Flight of rocks thrown through the air

[merrius]

This has been bugging me for 30 years, so I hope someone here can help.

I was at the base of a local Dam that had a fish hatchery on top, and a HUGE flock of hawks circling above, waiting to get lucky. Yum!
In a totally non-PC way, I started throwing rocks at the hawks. My 'lead' on them was perfect, but I wasn't even getting close; the rocks would just reach a peak and tail off, and the hawks didn't even flinch. So, I threw harder. Now, the rocks (the 'smooth, flat, slick, river rock' kind, as Brother Dave Gardner would say) would reach their peak, fall off a bit, maybe 10-20 feet, and then go UP AGAIN and finally fall away.
Question: why should this be? Why the double peak? I don't think it was due to thermals or air currents, as I've observed the same phenomenon when I throw rocks parallel with the ground...but only when I throw really hard.

Thanking you in advance for your assistance...
Merrius

 

[lisab]

Hmm...but a thermal would explain why the hawks were circling; they may have been riding them.

Do the rocks you throw have a lot of spin? Or do you throw them like a knuckleball?

 

[cronxeh]

lisa I don't even understand the meaning of the words you put together

 

[lisab]

lisa I don't even understand the meaning of the words you put together

lol...sorry.

I think there may have been thermals because hawks (and other birds) will increase their altitude by circling around in a thermal updraft. It's much more energy efficient than flapping their way up...sort of like taking the escalator vs. taking the stairs.

The presence of circling hawks indicates to me that there may have been a thermal updraft there. If merrius threw a rock into a strong thermal, it may have been enough to change the path of the rock slightly.

Another thing that might change the path of a thrown rock is spin, the same way a baseball pitcher spins a baseball to make it curve. But a baseball doesn't necessarily need spin to change its path...some pitchers throw the ball with no spin at all, and that's called a knuckleball. The irregularities on the baseball (specifically, the stitches) change the air flow around the ball, making its path very unexpectedly irregular. So even without a thermal, merrius may throw rocks in a way that makes them deviate from the expected ballistic path (i.e., neglecting air resistance)

Sorry, I shouldn't use baseball analogies on a forum with such an international readership .

 

[cronxeh]

that.. was.. one hell of a sexy explanation

 

[lisab]

that.. was.. one hell of a sexy explanation

I bet you melt in physics lectures .

 

[KalamMekhar]

Well it is a dam, and dams can get a lot of air to move upwards (just like sky scrapers)

 

[merrius]

Thanks for the sexy replies...we must be on the internet, eh?

Lisab, yes the rocks I throw always have tremendous spin, since they're smooth, flat, slick river rocks...and I guess I could go for the thermals, but as I stated, I have observed the same effect on rocks thrown parallel to the ground in an open area...but the double peaking was slightly lessened, and only occurred if I threw really hard; if I just threw normally, there was no double peak.

 

[Ivan Seeking]

Thanks for the sexy replies...we must be on the internet, eh?

Lisab, yes the rocks I throw always have tremendous spin, since they're smooth, flat, slick river rocks...and I guess I could go for the thermals, but as I stated, I have observed the same effect on rocks thrown parallel to the ground in an open area...but the double peaking was slightly lessened, and only occurred if I threw really hard; if I just threw normally, there was no double peak.

Keep in mind that even baseballs don't "rise" after pitched. What happens is that the ball falls less quickly than it normally would, which tricks our minds into thinking that it rises. This was a contentious point of debate for baseball fans, for decades.

IIRC, it has been shown that a true "riser" would require something like a 200 mph pitch

Assuming the rocks have some aerodynamic lift, it would make sense that you only see the effects of lift when the velocity of the rocks is maximized by a hard throw. [Ideally, the most flat surface of the rock should be facing down, with the most curved surface facing upwards.]

In thinking about how a rock a thrown, it might even make sense that a rock would tend to align for maximum lift at the peak of the trajectory. This in turn could create the illusion that the rock peaks a second time, just after the first peak, and just before the forward velocity drops below some critical value. It could also be that not long after the peak, the angle of the rock changes enough as it follows the trajectory, so that the lift becomes insignificant [points in the wrong direction].

One more possibility is that there is no significant lift; rather, your mind is being tricked by the fact that zero vertical velocity occurs at the peak. For a moment, it doesn't fall at all. For a short time after that, it is falling relatively slowly. This alone might trick the mind in the same way that a "riser" does in baseball; giving the impression of a second peak.

