# Does a rotating plank only acquire rotational kinetic energy?

I have another doubt with a Kleppner problem :(.

A thin plank of mass M and length l is pivoted at one end. The plank is released at 60$^{\circ}$ from the vertical. What is the magnitude and direction of the force on the pivot when the plank is horizontal?

My process differed from this one in quite a few ways (not just the OP's process; the process approved and recommended by the other PF users).

What I did is essentially begin by assuming that there will be two different forces on the pivot: one which is the reaction force from the centripetal force on the plank's CM, and another one which will be the reaction force to an upwards force exerted by the pivot (I'll justify this later- this wasn't mentioned in the PF post).

I figured out the centripetal force on the plank when it's horizontal. I began by finding its rotational velocity at that point using conservation of energy. I said that at first, the plank had only gravitational potential energy (setting potential energy at zero at the horizontal position), and when it's horizontal, it has only rotational kinetic energy. Why only rotational? Because the movement the plank is doing is purely rotational, so it won't have any linear kinetic energy (this is one point where I disagree with that PF post).

So,

$$Mg(\frac{l}{2})\cos(60)=\frac{1}{2}I\omega^2$$

$$\omega=\frac{2V_{CM}}{l}$$

Which gives the speed of the center of mass when the plank is horizontal:

$$V_{CM}^2=\frac{3gl}{8}$$

Thus the centripetal force on the center of mass is

$$F_C=\frac{3mg}{4}$$

Now, let's figure out the upwards force that the pivot exerts on the plan (if it exists!):

The torque on the plank about the pivot, when the plank is horizontal, is

$$\tau=\frac{Mgl}{2}$$

And the plank's angular acceleration at that point will be

$$\alpha=\frac{\tau}{I}=\frac{3g}{2l}$$

So the vertical acceleration of the CM will be

$$a=\alpha R_{CM}=\frac{3g}{4}$$

Therefore, the upwards force exerted by the pivot on the plank is

$$F_{up}=\frac{mg}{4}$$

Now using Newton's third law and taking the forces' directions into account, we find the net force $\vec{F}_T$ on the pivot:

$$\vec{F}_T=\vec{F}_{up}+\vec{F}_C=\frac{mg}{4}\left(3\hat{x}-\hat{y}\right)$$

Am I right in saying that the plank only acquires rotational kinetic energy as it rotates, and not any linear KE?

Now that I think about it, maybe I should have taken linear kinetic energy into account, since the CM is being translated... However, I still argue that the motion the plank is undergoing is purely rotational (with the complication that it's about an axis not through the CM), thus the plank should only acquire rotational KE.

• The parallel axis theorem is your friend in this business. Once you understand (really understand) it there will be no mysteries left in this problem. – dmckee --- ex-moderator kitten Jul 13 '14 at 0:57
• Ah, thanks very. I'll try to understand it more clearly. Now that you mention it, I realize that the only proof I really had trouble understanding (and maybe never understood) in Kleppner was the one concerning the parallel axis theorem. I'll get back to you if I need more help. – Physics Llama Jul 13 '14 at 1:23
• Okay, so for my first doubt in the question (regarding KE): the linear kinetic energy should be taken into account only if I consider rotational kinetic energy using the CM moment of inertia as the moment of inertia of the plank. Otherwise, if I consider the moment of inertia about the pivot, I don't need to talk about linear KE. Is this correct? As for the second doubt (whether or not the pivot exerts an upwards force or not); I'm not sure how a deeper understanding of the parallel axis theorem can help me there. Any thoughts? – Physics Llama Jul 13 '14 at 3:50
• Hi Physics Llama! We prefer to have one conceptual physics question per post, so I edited out your first question to make this just about the energy. Feel free to post the other question separately. Also, something to keep in mind when posting: titling your post with something like "K&K problem 2.8" (or whatever) isn't very useful - it's better to actually pose the conceptual question you want to ask in the title, as I've edited this to do. – David Z Jul 13 '14 at 4:48
• Ah, sorry- I wasn't aware of this. Thanks for editing it and informing me. I'll post the other question separately. – Physics Llama Jul 14 '14 at 1:01