# Lever rotating around an axis.

by clicwar
Tags: axis, lever, rotating
 P: 6 1. The problem statement, all variables and given/known data A lever of length 3L can rotate freely around a horizontal axis O. The end of each arm "2L" and "L" of the lever has one mass : "4m" and "9m" , respectively. Find the velocity of mass "9m" at the bottom. Consider that the lever was initially at rest in the horizontal position. 2. Relevant equations The problem is in a chapter of work and energy, before the conceps of torque and moment of inercia, so i guess that only energy and work can solve this. But any solution/hint with any method will be welcome. 3. The attempt at a solution I'm trying to apply conservation of energy, but i'm confused if the whole system is conservating energy or only part of it. I really need a hint of how start this problem or if possible a solution of it. Thanks in advance! Attached Thumbnails
 HW Helper P: 1,471 What forces are acting on the system?
 P: 6 I believe that only the gravitational forces are acting on the system "lever+masses". I can't see any other possible force able to do work.
Mentor
P: 41,563
Lever rotating around an axis.

 Quote by clicwar I'm trying to apply conservation of energy, but i'm confused if the whole system is conservating energy or only part of it.
Since everything is connected, treat the system as a whole. How does the mechanical energy change as the system rotates?
 P: 6 The mechanical energy of the system as whole is doesn't changing ... right? (Does the support O of the lever is doing work?) .
Mentor
P: 41,563
 Quote by clicwar The mechanical energy of the system as whole is doesn't changing ... right?
Right.
 (Does the support O of the lever is doing work?)
I interpret "rotate freely" as saying that there is no friction, and thus no mechanical energy lost to friction. The support O does no work, since it doesn't move.
P: 6
 Quote by Doc Al I interpret "rotate freely" as saying that there is no friction, and thus no mechanical energy lost to friction. The support O does no work, since it doesn't move.
Thank you very much for the help DocAl. The sentence quoted above is exactly what i was looking for.

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