Dot Physics

I don’t need to explain that I think the Mythbusters are awesome (do I?). I just finished watching their latest episode. In it, they tested the scene from Indiana Jones and the Last Crusade where Indy puts a flag pole in another motorcycle wheel. The result in the movie is that the motorcycle goes flipping in the air. I have no problems with their test – it seems like they did a decent job. However, in the end, Adam says something to the effect:

*”Look how high that bike went. It took a lot of energy. There is no way that energy could come from this stick I shove into the wheel.”*

I am sure many people will attack him for saying the energy came from the stick. He was probably just excited from blowing up a motorcycle that his tongue slipped (that kind of stuff happens to me all the time). Maybe he meant to say that there was no way that bike could get that high without explosives? I don’t think this is correct. The motorcycle has kinetic energy, probably a lot. So the question is: if a motorcycle is traveling at 40 mph (like in the show) how high could it go? The easiest answer is to think about a ramp. If the motorcycle interacted with a ramp that shot it vertically, how high would it go?

(you know I have to have a picture)

![Screenshot 28](http://scienceblogs.com/dotphysics/wp-content/uploads/2008/10/screenshot-283.jpg)

If I consider the motorcycle plus the Earth as the system, then the external work on it during this motion is zero. This gives:

![Screenshot 29](http://scienceblogs.com/dotphysics/wp-content/uploads/2008/10/screenshot-291.jpg)

Here I am assuming at the highest point, the velocity (v2 is zero) and the initial y (y1) is zero. Also, note that the mass cancels. So, if the motorcycle is going 40 miles per hour ([according to google calculator, that is 17.9 m/s](http://www.google.com/search?q=40+mph+in+m%2Fs&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a)), then I can plug in:

![Screenshot 30](http://scienceblogs.com/dotphysics/wp-content/uploads/2008/10/screenshot-302.jpg)

I don’t know how familiar you are with distances, but 53 feet is pretty high. In the movie and in the show, I don’t think the motorcycle went higher than about 10 feet. So, it surely has enough energy to do that move.

Comments

  1. #1 Uncle Al
    October 30, 2008

    Good magnitude, bad direction. Transitioning from horizontal to vertical must also conserve angular momentum – and the stick is insufficient mechanism. Popping a wheelie won’t get you lofted absent a ramp or other launcher. Then there is the matter of impulse implicit in a quick transition.

  2. #2 Dave
    October 31, 2008

    Uncle Al has touched on one factor which you didn’t analyse. You didn’t factor in the rotational energy/momentum stored in the rotating wheels. That might be a rather interesting analysis to perform. When the one wheel, which has been rotating, is stopped, via the flag pole jammed into the spokes, the rotational momentum has to be transferred somewhere (into the body of the bike). How high will that cause the bike to flip?

    Dave

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