- #1
Order
- 97
- 3
Homework Statement
A plank of lengt 2l leans against a wall. It starts to slip downward without friction. Show that the top of the plank loses contact with the wall when it is at two-thirds of its initial height.
Hint: Only a single variable is needed to describe the system. Note the motion of the center of mass.
Homework Equations
[tex]\tau=I\alpha[/tex]
Energy conservation
The Attempt at a Solution
I don't get very far at all.
First of all I evaluate the torque of the system: [tex]\tau=lMg\sin \theta=I\ddot{\theta}=\frac{Ml^{2}}{3}\ddot{\theta},[/tex] which leads to [tex]\ddot{\theta}=\frac{3g}{l}\sin \theta.[/tex] This is not possible to solve exactly and therefore is of no help.
Next, I try energy conservation to evaluate the angular velocity at a height of two-thirds of the original: [tex]Mgl\cos\theta=\frac{1}{2}\frac{Ml^{2}}{3}\omega^{2}+\frac{2}{3}Mgl\cos\theta,[/tex] which leads to the result [tex]\omega^{2}=\frac{2g}{l}\cos\theta.[/tex] Now this is of no use either since it cannot be combined with the first equation. It is a relation relative to the original angle, whereas the first equation above is a general differential equation.
Now if neither torque evalutations nor energy conservation leads anywhere, I don't know how to go on. Does anyone have a hint?