Somebody showed me a neat trick to do with school supplies in 7th or 8th grade, and I’ve wondered since how it works.
To do this you need a hollow cylinder - what we used were ballpoint pens that had that little black cap at the end opposite the point, so you could pull both off and have a hollow plastic cylinder. Once you have done that, you lay it on a flat surface, put your thumbs on it, and push down so it shoots forward but spins backwards. If you do it right the pen will take off of the table, rise several feet, and sometimes even do little loops.
Why does it do this? I have a hypothesis - since the pen is spinning backwards, the air on the top surface has less resistance from the surface of the pen than the bottom, which is spinning against the wind. I have my doubts about that, though, as the pen behaved in ways that made it seem like it had lift even when it started moving back towards me.
No, it’s definitely some kind of aerodynamic effect, not recoil. It will go forward a ways before lifting up, and will wiggle and loop around in the air, rising several feet sometimes.
BTW, the type of pen I just did it with was a Bic round stic, in case anyone wants to recreate it. It may take a few tries, but it’s not hard, once you manage to get that plug out of the end (molars work well).
I’ll back Badtz up on this - I’ve done it myself. Unfortunately, I’m too tired to do the research this calls for tonight.
But while you’ve got your pens apart, may I suggest cutting the shaft down to about two-thirds the normal length, and you’ll have a midget pen you can fit in your back pocket.
Same way a backspinning golf, tennis, table-tennis or whatever shot has more hang time.
I read a paper in Scientific American on this many years ago. As I remember it, the last part of the OP is almost right. The effect is due to uneven flow around the top and bottom of the pen (or ball or whatever). The top of the pen is rotating backwards, which makes for less turbulence over it. If conditions are right, the air flow over the pen will be laminar (straight and smooth). Conversely, the bottom of the pen is rotating into the air flow which if conditions are right will cause turbulent airflow under the pen.
Behind any object moving through the air is a “hole” (an area of comparatively low air pressure) that the air coming around the object rushes to fill in. But when the laminar flow over/turbulent flow under phenomenon occurs, the laminar flow over the top does not curl down behind the pen as well as the turbulent flow coming under the pen. This means that the area of low pressure behind the pen is offset upwards, and the imbalance of pressure causes the pen to rise, or hang or whatever.
I’ve seen a simlar effect in an experiment with a spinning balloon (a prototype lighter-than-air vehicle)
The spherical balloon was fixed in such a way that it could be rotated around a horizontal axis which was at right angles to the direction of travel, the vehicle was propelled forwards by turbo fans or some such, anyway, when the vehicle was travelling forwards, it could be made to rise or fall by spinning the balloon in one direction or the other.
I know the trick Badtz is describing, and I can assure you, it’s not the Magnus effect - it’s far too abrupt a leap.
I always reasoned that it’s due to the hexagonal cross section of the pen. The pen that works is those classic hexagonal bic ballpoints with the clear plastic sleeve, a plug in one end, tapered at the other, and a tiny little air hole in the side.
When you backspin it and force it across the table, the hex shape causes it to bounce ever so slightly, just a fraction of a millimeter. When it comes back in contact with the table, the first part to hit is the corner of the hex, which is moving pretty fast. Thus it bounces very high the next time.
After I read the OP I tried it. I used a normal round pen, not hexagonal or bumpy in any way. Try it, you can tell by the way it rises that it has nothing to do with compression of the pen or bouncing. Sometimes it will fly level at first, then rise. It works on the floor as well as off of a desk. Very cool trick.
I’ve never heard of this, but any aerodynamics textbook will tell you that a rotating cylinder will generate lift. Backspin for lift, just like a golfball. Sidespin on a ball will cause a hook or slice.
yo…I used to do this all the time in high school and I think its basic golf ball aerodynamics too…but try this stand facing a smooth wall and try it…the pen will loop right over your head and hit you right in the ass…my friends and I would take turns so we could see it happen…just had to chime in…peace