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Spring compression and escape speed

by ryoonc
Tags: compression, escape, speed, spring
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Feb22-09, 09:28 PM
P: 5
1. The problem statement, all variables and given/known data
A package of mass 5 kg sits at the equator of an airless asteroid of mass 6.3*10^5 kg and radius 48 m, which is spinning so that a point on the equator is moving with speed 2 m/s. We want to launch the package in such a way that it will never come back, and when it is very far from the asteroid it will be traveling with speed 227 m/s. We have a large and powerful spring whose stiffness is 3.0*10^5 N/m. How much must we compress the spring?

2. Relevant equations
Kf = Ui
(1/2) * m * v^2 = (1/2) * ks * s^2
v = sqrt[ (2 * G * M) / ri ]

3. The attempt at a solution
I've listed out the variables I'll be using in this equation:
m = 5 kg
M = 6.3e5 kg
k = 3e5 N/m
r = 48m

I'll get straight to the point, plugging in the variables was easy up to the point where I had to figure out what v was, where I find the escape speed. I used the equation I stated above ( v = sqrt[ (2 * G * M) / ri ] ) to find out the escape velocity needed to figure out how much compression is needed on the spring, and got 5.403e-6.

However, it seems to be a wrong answer, but I have no idea why, but I have a feeling it has something to do with my velocity (which I got a value of 1.3235e-3 m/s), but again I don't know what else to do with it.
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Feb22-09, 11:25 PM
HW Helper
P: 3,394
I don't think it is useful to use that v= formula when you are asked to provide not only the energy for escape velocity but also an additional speed of 227 m/s. Better to compute the total energy needed - the GmM/r plus the 1/2m*227^2. The spring energy must provide this, so you can now calculate the compression.
Feb23-09, 01:28 AM
P: 5
Thanks for your response, Delphi, I used the equation provided to find the total energy needed to launch it, but I came up with yet again, a wrong answer. Here's how I got my answer:

Find the total energy needed:
GmM/r + (1/2) * m * 227^2 = { [ (6.673e-11) * 5kg * 6.3e5kg ] / 48 } + [ (1/2) * 5kg * 227^2 ]
= 1.28823e5 m/s

Plug that value in to the kf = Ui equation:
(1/2) * m * v^2 = (1/2) * ks * s^2 = sqrt{ [ (5kg) * (1.28823e5)^2 ] / 3e5 }
= 5.259e2 m/s

Again, it's not correct.. Worse, I'm having trouble grasping the concept, which is what I'm really worried about.

Thanks for the help though, really. I've been scratching my head about this for a while now.

Feb23-09, 11:15 AM
HW Helper
P: 3,394
Spring compression and escape speed

1.28823e5 m/s should have units of Joules.
And be subbed into E = 1/2k*x^2 in place of E.
I get x = about 1 meter.

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