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Whilst thinking of a trick where a magician pulls a tablecloth directly from under a full set dinner (a classic) I wondered why friction doesn't increase as speed does. I get that they use a pretty sleek table cloth but even still I don't see how friction doesn't stop this motion even further, for instance the quicker one speeds around a race track, the quicker the tires bald. Is it perhaps the traction of the two which has this effect of decreasing friction or is there another force in play?

( if you think this question is off topic or unnecessary to the general public, just tell me so and I will edit it or delete it if need be, please do not vote as off topic or the like)

Qmechanic
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Diamond Louis XIV
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  • You won't be happy with a purely mathematical explanation? – HDE 226868 Nov 25 '14 at 02:19
  • It's not really the speed that kills NASCAR tires so quickly, it's the heat combined with drifting whilst rounding the corners. The effect of wearing the tire out quickly is caused by friction, but it's not the speed you attribute. – Kyle Kanos Nov 25 '14 at 02:19
  • I would delete it but because it has an answer I cannot, is there something else I should do to fix the apparent duplicate. – Diamond Louis XIV Nov 25 '14 at 17:51
  • @KyleKanos I would have preferred a comment saying that this question was a duplicate however I understand this is the standard procedure, hopefully next time it will be different though. – Diamond Louis XIV Nov 25 '14 at 17:59
  • @HDE226868 I would have no problem with and would enjoy a mathematical answer. – Diamond Louis XIV Nov 25 '14 at 18:03

1 Answers1

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Dynamic friction is constant, it doesnt change with speed. That is why the trick works. If you pull the cloth fast enough, the friction force will act for such a short time that it will not be enough to pull the stuff above it. In the case of the weels, the frictions force is also constant, but you make it last longer per unit of lenght because the wheels speed faster. In addition, most of the time you drive the friction is static, the point in contact with the asphalt is not moving relative to the tire. But if you make the weel rotate faster than the car moves, then the dynamic friction will waste it faster (like sandpaper).

  • Well more formally I'd say, because $\Delta v = a * \Delta t$, and static friction is $F_{friction} = k mg$, $a_{friction} = kg$, you have a lower $\Delta t$ under the trick so your $\Delta v$ is smaller. This is despite power being larger under the the fast case $power_{friction} = F distance / time$. $Power = energy/\Delta t$! Energy is the same. – atlex2 Jun 23 '19 at 15:33