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 (nobody)...and 611 guests Last 5 registered Oplandisks nothingstar N_loop yipe foxtrotromeo Browse members... |
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A small source of light S is located at a distance L from a vertical wall. An opaque object with a height of h moves toward the wall with constant speed v. At time t=0, the object is located at the source S. Find an expression for the speed v_shadow of the top of the object's shadow at time t. Express the speed of the top of the object's shadow in terms of t, v, L, and h. ------ hints i got: Hint 1. Calculate ratios of triangles Hint 2. Calculate the derivative Hint 3. Find dx/dt Hint 4. Substitute x ------ this is all the info i have. ive tried entering a gazillion of formulas but the goddamn program doesnt like one of them. its supposed to be pretty simple but i HATE working off a screen, and im used to questions in dutch, not english. if anyone has the time to help me solve this id be most grateful. thanks, hugs, kisses, Santa Claus |
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i done some inverse square law stuff for lighting about a year ago i could try and find some of my old notes gimme about an hour |
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i gave 5 wrong answers so should i ever come up with the correct answer i expect my score to be negative :\ |
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I am gonna try |
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youre an arcitect, arent you? |
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got it (i think), give me time to type it out in short: vs = - ( (h*L)/v ) * (1/t^2) |
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thats awesome!!! thanks a lot i scored 0 points because of my previous wrong answers, but at least i can continue to the next problem. |
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working with a pencil and crystal white paper sheet brings the joy that no other thing on earth could supply. |
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ok, so for the explanation: first we define the x- and y-axes, both start in the object S, the x-axis is directed to the right, x=0 being the place of the light object, x=L the basis of the wall. the y-axis is perpendicular to the x-axis, y=0 is the also placed on the light-object, the positive side is upwards. for the position of the wall with height h we get: x_wall(t) = v*t, with 0 <= t < L/v (1) now we try to find the y-position of the shadow of the moving wall. at t=0 it will be infinite, at time L/v it will be h. this will be used further to check if we got the right formula for the position. using the fact that we have congruent (or some like that) triangles, you can write: x_obj/L = h/y_s or y_s = h*L/x_obj and substituting (1) gives the results as expected above y_s = h*L/(v*t) taking the derivative to t yields the speed v_s = -h*L/(v*t^2) (if my derivating skills are still allright) this is of course a negative result, but has all to do with the direction of the y-axis damn i've got too much time on my hands |
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yeah im gonna keep my computer on for a minimum of time and just print out the problems next time. every minute the pc is on takes one point away from my IQ. |
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no no thanks a lot :) that explaination is helpful too. i noticed the negative thingey |
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i thought you finished school last year? |
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this is for university. and because the guy who teaches physics is an american, its all in english and from a computer screen (he seems to think its MUCH EASIER to solve a problem from a screen) |
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have you met hermonia already ? |
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no but the flapkeizer keeps jerking off in public over it |
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meh, i did it with the good old pen and paper, i hate courses given online |
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i would contact Ms Flamonna von RookSzpooken to make it stop. |
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the difference between my original ramblings was only the squared t because of the derivation. shows how stupid computers make me. |
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you're on erasmus? |
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TU Delft.. they dont give chemical engineering and such there i think |
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this reminds me of my last high school year... i had took a scientific class with physics speciality (mostly because i didn't know what to take), i was getting marks like 3/20 and 4/20 all the time.... i had good marks in chemistry which helped me not to get a TOO awful average in physics-chemistry (it's "combined" in france), but still i didn't get shit in physics - it wasn't even complicated, i don't know why i was "blocking".... anyway, good luck : ) |
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my brain is not what it used to be |
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I thought you were doing chemistry? |
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when im done with this histroy crap im gonna do physics for fun |
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is the answer 4? |
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i only had physics in my first year...it's just not interesting to me, although i can see why some people like it. |
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yeah, but youve got to understand some basic physics, thermodynamics, math etc etc stuff to understand the more complex chemical processes. them natural sciences cant be strictly seperated, its all connected :\ pogo: yeah but only for h=-2, L=2, v=1 and t=1 |
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