Yearly Archives: 2011

Centre of mass in extreme sports

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All projectiles follow similar paths (trajectories) called parabolas.

The exact trajectory followed depends on the launch angle and speed, but all have similar characteristics. In order to travel as far as possible the launch angle should be 45°.

These parabolic paths are evident in the motion of the centre of mass of any object that falls under gravity, whether it is spinning, twisting or otherwise in motion.

Here the centre of mass is on the motorbike itself, as it is so much heavier than its rider. All motorcycle jumps, regardless of the position of the rider, follow much the same path.

All photographs from the Red Bull X-Fighters website.

The physics of the Kinect

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The Microsoft Kinect is a peripheral for the Xbox 360 that does away with the need for a conventional controller – instead the player’s body and voice become the controller.

The Kinect sensor consists of:

  • 640×480 pixel visible light camera
  • 640×480 pixel infrared camera
  • Four-microphone sound sensing array
  • Class I infrared (780nm) laser diode

iFixit teardown of Kinect

The major advantage of the Kinect is that it works in 3D. Previous console vision systems (such as the PlayStation’s structured light” created by a beam of infrared laser light passing through a diffraction grating. This projects a grid of 50000 infrared dots across the playing area. These infrared dots are visible on many cameras with a “night vision” mode.

By comparing how the dot pattern looks, and how it should look, the Kinect can measure the distance between the sensor and the player – producing a “depth map” in the process.

Images from Matthew Fisher. Objects in red are closest to the screen; colours then move through the spectrum to purple objects that are furthest away.

Unlike Sony, who have cracked down on anyone trying to hack the PlayStation; Microsoft have been very open to Kinect hackers; including the team from Cox Lab at Harvard who have developed a portable Kinect-based 3D camera.

* It has been shown that the depth resolution is non-linear and that the further an object is from the sensor the less information is available about its true distance.

Percent, permil and basis points

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I only recently discovered the permil (cf. percent), a typographic character that enables you to give a fraction equal to one part in one thousand without using a decimal point. For example 12.3% = 123‰ (“twelve-point-three percent is equal to one hundred and twenty-three permil”).

There is also a symbol (‱) for basis points (aka permyriad), parts in ten thousand. For example 12.34% = 123.4‰ = 1234‱ (“twelve-point-three-four percent is equal to one hundred and twenty-three-point-four permil or one thousand, two hundred and thirty-four basis points”).

A large number of fonts are unable to render the permil and/or basis point symbols correctly, so the post above may be missing some symbols.

Whole aeroplane parachutes

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Some pilots wear parachutes when they fly, in case they have to bail out of a malfunctioning aircraft. But what if you wanted the aeroplane itself to bail out?

Unlike parachutes for humans, whole aeroplane parachutes are deployed balistically; they are “fired” out of their containers by solid-fuel rockets, rather than pulled by air resistance.

A vertical launch whole plane parachute mounted on an aeroplane’s roof. The thin black container contains the solid-fuel rocket that will pull the parachute out of the white container.

Wellcome Trust Science Writing Prize

I entered the Wellcome Trust’s Science Writing Prize that ran in conjunction with the Guardian and unfortunately didn’t make the shortlist. I’m quite proud of the piece I put together so I’ve put my entry, SNIFing out Rogue Nuclear Reactors, up on this site.

I figure that there must be quite a lot of people in a similar situations and I don’t want good science writing to go to waste, so I’ve decided to build a list of links here. If you’d like me to include a link to your piece then tweet me @alby using the #swploser hashtag or drop the link in a comment below. If you don’t have a site of your own then I’m happy to host it here for you, again just post in the comments below.

Check Mate by Tom Houslay on the reproductive “arms race” between the sexes.