 

[Pythagorean]

If the rocks are disc shaped, it's not so hard to imagine them actually peaking.

 

[Ivan Seeking]

If the rocks are disc shaped, it's not so hard to imagine them actually peaking.

Even baseballs don't actually rise. Yes, it is very hard to imagine. That would be many grams of lift over 2 or 3 square inches.

 

[Ivan Seeking]

I should add that a baseball generates lift through its forward spin. A human cannot cause the ball to spin that fast. I remember the 200 mph number from somewhere as a practical number, but I also remember seeing baseball spun on a shaft, in a wind tunnel, such that the lift could be measured. The rate of spin required was far above the ability of a human manage, in a throw.

 

[merrius]

At the Dam, the rocks reached their peak, started down for an estimated 10-20 feet, just like regular throws, then suddenly jumped up again to a new peak. It was neat to watch, since I had never seen that before, and people were wanting me to keep throwing, but after 8 or 10 of those 'super throws,' my arm gave out. From the ground, it looked as if the second peak MIGHT have been even higher than the first...

My arms are on the short side, but if I had long arms like the Johnsons, (Walter and Randy) I would be able to throw even harder, and I bet I could have scared those hawks.

 

[Ivan Seeking]

At the Dam, the rocks reached their peak, started down for an estimated 10-20 feet, just like regular throws, then suddenly jumped up again to a new peak... From the ground, it looked as if the second peak MIGHT have been even higher than the first...

Based on what reference point? How do you know that it actually rose again?

 

[merrius]

Ivan: I said the second peak MAY have been higher than the first. It only SEEMED as if it could have been; but there was definitely a jump up from it's previous descending phase, even if it didn't surpass the first peak.

 

[russ_watters]

Even baseballs don't actually rise. Yes, it is very hard to imagine. That would be many grams of lift over 2 or 3 square inches.

Baseballs produce very little lift because they are spheres. A flat rock can produce a significant amount of lift - kinda like a heavy frisbee.

What the OP is describing sounds to me like the rock's aerodynamics make it want to pitch up, which gives it lift, which makes it rise or appear to rise*. Then it stalls and falls again, increasing speed...which makes it pitch up again.

A paper airplane that isn't trimmed properly does the same thing.

*Whether it actually rises or it just appears to rise because the downward trajectory has been significantly altered doesn't really matter. It is just different degrees of the same phenomena.

 

[Ivan Seeking]

Baseballs produce very little lift because they are spheres. A flat rock can produce a significant amount of lift - kinda like a heavy frisbee.

What the OP is describing sounds to me like the rock's aerodynamics make it want to pitch up, which gives it lift, which makes it rise or appear to rise*. Then it stalls and falls again, increasing speed...which makes it pitch up again.

A paper airplane that isn't trimmed properly does the same thing.

Well, in principle, but what is the mass density of a paper airplane as compared to a rock? I doubt any hard calculations would show this to be siginficant.

*Whether it actually rises or it just appears to rise because the downward trajectory has been significantly altered doesn't really matter. It is just different degrees of the same phenomena.

Not true. If enough lift was generated to actually cause it to rise, then we have a completely different family of curves in play. You can only get a second peak if enough lift is present to cause it to rise, producing a curve having a positive concavity. Without enough lift to rise, it would still follow a curve having a negative concavity; and there would be no true second peak.

 

[Ivan Seeking]

Ivan: I said the second peak MAY have been higher than the first. It only SEEMED as if it could have been; but there was definitely a jump up from it's previous descending phase, even if it didn't surpass the first peak.

Again, how do you know "definitely". Why do you reject the notion that it could have been an illusion?

Without taking some time to lay this out formally, I don't know if you could generate enough lift or not. My objection is: Neither does anyone else. It simply may not be possible to change the vertical direction of motion through lift alone. So it is reasonable to ask how you can be so sure. Without a proper reference for you to judge elevation, it seems more likelly to me that it was an illusion.

If it really did rise, then based on your description of things, it would have to be explained by the basic aerodynamics referenced already.

