Catapult science fair project

[stscherer]

I am trying to come up with a hypothesis on wheter a ping pong ball with holes or without holes will be thrown farther from the catapult in the science fair project "Bombs Away" I could not find any research on the affect of the holes on the distance? Is it that it will increase the air resistance thus not be thrown as far? Please help give me some info on this. Thanks,

[deleted-71417]

Hi,

Here is something related to your question:
“
Jearl Walker www.flyingcircusofphysics.com
December 2006 Golfers have long realized that a dimpled golf ball will fly much farther than a smooth ball because the dimples somehow reduce the air drag on the ball. It is that drag force that opposes the ball's motion and drains energy from it. Understanding how the dimples decrease the drag force has been very challenging because the experiments with air flow past a ball are difficult to see or measure. Up until now, that is.
The air drag is primarily due to a difference in the air pressure between the front and rear of the ball. Let's take the perspective of a smooth ball, as if we rode along with the ball and felt the air streaming past us. As the stream moves around the surface of the ball, the air layer rubbing against the surface slows until it reaches a stagnation point, and then the stream breaks free of the surface. On a smooth ball, the break-away point occurs well before the air reaches the point on the rear that is opposite the impact point on the front.
This break-away of the air stream creates a vortex-filled wake behind the ball. Because the air pressure in a vortex is low, this condition means that the ball has high pressure along its front surface and low pressure along its rear surface. The difference in the pressures on front and rear is the air drag that slows the ball.
Dimples change the picture dramatically because somehow they delay the stagnation of the layer of air sliding past the surface of the ball. So, the layer clings to the ball until it reaches the point almost directly behind the front impact point. The break-away point (or the stagnation point) is said to be delayed because it occurs farther back on the rear surface of the ball.
The result is that the vortex wake is much narrower and so the pressure across the rear surface of the ball is not so low. That means that the pressure difference between the front and rear is lower than with a smooth ball, perhaps 50% lower, and so the drag force is less by that same amount. What matters to the golfer is that a long drive goes much farther toward the hole.
I've known all this since I wrote the first edition of The Flying Circus of Physics. For all those years the nagging question has been: "Yes, but why do the dimples delay the break-away point?"
Jin Choi, Woo-Pyung Jeon, and Haecheon Choi of Seoul National University in Seoul, Korea, have now published an answer based on experiment because they figured out a way to measure the speeds down within and just above the dimples on a golf ball. A dimple causes turbulence in the air flow next to the ball's surface. Bringing faster air down next to the surface prevents the air next to the surface from slowing, stagnating, and then breaking free of the surface. Thus, we have the seemingly contradictory statement that the dimples lower the air drag on a golf ball by creating turbulence in the air flowing past the ball. For a golfer, then, turbulence is a good thing.

http://www.cookeassociates.com/seesite/ ... ground.htm golf ball (go down to the photos showing smoke tracers moving past a tennis ball)

· Aoki, K., A. Ohike, K. Yamaguchi, and Y. Nakayama, “Flying characteristics and flow pattern of a sphere with dimples,” Journal of Visualization, 6, No. 1, 67-76 (2003)
· · · Penner, A. R., “The physics of golf,” Report on Progress in Physics, 66, 131-171 (2003)
· Won, S. Youl, Q. Zhang, and P. M. Ligrani, “Comparisons of flow structure above dimpled surfaces with different dimple depths in a channel,” Physics of Fluids, 17, article # 045105 (2005)
· Choi, J., W.-P. Jeon, and H. Choi, “Mechanism of drag reduction by dimples on a sphere,” Physics of Fluids, 18, article # 041702 (4 pages) (2006)
· · Libii, J. N., “Dimples and drag: Experimental demonstration of the aerodynamics of golf balls,” American Journal of Physics, 75, No. 8, 764-767 (August 2007)”

This quote is from this website:

http://www.flyingcircusofphysics.com/pd ... ef_Com.pdf


You might find this discussion of golf ball aerodynamics helpful:

http://en.wikipedia.org/wiki/Golf_ball

http://www.ihed.ras.ru/flucome10/cd/papers/221.pdf

http://www.springerlink.com/content/q8646350j3h46246/


I had the same problem you have had finding comparisons between the aerodynamics of smooth spheres and spheres with holes in them. The closest thing I could find was the comparison between smooth spheres and spheres with dimples in them. But for purposes of forming a hypotheses dimples may be close enough to holes qualitatively. See what you think.

Best regards,

Barrett L Tomlinson

[deleted-71939]

In addition to the dimpled golf balls, here is some information regarding actual holes in a golf ball, or in your case, a ping pong ball...
Your first instincts were right! A ping pong ball with holes would have a much higher air resistance so it would require a lot more force to get it the same distance as a regular ping pong ball...So your hypothesis can definitely mention this part and it would be correct...The more streamlined and solid the shape of an object is, the less air resistance there will be on it...
Another factor you can also consider is mass which can have an affect on how far things will go...this has to do with inertia which basically means that objects resist any change to their motion...the more an object weighs, the more inertia it has so it takes a lot more force to get it moving but once it does, it is difficult to stop it as well...this works in reverse as well...the less an object weighs, the less inertia it has so it take little force to get it moving but also take very little force (like air resistance) to get it to stop moving...
This is why its easier to throw a baseball farther than a ping pong ball because a baseball has more mass and thus more inertia so air resistance has less of an effect on it..heres a nice article on inertia: http://www.mansfieldct.org/schools/mms/ ... nertia.htm
this is just another factor you might want to consider but what you have so far looks great!

Hope this helps and feel free to ask any more questions!