Oscillating system (small board on a halfpipe)

[pinsky]

Hello there!

I have a situation as follows:

[PLAIN]http://img821.imageshack.us/img821/4026/kuglananagibu.gif

I have to find the period of oscillation of the system. I've know how to set the equations for when solving the equations with forces, but am lost when trying to solve it with energy.

There is no friction between any of the surfaces.

My assumption is that there should be three energies interchanging here.

Rotationl energy of m (around the center of the circle with radius R)
W_{rot}=1/2 mR^2 \omega

And translational energies of M and m.
W_{trans}=1/2 Mv^2 + 1/2m v^2\omega

Any ideas where to go form here?

 

[Delta2]

mgR(1-cos(\phi))+\frac{1}{2}mR^2{\omega}^2=c_0 with \omega=\frac{d\phi}{dt}.

You also might find usefull the trigonometric identity cos(\phi)=1-2sin^2(\frac{\phi}{2}). If you consider small amplitudes then you can also take the approximation sin^2(\frac{\phi}{2})=(\frac{\phi}{2})^2. If you use both you ll end up with a differential equation that doesn't look simple , yet it has solutions of the form \phi(t)=asin(bt)

 

[Spinnor]

Why do you include the kinetic energy of the half-pipe, does it move?

What you have here is similar to a physical pendulum? Do a Google search,

"period of a pendulum large amplitude oscillation" or something like that.

 

[pinsky]

Thank you for your replays.

I should have metnion more explicitely, the halfpipe has a constant mass M and lies on a frictionless surface. Therefor is also oscillates.

 

[Spinnor]

Thank you for your replays.

I should have metnion more explicitely, the halfpipe has a constant mass M and lies on a frictionless surface. Therefor is also oscillates.

Can you assume small amplitude motion?

 

[Spinnor]

Can you write down the Lagrangian for this system, T - V, and then solve the Lagrange equations of motion? What textbook are you using?

 

[Delta2]

Thank you for your replays.

I should have metnion more explicitely, the halfpipe has a constant mass M and lies on a frictionless surface. Therefor is also oscillates.

Hm now it becomes really interesting. You have to add \frac{1}{2}(M+m)v^2 to the left hand side of the equation(i guess M oscilates horizontaly so his potential energy doesn't change). But apparently we need one more equation and i can think only one that uses forces though. If F_n is the force between m and M then F_n-mgcos(\phi)=m{\omega}^2R and F_nsin(\phi)=M\frac{dv}{dt}

 

[pinsky]

Can you assume small amplitude motion?

Yes.

Can you write down the Lagrangian for this system, T - V, and then solve the Lagrange equations of motion? What textbook are you using?

I'm not on Lagrangian mechanics yet, it's first year physics.

Hm now it becomes really interesting. You have to add \frac{1}{2}(M+m)v^2 to the left hand side of the equation(i guess M oscilates horizontaly so his potential energy doesn't change). But apparently we need one more equation and i can think only one that uses forces though. If F_n is the force between m and M then F_n-mgcos(\phi)=m{\omega}^2R and F_nsin(\phi)=M\frac{dv}{dt}

Shouldn't there also be a force component of the system force on the board? (Due to the acceleration of the halfpipe).

I don't see how including forces into energy equations could help. Maybe they could be connected by

Wk = x F (for the kinetic energy of the halfpipe)

it's just, I've never seen a scenario in which those two interact. Could there a purely energy solution be found by using the center of mass? Or perhaps that the center of mass doesn't move in the x direction?