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Post by BertL on Jul 8, 2007 20:46:34 GMT -4
Hey guys, As you might know I like to do analysis. I did some more today for a debate currently held on one of my YouTube videos. bertl.hostbrickmovies.com/stuff/analysis/Here is the analysis. I would really like to hear what you guys have to say about it. - BertL
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Post by gwiz on Jul 9, 2007 5:23:31 GMT -4
[rocky]they could have filmed it at double the speed[/rocky]
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Post by BertL on Jul 9, 2007 5:35:46 GMT -4
It's easy enough to calculate the astronaut's length according to rocky, then.
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Post by Tanalia on Jul 9, 2007 6:55:48 GMT -4
Just to play Devil's Advocate, from your own post, changing the frame rate will make it appear as if the gravity is different. If we assume it was filmed on Earth with this in mind, then instead of 30 fps we get 30x2.47 = 74 fps. This means the arc would have lasted 98/74 = 1.324 seconds, or 0.662 seconds to go up (and then the same down). Plugging this in with Earth gravity gives a distance of 2.148 m, well within the error tolerances mentioned. So while the clip certainly is consistent with being filmed on the Moon, it can also be construed ( if you only consider this one element in isolation) as being filmed on Earth with a different frame rate. eta: yes, I know it's video, I'm just using "filmed" in the same common, if slightly outdated sense, as similar terms like "taped"
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Post by BertL on Jul 9, 2007 13:01:58 GMT -4
You're right, Tanalia. However the other side of the debate (AngeAzrael) hasn't given "They changed the framerate" as an explanation for how it was faked (he hasn't given any explanation, for that matter). Therefore, I didn't include it in the analysis.
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Post by Count Zero on Jul 10, 2007 6:39:45 GMT -4
Hi Bert, Excellent work, as usual. A couple of minor points: 1.) Gemini astronauts were shorter than average. John Young is 1.75 meters tall (5' 9", according to the Apollo 16 press kit). Using Young's nose as a ruler in this picture, and the distance between his eyes in this picture, I estimate the suit adds ~6cm (0.06m) to his height, for a total of 1.81m. At first glance, your calculation looks off, however...B.) For best accuracy, you really should use the center-of-mass (rather than the top) of the object as your measurement point. It's not hard to see where it is at the apex and as it's coming down; the bright spot around which the object is rotating will serve nicely (it also shows up better than the dark, blurry edges). On the way up you'd have to estimate, however the frame after the one you used looks like a pretty good (and easily visible) split. III.) Here is a parabola ( larger version). Use it wisely. This brings me to a question: Why are people saying a ballistic trajectory is a perfect parabola? Isn't it actually an elliptical section?
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Bob B.
Bob the Excel Guru?
Posts: 3,072
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Post by Bob B. on Jul 10, 2007 8:32:34 GMT -4
This brings me to a question: Why are people saying a ballistic trajectory is a perfect parabola? Isn't it actually an elliptical section? Correct, the path it traces is really the end of a highly elongated ellipse (eccentricity <1). A trajectory is parabolic only when the object is thrown precisely at escape velocity (eccentricity=1). If it is thrown faster than that the trajectory is hyperbolic (eccentricity>1).
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Post by petereldergill on Jul 10, 2007 9:17:47 GMT -4
Then, even in a perfect vacuum, the equation d = vt + .5 at^2 is false?
(Or, not perfectly accurate?)
Pete
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Bob B.
Bob the Excel Guru?
Posts: 3,072
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Post by Bob B. on Jul 10, 2007 9:39:56 GMT -4
Then, even in a perfect vacuum, the equation d = vt + .5 at^2 is false? (Or, not perfectly accurate?) Gravity always acts toward the center of mass of a body and is proportional the the inverse of the square of the distance. Therefore as an object is thrown and its position changes, the direction and magnitude of the gravity vector acting on it also changes. d = vt + .5 at^2 is accurate but acceleration is not constant -- for high precision you must take into account that the acceleration of gravity is constantly changing as the relative positions of the bodies change. This is of course neglibile when you toss an object only a few meters. EDIT: In high school physics it is common to work problems dealing with ballistic trajectories using d = vt + .5 at^2. It is usually assumed that the ground is a flat surface and gravity is always acting straight downward at constant magnitude. In this simplified case the trajectory is a parabola. Now visualize that the surface is curved and gravity is directed radially inward toward the center of curvature and weaker with increasing distance. Your perfect parabolic trajectory is now an ellipse.
