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Offline fugibo

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« on: November 29, 2009, 05:04:20 PM »
I just had a Physics problem that read:
"A pole vaulter clears 6.0 meters. What is velocity just before he hits the ground?"

I could be horribly off, but something tells me that since they gave nothing else and I don't know what his altitude was the moment he "cleared 6.0 meters" or when his maximum altitude would be, I can't solve this problem.

Idiots.
« Last Edit: November 29, 2009, 07:21:46 PM by Longfellow »

Offline winkio

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Re: ...
« Reply #1 on: November 29, 2009, 05:13:13 PM »
There are a ton of factors you need:
distance from the 6 meter mark to his center of mass when he clears it
horizontal velocity
angular velocity

And yes, as you said, whether or not he clears the 6 meters at the apex of his jump.

Offline Holyrapid

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Re: ...
« Reply #2 on: November 29, 2009, 06:43:17 PM »
Now, i´m just a second stage (i´m learning to become a datanome, in profession school [ok, it´s not high school, but the place where you go study a real job, and it´s a school of the same level, after comprehensive school i think...] god, i´m bad with words...) but even i think you´d need to know more than that he cleared six meters.
I think what you´d at least need is:
How high is he in the air, how heavy is he, is he tall or short (affects somewhat the location of his center of gravity), and how fast was he going when he pole vaulted. (his starting speed...)

Offline fugibo

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Re: ...
« Reply #3 on: November 29, 2009, 06:52:33 PM »
Now, i´m just a second stage (i´m learning to become a datanome, in profession school [ok, it´s not high school, but the place where you go study a real job, and it´s a school of the same level, after comprehensive school i think...] god, i´m bad with words...) but even i think you´d need to know more than that he cleared six meters.
I think what you´d at least need is:
How high is he in the air, how heavy is he, is he tall or short (affects somewhat the location of his center of gravity), and how fast was he going when he pole vaulted. (his starting speed...)

You don't need his weight (since all objects fall at the same speed), though technically you would need his center of gravity – however, I don't think this class will be looking for ragdoll physics, so we'll just assume it's a block with a jetpack.

Offline Diokatsu

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Re: ...
« Reply #4 on: November 29, 2009, 06:53:58 PM »
Now, i´m just a second stage (i´m learning to become a datanome, in profession school [ok, it´s not high school, but the place where you go study a real job, and it´s a school of the same level, after comprehensive school i think...] god, i´m bad with words...) but even i think you´d need to know more than that he cleared six meters.
I think what you´d at least need is:
How high is he in the air, how heavy is he, is he tall or short (affects somewhat the location of his center of gravity), and how fast was he going when he pole vaulted. (his starting speed...)

You don't need his weight (since all objects fall at the same speed), though technically you would need his center of gravity – however, I don't think this class will be looking for ragdoll physics, so we'll just assume it's a dude with a jetpack.

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Offline fugibo

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Re: ...
« Reply #5 on: November 29, 2009, 07:01:10 PM »
The answer they were looking for was apparently 10.8 m/s (rounded from ~10.84...)

He's six meters up, and his velocity is -9.8 * t m/s. Integrate to get -4.9 * t^2 + 6 m, find the zero (the time at which he will land) and you get (6 / 4.9)^(1/2) s, plug it into the velocity function and you get -9.8m/s^2 * (6 / 4.9)^(1/2) s, which, according to irb, is -10.8443533693808 m/s. The idiots who wrote the quiz apparently forgot that downward velocity is negative.

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Re: ...
« Reply #6 on: November 29, 2009, 07:15:09 PM »
I'd solve it like this:

Since his mass isn't defined, I'll just say he weighs 0 kg and I have the energy of 0 J. Solved like 99% of everything in the real world. "I assume."
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Offline fugibo

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Re: ...
« Reply #7 on: November 29, 2009, 07:21:24 PM »
I'd solve it like this:

Since his mass isn't defined, I'll just say he weighs 0 kg and I have the energy of 0 J. Solved like 99% of everything in the real world. "I assume."


aglsaglagksdgad, my bad – it's supposed to velocity, not kinetic energy. X|

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Re: ...
« Reply #8 on: November 29, 2009, 07:52:32 PM »
LMAO! Yeah, then I'd do

t = v / a
=> dt = dv / a
v = s / t
=> dv = ds / t
=> dt = ds / (a * t)
=> tdt = ds / a
=> t^2 / 2 = s / a
t = SQRT(2 * s / a) # a being g, s being -6
t = SQRT(2 * (-6) / 9.81)
t = -10.84

and get your result.
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Offline winkio

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Re: ...
« Reply #9 on: November 30, 2009, 01:11:43 AM »
yeah, I was hoping so.  Because KE takes into account rotation as well, which the problem totally ignored.

Offline Blizzard

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Re: ...
« Reply #10 on: November 30, 2009, 01:56:24 AM »
I can still assume that I may ignore everything else. xD
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