Naked Science Forum
Life Sciences => The Environment => Topic started by: ricbritain on 19/06/2010 21:34:15
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In case you didn't know it is the football/soccer world cup in South Africa right now, and my question relates to it.
The tournament is being held in some places of high altitude. A couple of work mates commented on how the 'thin air' is effecting the movement of the ball through the air. I am a little skeptical of this and can't imagine it to be true. Surely something the size of a football move at speed will not be effected in any tangible way. Has anyone got any idea if I am right to be skeptical or if my colleagues are correct?
Thanks
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Yes, I think it's true. Johannesburg is at quite an elevation (around 5750 feet), so the air there will be quite a bit "thinner" (less dense.) A ball will experience less drag due to the air than it would at sea level. For a given force of kick, the ball will travel a greater distance.
Of course, if the players are not used to playing at high altitudes, they'll also be be finding they are a bit more breathless than usual.
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Yes, I think it's true. Johannesburg is at quite an elevation (around 5750 feet), so the air there will be quite a bit "thinner" (less dense.) A ball will experience less drag due to the air than it would at sea level. For a given force of kick, the ball will travel a greater distance.
Of course, if the players are not used to playing at high altitudes, they'll also be be finding they are a bit more breathless than usual.
Would the barametric pressure differential affect the air in the ball and the size of the ball?
Or is that adjusted
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It affects the players more than the ball, which is inflated to the same gauge pressure relative to the local ambient pressure, thus it is pretty moch going to play the same. The players will suffer if they are not acclimatised, as they will be getting a lower mass of oxygen with each breath, although the volume of air inhaled is the same.
Makes for some interesting games as teams travel from there to the sea level stadia and perform markedly different.
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thus it is pretty moch going to play the same.
I don't think so. The reduced drag on the ball will be quite real. It might take the players a while to learn how to compensate for this effect, or maybe they can adapt very quickly.
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I remember years back when the News media interviewed the Olympic athletes. They mentioned they went to higher ground to condition themselves for endurance. They spent months on end training in the low air density. If they needed time to adapt why not anyone else?
I understood it was not a quick transition. Is this old thinking? Did man evolve in that short of time to quickly adapt?
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I understand the effects of high altitude on an athlete. This is well known and easy to understand. I still am not sold on the effect on the ball though. If it effects the ball would it not effect everything else that moved through it?
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One way of approaching this is from an aircraft perspective. Aircraft instruments measure something called indicated airspeed, this is the same as the true speed through the air at ground level and a certain pressure and temperature.
A rule of thumb is that the IAS under-reads by about 2% per 1000ft of altitude. The highest stadium is in Polokwane at about 4000ft, so the IAS would be 8% wrong. So a football will experience drag as if it was going about 8% slower than it is actually moving.
What makes it more complex is that the drag force on a football drops radically over a certain speed (the air becomes turbulent which acts like dimples on a golf ball making the ball fly further). Footballers take advantage of this effect to confuse goalkeepers.
http://physicsworld.com/cws/article/print/1533
So at ground level if you can kick the ball at about 25m/s it will go a lot further than if you kick it at 20m/s, however in a high stadium this transition will occur at an 8% higher speed. If you can only just kick the ball fast enough normally, instead of the ball moving in the low drag regime it could well be in a high drag one, and not go nearly as far, or slow down more than you were expecting.
The transition between the two types of flow is also used in curving shots to confuse the goalkeeper - if a curving shot suddenly moves into the high drag regime near the goaley it will suddenly both change speed, and if it is spinning start swerving radically, - making it very hard to catch. However if it never got into the low drag regime it won't go through the transition and will be much easier to catch.
There is of course also an effect on the players which may well be larger, but it sounds plausible that a ball could be moved a few 10s of cm by the pressure difference which when shooting into the corner of the net may make all the difference.
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That's an excellent answer; thanks.
Chris