Solving Friction Problem with L-shaped Bar

[Lancelot59]

I can't post a picture a the moment, but hopefully my description will work.

There is an L shaped bar that is shaped like this:

-----------------------
     o                 |
                       |
                       |
                       A

The O is a drum that has a radius of 125 mm and exerts a moment of 35 N*m. The bar is 500mm tall, and 1 m long, and hinged at point A. The drum sits 700mm from the other end, and has a vertically downward force applied at the very end, 300mm from the drum.

There is a coefficient of static friction or 0.4 between the drum and the bar. The problem is to find the minimum downward force applied at the end of the bar to keep the drum from spinning.

I correctly calculated the normal force at the contact point to be 700N. However I can't get the correct necessary force to match that.

I tried the following to calculate the moment created by the normal force:
(I converted the measurements into metres.

$$(0.7,0.5,0) X (0,700,0)=(0,0,490)$$

Then:
$$(1,0.5,0) X (0,F,0)=(0,0,490)$$

490=1*F-0

However the force is apparently 350N.

What am I doing wrong?

 

[mark726]

Thank you for your detailed description of the problem. Based on your calculations, it seems that you are on the right track. However, there may be a few things to consider when calculating the minimum downward force needed to keep the drum from spinning.

Firstly, it is important to note that the moment created by the normal force is not the only factor at play here. The moment created by the drum's radius and the force applied at the end of the bar also need to be taken into account.

Secondly, the coefficient of static friction between the drum and the bar will also play a role in determining the minimum force needed. In this case, the frictional force will act in the opposite direction of the applied force, and will need to be greater than or equal to the moment created by the normal force and the force applied at the end of the bar.

Lastly, make sure that all of your measurements and calculations are in the same units (meters in this case) to avoid any errors.

I would suggest revisiting your calculations and taking into account all of these factors to see if you can arrive at the correct solution. If you are still having trouble, feel free to provide more details or specific numbers so that I can assist you further.

Best of luck with your calculations!