Thursday, December 07, 2006



After lengthy reading of Bill Bryson's The Life and Times of the Thunderbolt Kid has lead me to also reminisce of my own child hood and the stupid moments in growing up… entitled flight, projectile motion and dislocated shoulders…

Science was always a big part of growing up: how did things work, could I reproduce similar results from experiments and TV shows I had encountered? This lead to the fascination with flight…

What I wanted to happen:


How lift is produced. In hind sight this was the information a child needed to reduce the danger from homemade hang glider injuries.

Lift is produced by an aerofoil. You will notice the odd shape of the aerofoil: it is curved on the top surface and flat on the bottom surface. This is used to exploit a difference in air pressure as follows… Analogy: picture a running track, you are running next to your friend. If the two tracks were the same distance you would have to run at the same speed to arrive at the finish at the same time.

Now we lengthen one track by curving it outwards. Two runners start at the same position and must cross the finish line at the same time… In order for both runners to arrive at the same time the runner with the longer track must run at a faster pace, to cover the longer distance.

Under pressure (also an awesome track by queen). Pressure, in ley terms, is the amount of collisions per molecule per second on a surface. Air pressure is no different if you compress gas molecules you reduce volume and increase the number of collisions with the side of the container. Now if we pass a gas over a surface, very slowly we get a lot of collisions and a lot of pressure or quickly a few glancing blows and a low pressure.

It we apply this to our running track, on the longer track the molecules move faster therefore low pressure (few collisions). On the shorter track air moves slowly, more pressure (More collisions). Put the shorter track to the bottom of the wing. More pressure below than above, net upward movement, and therefore lift is generated.

What actually happened…
Projectile motion:

Throw a ball straight out in front of you, what happens? The ball takes a curved trajectory to the ground.

Now picture a 12 foot high retaining wall with a young man running full pelt (approx 12 km an hour) off the top of it, strapped to his back two large white shopping bags and length of wood, of a 3 meter wing span…
Time for some maths….

We need:
acceleration due to gravity 9.8 ms-2
Initial velocity in ms-1 12km equals 3.3 ms-1
Height in meters 4m (approx)
Ignoring the following
Air resistance
Number of poles from the near by veranda the wings clipped rotating our hero through 720 degrees bringing the shoulder more forward of its previous position.

Over to Newton..

The time to fall 4 meters will be given by the formula…

H= ut +.5aT^2 where h is equal to height (4m), u is the initial downwards velocity 0, a is the acceleration due to gravity 9.8 ms-2 and T is the time taken to fall.

To fall 4m

4=0t +.5(9.8)T^2
4=4.9t^2
t=0.9 seconds
(funny it felt a lot longer)

The distance covered on land will be given by

Ut=d where d is the distance travelled horizontally, u is the initial (unchanging) velocity and t the time taken to do so.

D= 3.3*.9
2.97 meters flight distance, slightly shorter than needed for the Atlantic crossing.

In short, 4 meters down 3 meters horizontal clipped a post on a veranda turned through 720 degrees and nearly popped out my shoulder. Realised that two shopping bags and a plank of wood was not a suitable substitute for a glider. Found, however, that 6 bags and jumping from a tree could act as a parachute as long as I landed on a mate's sister to cushion the landing.

4 Comments:

Blogger Keshi said...

**Found, however, that 6 bags and jumping from a tree could act as a parachute as long as I landed on a mate's sister to cushion the landing.

LOL!

Keshi.

9:28 PM  
Blogger Homo Escapeons said...

WOW!
Professor Aidan is in da house!
Excellent. I am not sure that my brain is still supple enopugh to absorb or remember all of this information..but I love it.

It is a bit scary when you break it all down..if only there were formulas to predict human behavior.I would love to be able to predict when my son will be climbing the tree in the backyard with a load of bags..any ideas..how old were you?

9:32 AM  
Blogger Aidan said...

Sorry about the delay in commenting, trying to organise a 2nd honey moon:) but the car battery died this morning..... ill fated maybe just set up a tent in the living room.

Keshi: thanks i am pretty sure she had a crush on me so she didnt mind too much..

HE i was around 8 or 9 never realy learned my lesson though. Currently redesigning it i have the wifes permission to injure myself for the course of science... Going to have to wait til after christmas though.. should make a good post.

WE can predict human behaviour to an extent (no free will remember), just need a bit more info and we will work it out.

2:33 PM  
Blogger Keshi said...

She had a crush on ya so she wudnt mind u 'crushing' her with ur landing LOL!

Keshi.

6:16 PM  

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