 

[sardonicus]

I've skipped a lot a rocks in my time. The point is to throw a rock (flatter the better) with spin along the surface of the lake or river and get it to bounce as many times as possible off the water. Over the years I've come across a lot of different rocks and have seen a lot of odd trajectorys, don't recall seeing one as described here but these a parallel to the water. Another shoreline activity when ones arm gets sore is to find a suitalbe bat of driftwood (abundant in some locations), then one-armed toss rocks up and bat them out into the lake. Obviously nice shperical stones go the best but curiosity results in batting all sorts of rock fragments that make cool whirring and whining noises and wild twisting trajectorys. I can't deny it but I do not recall seeing that the type of flight described.

Next time I'm out I will try to remember to see if I can recreate the phenomenon, I'll try with and without hawks, if possible, do you think seagulls would do?

 

[russ_watters]

Not true. If enough lift was generated to actually cause it to rise, then we have a completely different family of curves in play. You can only get a second peak if enough lift is present to cause it to rise, producing a curve having a positive concavity. Without enough lift to rise, it would still follow a curve having a negative concavity; and there would be no true second peak. [emphasis added]

Wrong. If the general trajectory is downward, you can be concave up, but still descending.

 

[cronxeh]

Well, in principle, but what is the mass density of a paper airplane as compared to a rock? I doubt any hard calculations would show this to be siginficant.



Not true. If enough lift was generated to actually cause it to rise, then we have a completely different family of curves in play. You can only get a second peak if enough lift is present to cause it to rise, producing a curve having a positive concavity. Without enough lift to rise, it would still follow a curve having a negative concavity; and there would be no true second peak.

The mass is unimportant, as 10 pound airplane of same shape as 1 ounce airplane will fall at the same rate.

The rock's lift depends on velocity² and the faster the rock goes the more lift it will generate, once the angle of attack is great enough it will literally defy gravity and soar up until the velocity drops below critical level and the rock 'stalls' plummeting down again. Thus you see double peaks. There might be more, depending on how fast the rock was thrown off a cliff and height

 

[mugaliens]

Even baseballs don't actually rise. Yes, it is very hard to imagine. That would be many grams of lift over 2 or 3 square inches.

Actually, http://www.youtube.com/watch?v=AWzciMY8pbU". But they either have to be a wiffle ball, or they have to ricochet off the back of a lawn chair. In the case of the my linked video, it's both.

 

[loseyourname]

It's also possible that the http://www.physics.armstrong.edu/faculty/mullenax/research/riseball.html" thrown in fastpitch softball can actually rise.

 

[Ivan Seeking]

Wrong. If the general trajectory is downward, you can be concave up, but still descending.

A positive concavity means a positive second derivative in the equation of motion. If something has a constantly negative [downward] vertical velocity after the peak, the second derivative is negative. Check your calc I book. - ie, the acceleration vector [due to gravity] always points downwards; ignoring acceleration due to air friction.

 

[Ivan Seeking]

The mass is unimportant, as 10 pound airplane of same shape as 1 ounce airplane will fall at the same rate.

Incorrect. The downward force due to its mass must be exceeded by the lifting force, in order to rise. How fast it falls has nothing to do with it.

 

[Ivan Seeking]

Actually, http://www.youtube.com/watch?v=AWzciMY8pbU". But they either have to be a wiffle ball, or they have to ricochet off the back of a lawn chair. In the case of the my linked video, it's both.

It's also possible that the http://www.physics.armstrong.edu/faculty/mullenax/research/riseball.html" thrown in fastpitch softball can actually rise.

Note that I said a baseball, not a wiffle ball or a softball - the key point being that a baseball most closely approximates a rock.

From your link, lyn:

Please note that the fastball does not rise though; it simply falls slower than a ball without a backward rotation.

 

[boit]

I have to perfom the experiment seeing that the lake is less than ten minutes walk away. But first I need to see birds gliding about a centre. Now if I just had a catapult.. and a high speed camera..

 

[david-scott]

What instead of a double peak it would have plateaued then peaked?

This, which others have mentioned, is affected by the observer.

If there was a thermal pattern pushing air upward from the instance the rock leaves the hand thrown this would have added to the vertical velocity.
After the rock was out of this air pattern it would slightly decrease in speed, because it is now out of its previous medium.
The rock would then continue upward out of the first medium until it hits its peak.

If this was done right by the side of the dam, I could see the air being just like the upward air around a skyscraper.