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Post by ishmael on Jul 10, 2007 10:16:24 GMT -4
Then, even in a perfect vacuum, the equation d = vt + .5 at^2 is false? (Or, not perfectly accurate?) Pete This equation assumes the acceleration due to gravity is constant. For a small range of motion, it will be almost constant. But for a large motion, the distance and direction of gravitational acceleration will change, as the distance of the object (and direction) from the center of the earth (or other object) changes. EDIT Bob B. already answered.
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Post by petereldergill on Jul 10, 2007 11:32:28 GMT -4
I was thinking more of the tossing of the objects in Bert's video.
I do talk about the changing graviational field with my physics students, but more in terms of GPS, or weighing yourself at the poles vs. the equator
At what point would you need to consider these changes as a factor? I'm guessing in my physics class (grade 11), it probably would not be wise as it would probably confuse the students more than anything.
I do talk about the shuttle takeoff and assume that the thrust of the engines is constant during the first burn, and discuss what happens as the shuttles mass decreases. Does the changing gravitational field affect this enough for engineers to worry about? I'm assuming if you would need to include it, there would be some sort of integration involved for the gravitational field
Is the shuttle's thrust constant during each of the stage burns?
Pete
Edit: I guess for a long enough ballistic, you'd also have to take into account the corriolis effect?
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Bob B.
Bob the Excel Guru?
Posts: 3,072
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Post by Bob B. on Jul 10, 2007 12:15:15 GMT -4
I was thinking more of the tossing of the objects in Bert's video. In that case the changes in acceleration are so small that they are negligible. At what point would you need to consider these changes as a factor? In rocketry you definitely do. I suspect that even long range artillery might be affected enough to consider it, though drag has a much larger influence. I'm guessing in my physics class (grade 11), it probably would not be wise as it would probably confuse the students more than anything. I've never taught so I don't have experience to draw from, but I suspect you might be right. I do talk about the shuttle takeoff ... Does the changing gravitational field affect this enough for engineers to worry about? Yes Is the shuttle's thrust constant during each of the stage burns? No. #1 - the main engines are throttled up and down at different points in the ascent. #2 - an engine's thrust is affected by the changing ambient air pressure. #3 - solid rocket motors are high variable in thrust as the geometry of the burn surface changes.
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furi
Mars
The Secret is to keep banging those rocks together.
Posts: 260
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Post by furi on Jul 10, 2007 12:19:36 GMT -4
Wahhh, can't find my file, might be on home comp, I have a beautiful WWII Naval gunnary training manual that lists the corrections for Lattitude and firing angle and distance direction, Giving corrections for Coriolis and Gravity, I was very suprised at how much data there is in regards to Hi Angle Firing, and how accurately it was calculated (Paris Gun I believe was the first gun to need correcting due to visible coriolis effect)
Will Post link once I find it again
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Bob B.
Bob the Excel Guru?
Posts: 3,072
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Post by Bob B. on Jul 10, 2007 13:36:16 GMT -4
Wahhh, can't find my file, might be on home comp, I have a beautiful WWII Naval gunnary training manual that lists the corrections for Lattitude and firing angle and distance direction, Giving corrections for Coriolis and Gravity, I was very suprised at how much data there is in regards to Hi Angle Firing, and how accurately it was calculated (Paris Gun I believe was the first gun to need correcting due to visible coriolis effect) Will Post link once I find it again Please do post a link when you have it. In addition to being a space buff, I'm also very much interested in naval warships. I'd definitely like to get a look at a gunnery manual.
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Post by sts60 on Jul 10, 2007 14:31:10 GMT -4
Pete, just to expand a bit on one of Bob's points - one reason this is done is to control the aerodynamic loading on the vehicle. You'll hear the PAO (voice commentator) mentioning the SSMEs throttling down as "Max Q" is approached.
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