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So this guy was the first to run a loop and in this (german) article (and also in the video) a certain speed (13.8km/h) is mentioned.

Why must he run at this speed and not just "as fast as possible"?

My intuition says it has to do with to much centrifugal force and him not being used to it $\rightarrow$ trip hazard.

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Presumably the minimum velocity they calculated would just perfectly balance out gravity which wouldn't leave any additional force left for him to actually run. If you watch the video, when his feet reach the apex he's barely going fast enough and can't really push off of the top but forward momentum keeps him going. He must have lost a lot of velocity while transferring his forward momentum upward. –  Brandon Enright Feb 26 at 7:50

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When they mention that speed they are speaking of the minimum speed he will need to run in order to keep contact with the loop. I believe they calculated that number simply as a reference point so that 1) they knew looping the loop was theoretically possible, and 2) so he had an estimate of about how fast he needed to run. Imagine trying this stunt all day just to find later that in order to run around the loop you would have needed to achieve a speed of $50 {km \over h}$ for the stunt to be even theoretically possible.

In fact, if you watch the video you can tell that he does run faster than what he theoretically needed to in order to run safely around the loop. Theoretically, the minimum velocity needed at the apex of the loop is the velocity which will result in a normal force of zero at that point. This means at the very top of the loop his feet would not be exerting any force upon the loop at all (meaning he would just barely make it through the apex without falling). You can see from the video that this is not the case. If you watch the slow motion clip his foot clearly presses against the track very near to the apex of the loop, meaning he must have been traveling faster than the theoretical minimum velocity.

So, in answer to your question, yes, running as fast as he could would have worked just fine, since that would clearly have been faster than the minimum velocity. Of course, the faster he runs the more leg strength he would need at the apex to resist the normal force of the track pushing down on him.

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