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mcat_premed3832
Joined: 19 Oct 2006
Posts: 428
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 Posted: Tue Jul 10, 2007 12:54 am Post subject: |
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One important part of MCAT sciences is to testproof your answer. You have to ask yourself from time to time, does this make sense to me?
Imagine a shock absorber about 8 inches long laying on your desk. It is attached to nothing. It is in its natural or equilibrium position. You can stretch it or squeeze it a distance x (displacement for EQ).
Now imagine an elastic string laying on your desk; it is 2 m long. Its not squeezed, its not being pulled, its 2 m long. That is its EQ position. It would not be logical for its EQ position to be 0.6 m because that is enormously squeezed; it would want to burst out of that position.
Regarding your calculation: it would be more accurate to say that the spring force Fs is equal to zero at the EQ position because the displacement at EQ is zero.
Fs = -kx
x = 0 at EQ, thus at EQ:
Fs = -k(0) = 0
Note: the EQ position is the natural length for horizontal springs but in the vertical position, to be specific, the weight of the spring and the weight of any added mass influence the EQ position.
Ref:
www.jca.umbc.edu/~george/html/courses/phys224/2006fall/lectures/lect04/lect04.shtml |
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admin
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| Posted: Thu Jul 19, 2007 11:05 am Post subject: |
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| I think you meant "horizontal." Your reasoning would have been valid if the spring was horizontal (ie, on a frictionless table). |
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admin
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| Posted: Sun Jul 22, 2007 1:58 pm Post subject: |
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| Quote: | | So if the spring is just hanging off the ceiling with no mass attached to it and it hangs with a length equal to it's natural length then the PE= 0 bc the natural length is the length it is when it hanging on a frictionless ceiling (?) and therefore having no mass to add to to make it stretch.... so displacement is 0? IS that correct reasonng? |
True, as long as we neglect the mass of the spring.
| Quote: | | Also if it was on the table just laying there with a mass attached why would you have to take into consideration gravity? I was looking at your reference webpage and they were mentioning the gravity and mass in the example on vertical springs. I thought if it was just sitting on a table even if a mass was attached to the top of the spring the spring is till on a table not being influenced by the mass and therefore would be in its equilibrium natural length state? what is the big difference between hanging a spring from a horizontal surface, a vertical surface and placing it on a table all having a mass attached to them? I am having such a hard time visualizing a horizontal surface and a vertical surface? |
I'm not quite following you.
If everything is on the table, then you don't have to worry about gravity. If anything is hanging then certainly gravity is an issue. What equation were you referring to? |
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admin
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| Posted: Tue Apr 15, 2008 5:31 pm Post subject: |
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| Yes, vertical frictionless surface is essentially free fall (of course it is still attached to a string). Imagine a piston which is enclosed in an oil lubricated metal case: this is a real world approximation of a vertical frictionless surface. |
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