What forces are at work here?

[rivan]

Surely this only barely qualifies for HT, but here it is:

- Take a stick pen and strip it to the barrel.
- Put the barrel on a table or other flat, smooth surface, put your index fingers together in the center of the barrel.
- Push down as hard as you can, then let the barrel pop forward out from under your fingers. Giving the barrel spin is more important than forward motion. Equal pressure on each finger will yield a straighter flight.

The barrel should fly forward, arc up into a loop, finally arcing almost back it's original flightpath. A good shot from a desk can get a loop as high as about 5 feet, and the pen can travel forward 15 feet or so before hitting the ground.

So the part I don't get is the loop. Why does the barrel spinning on it's axis end up getting us a loop in midair? Why only one? Why does the pen - after the loop - then fly almost exactly in the direction it was originally shot in, at the level it was originally shot in?

 

[BrownTown]

whats a stick pen?

 

[imported_inspire]

Originally posted by: rivan
Surely this only barely qualifies for HT, but here it is:

- Take a stick pen and strip it to the barrel.
- Put the barrel on a table or other flat, smooth surface, put your index fingers together in the center of the barrel.
- Push down as hard as you can, then let the barrel pop forward out from under your fingers. Giving the barrel spin is more important than forward motion. Equal pressure on each finger will yield a straighter flight.

The barrel should fly forward, arc up into a loop, finally arcing almost back it's original flightpath. A good shot from a desk can get a loop as high as about 5 feet, and the pen can travel forward 15 feet or so before hitting the ground.

So the part I don't get is the loop. Why does the barrel spinning on it's axis end up getting us a loop in midair? Why only one? Why does the pen - after the loop - then fly almost exactly in the direction it was originally shot in, at the level it was originally shot in?

What office do you work for? I'd like an application - it seems we need more guys on the payroll to figure this one out.

 

[imported_inspire]

Sorry - double-post.

 

[rivan]

The kind you can remove all the parts to and end up with a somewhat flexible plastic cylinder.
http://www.theadvertisingstore.com/pens/pen_images/pens.jpg

edit - another, slightly better image

 

[rivan]

Originally posted by: inspire
What office do you work for? I'd like an application - it seems we need more guys on the payroll to figure this one out.

 

[BrownTown]

hmm, hard to visualise what you are saying without seeing it, but it likely has to do with the rotational inertia of the pen.

 

[nova2]

http://images.google.com/images?as_q=stick%20pen&safe=off

 

[BrownTown]

no, i get the stick pen now, its just some fancy name for a normal pen. What I'm not sure about is the trajectory of the pen in flight after launched.

 

[rivan]

It'll fly like this.

The arrows on the first circle show the direction of rotation as it leaves your grip. There's a good amount of forward motion when you let it fire out from your fingers, and ideally there's quite a bit of rotation as well.

 

[CTho9305]

My guess is http://en.wikipedia.org/wiki/Magnus_effect

 

[CycloWizard]

Originally posted by: CTho9305
My guess is http://en.wikipedia.org/wiki/Magnus_effect

:thumbsup:

It's just like curving a soccer/tennis/ping pong ball. If you put spin on it, the drag created by the combination of the spin and translation cause the fluid to curve around the object (from a Lagrangian perspective) or the object to take a curved path in the fluid (from an Eulerian perspective). The fluid will match the object's velocity at the surface (no-slip boundary condition), creating a shear stress gradient near the object's surface.

 

[rivan]

Thanks for the answers guys - though I'm still a little baffled by this particular instance of the effect. Is it just chance that the pen has enough rotational inertia to make it through a single loop before evening out, usually to very near it's original path? Is the size of the loop directly proportional to the amount of rotational energy given to the barrel, and if so, how does it happen that the pen tends to complete a single loop then continue on? The Magnus effect I can see getting the pen into and through the loop, but the almost straight flight at the bottom/after of the loop doesn't seem to be accounted for by the effect.

...or is it, and I'm just not following it through all the way?

 

[CycloWizard]

Originally posted by: rivan
Thanks for the answers guys - though I'm still a little baffled by this particular instance of the effect. Is it just chance that the pen has enough rotational inertia to make it through a single loop before evening out, usually to very near it's original path? Is the size of the loop directly proportional to the amount of rotational energy given to the barrel, and if so, how does it happen that the pen tends to complete a single loop then continue on? The Magnus effect I can see getting the pen into and through the loop, but the almost straight flight at the bottom/after of the loop doesn't seem to be accounted for by the effect.

...or is it, and I'm just not following it through all the way?

It's probably more a coincidence in that the force required to overcome the static friction is such that you apply a specific rotational velocity and translational velocity and get a nice loop out of it. If you spun it faster and could keep the translational velocity constant, the loop would become smaller.