- #1
Philethan
- 35
- 4
Homework Statement
A block of mass M is connected to a spring of mass m and oscillates in simple harmonic motion on a horizontal, frictionless track. The force constant of the spring is k and the equilibrium length is l. Assume that all portions of the spring oscillate in phase and that the velocity of a segment dx is proportional to the distance x from the fixed end; that is, vx = (x/l)v. Also, note that the mass of a segment of the spring is dm = (m/l)dx.
(a) Find the kinetic energy of the system when the block has a speed v.
(b) Find the period of oscillation.
Homework Equations
[tex]-kx=ma[/tex]
[tex]T=2\pi \sqrt{\frac{m}{k}}[/tex]
The Attempt at a Solution
I've solved (a).
[tex]\frac{1}{2}Mv^{2}+\int_{0}^{l}\frac{1}{2}(\frac{m}{l})dx[(\frac{x}{l})v]^{2}=\frac{1}{2}(M+\frac{1}{3}m)v^{2}[/tex]
For (b), I think I sill can, by the net force of the M block, get the SHM period.
No matter how much mass the spring has, the net force of the M block is still :
[tex]F_{net}=-kx=Ma[/tex]
So, I think the period is still
[tex]T=2\pi \sqrt{\frac{M}{k}}[/tex]
But the answer is
[tex]T=2\pi \sqrt{\frac{M+\frac{1}{3}m}{k}}[/tex]
I think it's a little bit plausible because the mass in this equation is the effective mass.
However, I really want to know what happened to the net force of M block.
Can I still use Hooke's law? If I can't use it, then what's the meaning of spring constant k now?
Can I solve the period without using effective mass? I want to use Hooke's law and Newton's law to solve the
2nd order differential equation and get the answer.
Thank you very much!