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Flying circus of physics

Loop-the-loop by cars, bikes, skateboards, and (!) simply running

Saturday, November 01, 2014


Loop-the-loop by cars, bikes, skateboards, and (!) simply running
Jearl Walker
November 2014   A standard physics problem and demonstration involves an object moving into a vertical loop and maintaining contact to complete the loop. The object might be a block moving along the track without friction or a ball or disk rolling smoothly along the track. The object might be given an initial speed on a horizontal track or it might gain that initial speed by coming down from an initial height.

Normally, the problem focuses on this question: When the object enters the loop, what is the least speed that will allow it to maintain contact with the track and complete the loop? If it enters with more than that critical speed, it easily completes the loop, but if it enters with less than the critical speed, it falls off the track before reaching the top of the loop.

Let’s consider a ball sent rolling along a horizontal track toward the loop. The track supports the ball with an upward force called the normal force, where the adjective “normal” means that the force is perpendicular to the track. When the ball enters the loop and travels upward, the track continues to push on the ball with a normal force that is always perpendicular to the track at the ball’s location. That force deflects the ball’s travel so that the ball continues to follow the curvature of the track.

The tricky part is that as the ball travels upward, it slows and the normal force weakens. If the ball enters the loop with the critical speed, the normal force disappears just as the ball reaches the top of the loop, but it then begins to strengthen as the ball travels down along the rest of the loop. However, if the ball enters the loop with less than the critical speed, the normal force disappears before the ball reaches the top of the loop. That means that the ball loses contact with the track and flies off of it through the air.

So much for a ball moving through a loop in the traditional physics demonstration or homework problem. Are there other examples, perhaps dangerous or even potentially lethal examples? Well yes.


Calculating the critical entering speed is more difficult when the object is long compared to the radius of the loop. Here in this experimental setup and after several failures, the young men finally resorted to using a pickup truck to get enough entering speed.

Small motorcycle

Using a “pocket bike,” this young man experimentally discovers that he needs a long run-up distance to get enough entering speed.


Here Bob Burnquist first shows us what has become a standard skateboard trick. But then he raises the existential question: What happens if the top of the loop is missing? In a classroom demonstration, the ball would just fly through the air, crash into the far side of the loop, and then fall down. But could an expert skateboard regain his balance after hitting the far side?

Car and motorcycle

Can the trick be done with a car or motorcycle? Well, yes, though a mistake could be fatal. Fifth Gear, car loop two cars Top Gear, one vehicle, two loops (720 degrees) Top Gear, motorcycle, two loops (no protection)


Yes, just running. No car, motorcycle, or skateboard. Just running into the loop. Here is Damien Walters running a loop-the-loop. This is not only difficult to accomplish but also difficult to calculate. When Walters begins to climb the loop, he is propelling himself upward by his foot on the loop. None of the other examples involve such a propulsion. Also, he is not small relative to the loop’s radius, nor does he make contact all the way around. Rather, he almost leaps from contact point to contact point.

More videos waterslide real loop, put at end bike looping (long, bike near end) skateboard, good but long Hot Wheels documentary on doing the loop

Fifth Gear, full show part 1 part 2 part 3 part 4 part 5

Are you looking for the stories from last month? They are in the archives:

Nimitz Freeway collapse, 2.103

Warmth of greenhouses, deaths of children in closed cars, 4.84

Friction welding, 4.11

Pub trick --- opening a wine bottle without a corkscrew, 1.269



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