising the day.
http://www.eng.cam.ac.uk/events-and-outreach/schools-and-community-outreach
And Hugh was there.
http://www2.eng.cam.ac.uk/~hemh1/
The Cambridge University Rifle Association were super helpful and let us take over their range for most of the day! Katherine Fleck was our expert shooter with infinite patience.
http://cura.soc.srcf.net/
If you have other suggestions for calculating the speed of a bullet: let me know!
CORRECTIONS
- At 3:17 the close-up shows the sliding box already taped to a piece of paper. I only noticed when uploading that we had used this shot from later in the day by accident. It should have been the shot of the box sliding off the pendulum platform. Our solution to fix this problem was to tape it to a piece of paper.
- A few people have asked why we didn’t use a “chronograph”. We did actually use one which detects the movement of the bullet using infrared sensors. But the .22 bullet was too small to trip them.
- People have commented that we didn’t need to use the equation for the period as we know the length of the pendulum and the value of g. But the period is really easy to measure accurately by letting the pendulum swing for ages. Length could be measured fairly accurately (guessing the centre of mass) but g would require an extra assumption.
- Let me know if you spot anything else!
Thanks to my Patreon supports who do support these videos and make them possible. Here is a random subset:
Nicholas Main
Julian Metzler
Amanda Pagul
Christopher Wilson
Claire Greenhalgh
Support my channel and I can make more maths videos:
https://www.patreon.com/standupmaths
Music by Howard Carter
Filming and editing by Trunkman Productions
Sound production by Peter Doggart
Design by Simon Wright
MATT PARKER: Stand-up Mathematician
Website: http://standupmaths.com/
NEW BOOKk: http://wwwh.umble-pi.com
Nerdy maths toys: http://mathsgear.co.uk/")
