Lever rotating around an axis.


by clicwar
Tags: axis, lever, rotating
clicwar
clicwar is offline
#1
May24-07, 10:56 PM
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
imagem.jpg  
Phys.Org News Partner Science news on Phys.org
SensaBubble: It's a bubble, but not as we know it (w/ video)
The hemihelix: Scientists discover a new shape using rubber bands (w/ video)
Microbes provide insights into evolution of human language
andrevdh
andrevdh is offline
#2
May25-07, 03:48 AM
HW Helper
P: 1,449
What forces are acting on the system?
clicwar
clicwar is offline
#3
May25-07, 12:06 PM
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.

Doc Al
Doc Al is online now
#4
May25-07, 12:58 PM
Mentor
Doc Al's Avatar
P: 40,906

Lever rotating around an axis.


Quote Quote by clicwar View Post
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?
clicwar
clicwar is offline
#5
May25-07, 04:46 PM
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?) .
Doc Al
Doc Al is online now
#6
May25-07, 04:51 PM
Mentor
Doc Al's Avatar
P: 40,906
Quote Quote by clicwar View Post
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.
clicwar
clicwar is offline
#7
May25-07, 07:02 PM
P: 6
Quote Quote by Doc Al View Post
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.


Register to reply

Related Discussions
Why the earth is rotating along its axis? General Astronomy 30
Changing axis of a rotating wheel Introductory Physics Homework 0
Fixed Axis Rotating Advanced Physics Homework 2
A diatomic molecule rotating about it's axis of symetry Quantum Physics 5
Rotating around axis, surface area and volume Calculus & Beyond Homework 0