- #1
Jacobpm64
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Homework Statement
A particle of mass [tex] m [/tex] is at rest at the end of a spring (force constant [tex] = k[/tex]) hanging from a fixed support. At [tex] t = 0 [/tex], a constant downward force [tex] F [/tex] is applied to the mass and acts for a time [tex] t_0 [/tex]. Show that, after the force is removed, the displacement of the mass from its equilibrium position ([tex] x = x_0 [/tex], where [tex] x [/tex] is down) is
[tex] x - x_0 = \frac{F}{k}[cos \omega_0(t-t_0) - cos \omega_0 t] [/tex]
where [tex] \omega_0^2 = \frac{k}{m} [/tex].Homework Equations
Newton's 2nd law [tex] F = ma [/tex] and knowing how to solve second order linear ODEs.
The Attempt at a Solution
Okay, so I have no idea if what I'm doing is right, but I've been working on this (trying different things) for about 6 hours now.. It's painful. So, any assistance would be appreciated.
First, I split this thing into two parts.. one after the force is removed and one while the force is applied.
After force is removed: ([tex] t \geq t_0 [/tex] and [tex] t = 0 [/tex])
[tex] mg - kx = m \ddot{x} [/tex]
[tex] \ddot{x} + \omega_0^2 x = g [/tex]
We know [tex] x(t) = Acos(\omega_0 t - \phi ) + \frac{g}{\omega_0^2} [/tex].
We also know that at [tex] t = 0, x = x_0, [/tex] so:
[tex] x_0 = Acos\phi + \frac{g}{\omega_0^2} [/tex]
Now, the displacement from the equilibrium is:
[tex] x - x_0 = Acos(\omega_0 t - \phi ) - Acos\phi [/tex]
Simplifying, [tex] x - x_0 = A[cos(\omega_0 t - \phi ) - cos\phi] [/tex]
While the force is applied: ([tex] 0 < t \leq t_0 [/tex])
[tex] F + mg - kx = m \ddot{x} [/tex]
[tex] \ddot{x} + \omega_0^2 x = \frac{F + mg}{m} [/tex]
So, [tex] x_2 (t) = Acos(\omega_0 t - \phi) + \frac{F+mg}{m\omega_0^2} [/tex]
Now, at [tex] t = t_0 [/tex], we know [tex] x = x_2 [/tex], so,
[tex] Acos(\omega_0 t_0 - \phi ) + \frac{g}{w_0^2} = Acos(\omega_0 t_0 - \phi) + \frac{F+mg}{m\omega_0^2} [/tex]
But this gives me [tex] F = 0 [/tex] which is of course true, because [tex] t = t_0 [/tex] is the time when the force is removed. I just do not know where to go and if this is helpful at all. Eventually, i want to have [tex] A = \mbox{something} [/tex] so that i can plug that into my equation for displacement from the equilibrium [tex] x - x_0 [/tex] above.
What am I doing wrong? What should I do? Where should I go?