- #1
Philethan
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Homework Statement
A solid sphere sphere (radius = R) rolls without slipping in a cylindrical trough (radius = 5R) as shown in the following figure. Show that, for small displacements from equilibrium perpendicular to the length of the trough, the sphere excuses simple harmonic motions with a period [tex]T=2\pi \sqrt{28R/5g}[/tex]
Homework Equations
[tex]\vec{F}_{net}=m\vec{a}[/tex]
[tex]\vec{\tau }_{net}=I\vec{\alpha }[/tex]
[tex]\left | \vec{a} \right |=R\left |\vec{\alpha }\right |[/tex]
The Attempt at a Solution
Step1. Draw the Free body diagram.
[tex]Mgsin(\Omega )-fs=Ma[/tex]
Take the center of sphere as pivot.
[tex]f_{s}R=(\frac{2}{5}MR^{2})\alpha [/tex]
[tex]a=R\alpha [/tex]
Finally,
[tex]\therefore \, f_{s}=\frac{2}{7}Mgsin(\Omega )[/tex]
Step 3. Take the center of the cylindrical trough as the pivot.
[tex]\tau _{net}=[-Mg\times 4Rsin(\Omega )]+[\frac{2}{7}Mgsin(\Omega )\times 5R]=(\frac{2}{5}MR^{2}+M(4R)^{2})\frac{d^{2}\Omega }{dt^{2}}[/tex]
[tex]\therefore \: -\frac{18}{7}MgRsin(\Omega )=\frac{82}{5}MR^{2}\frac{d^{2}\Omega }{dt^{2}}[/tex]
[tex]\therefore \: -\frac{45}{287}\frac{g}{R}\Omega \approx \frac{d^{2}\Omega }{dt^{2}}[/tex]
[tex]\therefore \: T\approx 2\pi \sqrt{\frac{287R}{45g}}=2\pi \sqrt{\frac{31.\bar{8}R}{5g}}[/tex]
I've seen the correct solution but I still don't know why I was wrong.
I will be grateful for any help you can provide.