Moments and tilting

1. Nov 19, 2014

Fabian901

Say I have a uniform beam supported by 2 bricks and I attach an object with a certain weight at one end of the beam so it starts tilting. Why is the beam still in equlibrium (sum of moments about one of the bricks is equal to 0) if it is tilting?. I've seen this exercise on youtube and I'm still not understanding it. The link is below if you want to have a look.

Thanks!

2. Nov 19, 2014

Simon Bridge

In the example, the condition being calculated is for the case that the beam is just about to tilt. The moments exactly balance, so Newtons first law applies.

3. Nov 19, 2014

Fabian901

Okay, but does the weight on the edge have enough downward force to actually make it tilt or would I need to increase it? If it does have enough downward force to make it tilt then surely it is just a matter of seconds until the beam starts tilting and therefore the sum of moments would not equal 0.
Would this be correct or am I missing something?

4. Nov 19, 2014

Simon Bridge

What does Newton's first law say?
If the beam is stationary, it remains stationary - if it is in motion, it stays in motion - at a constant (angular) velocity.

Lets say it (in the example) is stationary - then, with the moments exactly balanced the slightest extra moment, for the tiniest fraction of a second, will set it in motion at a constant angular velocity. You can see if the moments remain in balance while tilting by redrawing the picture with the beam tilted at some obvious angle, draw in the forces, and calculate the moments. However, they don't have to be out of balance for the beam to keep tilting.

5. Nov 21, 2014

Fabian901

I see now! Thanks a lot for your help!

6. Nov 21, 2014

Simon Bridge

No worries - the idea that you have to push something for it to move is a hard one to get rid of.