Forces on rotating rod

[ymnoklan]

Homework Statement

A carbon fiber rod of length L = 1.0 m and of negligible mass can rotate without
friction about an axis through its midpoint. Two small blocks of lead with masses m1 = 2.0 kg
and m2 = 3.0 kg are attached to each end of the rod. You hold the bar in a horizontal position and
release the system.

What is the speed of the blocks when the rod reaches a vertical position? You can disregard
the air resistance.
How great are the forces in the rod when it is vertical and in which directions do they act?
How big is the force on the axis, and in which direction does it act when the rod is vertical?

Relevant Equations

Ep=mgh, Ek=1/2mv^2, a=mv^2/r

I get that the speed is 3.1 m/s, the forces in the rod are 38 N downward and 58 N upward, and that the force on the axis is 49 N.

 

[kuruman]

If you are asking for confirmation whether your answers are correct, please show your work.

We prefer that you use LaTeX to post equations. Please click "LaTeX Guide" above "Attach files" to learn how.

 

[ymnoklan]

I think the main problem is the last question: what do they even mean with “the force on the axis”? What is the difference between this and the forces on the rod?

 

[kuruman]

I think the main problem is the last question: what do they even mean with “the force on the axis”? What is the difference between this and the forces on the rod?

What is the difference between the force that the axis exerts on the rod and the force that the rod exerts on the axis?

 

[haruspex]

the forces in the rod are 38 N downward and 58 N upward,

Please post your working for that.

 

[ymnoklan]

Ok, so my notes are in Norwegian, but you’ll probably get the essence of it based on the equations. For the first question I basically just used conservation of mechanical energy and solved for the speed v. The second question, I used the speed from the first question and found the centripetal force on each end of the rod. Finally for the last question I just find that the only forces acting are the gravitational forces of the two blocks, but this feels kind of wrong(?) Please tell me what you think? How would you solve this kind of problem?

 

[kuruman]

Sorry, your notes are illegible, not to mention sideways. Also, "getting the essence" is not good enough because the Devil is in the details. Your strategy sounds fine for the first two parts but what about its implementation? I can't tell unless I can read your work . . . in English.

I just find that the only forces acting . . .

Acting on what system by what entity outside the system? That's an issue that it seems you have not settled in your mind. What do you think?

 

[TSny]

Welcome to Physics Forums!
Please read the guidelines for posting homework questions. Note item 5 regarding posting images of your work.

The left side of the equation is not correct for the magnitude of the change in potential energy. See if you can spot the error.

$\dfrac{m_1v^2}{r}$ gives the net force acting on $m_1$ when the rod is vertical. You want the force that the mass exerts on the rod. How many forces act on each mass? Draw a free-body diagram for each mass when the rod is vertical.