Physics Help Forum

Physics Help Forum Feed Site Feed

Go Back   Physics Help Forum > College/University Physics Help > Advanced Mechanics
Reply
 
Thread Tools Display Modes
  #1  
Old 10-21-2009, 09:24 AM
Junior Member
 
Join Date: Oct 2009
Posts: 7
Thanks: 1
Thanked 0 Times in 0 Posts
Bluekitten is on a distinguished road
Default How to find center of mass and moment of inertia for a non-symmetrical object?

If the object was symmetrical, I would be able to figure it out. With a non-symmetrical object, I honestly have no idea where to start. If anyone could tell me the relevant general equations I would really appreciate it. I know you guys don't generally give answers, but I'm kinda hoping that since I'm not asking for answers, just the general equations (with variables only) you all might help me out

A rod that is 2 meters long and weighs 2935 grams has weights attached to the end that weigh 405.1 grams (each weight is approximately 100g - the total of all weights added is 405.1g).

The weights have a diameter of approximately 3cm, and have been taped to the end of the rod in such a way that the end of the masses are at the end of the rod.similar to this:
=======OO (the rod extends under the masses, and ends at the same point as the masses end.)

There are two of the weights on the front and two of the weights on the back of the bar, so the cross section of the end with the weights looks like this:

0||0
0||0

The pivot point is .7 meters away from the end of the rod that has no additional weights.
Reply With Quote
Advertisement
 
  #2  
Old 10-21-2009, 09:58 AM
arbolis's Avatar
Super Moderator
 
Join Date: Apr 2008
Location: Argentina
Posts: 735
Thanks: 278
Thanked 255 Times in 184 Posts
arbolis is a jewel in the rougharbolis is a jewel in the rougharbolis is a jewel in the rough
Default

Quote:
Originally Posted by Bluekitten View Post
If the object was symmetrical, I would be able to figure it out. With a non-symmetrical object, I honestly have no idea where to start. If anyone could tell me the relevant general equations I would really appreciate it. I know you guys don't generally give answers, but I'm kinda hoping that since I'm not asking for answers, just the general equations (with variables only) you all might help me out

A rod that is 2 meters long and weighs 2935 grams has weights attached to the end that weigh 405.1 grams (each weight is approximately 100g - the total of all weights added is 405.1g).

The weights have a diameter of approximately 3cm, and have been taped to the end of the rod in such a way that the end of the masses are at the end of the rod.similar to this:
=======OO (the rod extends under the masses, and ends at the same point as the masses end.)

There are two of the weights on the front and two of the weights on the back of the bar, so the cross section of the end with the weights looks like this:

0||0
0||0

The pivot point is .7 meters away from the end of the rod that has no additional weights.
Hi and welcome to the forum!
I don't really get the picture of the system. If you're looking for a formal definiton of center of mass, check out this page : Center of mass - Wikipedia, the free encyclopedia , which involves a triple integral.
So if you haven't taken calculus III yet, I guess your problem can be simplified. But as I said, I'm not able to visualize the system.
Let's wait another helper help you. Good luck.
__________________
Isaac
If the problem is too hard just let the Universe solve it.
Reply With Quote
  #3  
Old 10-21-2009, 10:16 AM
Junior Member
 
Join Date: Oct 2009
Posts: 7
Thanks: 1
Thanked 0 Times in 0 Posts
Bluekitten is on a distinguished road
Default

Hmmm. Well, imagine a two by four, with four of these http://bodybuilderfitness.com/librar...se_weights.JPG taped to one end of the two by four. Two of the weights on the front (the wide end that is on top in this picture http://upload.wikimedia.org/wikipedi...ted_timber.JPG) and two of the weights on the back - imagine holding the two by four upright, with the weights facing you. The back would be directly behind the wood on the other side of the weights.

Here's a very badly drawn MS Paint diagram
Reply With Quote
  #4  
Old 10-21-2009, 10:48 AM
Banned
 
Join Date: Aug 2009
Location: UK
Posts: 240
Thanks: 57
Thanked 82 Times in 72 Posts
Paul46 will become famous soon enough
Default

It will be approx:

(1/6) x mass x (width (0.1 m)^2 + length from axis (1.3m)^2)
Reply With Quote
The Following User Says Thank You to Paul46 For This Useful Post:
Bluekitten (11-12-2009)

Donate to PHF
  #5  
Old 10-21-2009, 10:27 PM
PHF Helper
 
Join Date: Feb 2009
Posts: 1,348
Thanks: 236
Thanked 460 Times in 414 Posts
physicsquest is a glorious beacon of lightphysicsquest is a glorious beacon of lightphysicsquest is a glorious beacon of lightphysicsquest is a glorious beacon of lightphysicsquest is a glorious beacon of light
Default

The pivot point is .7 meters away from the end of the rod that has no additional weights

I dont understand this bit. If the pivot point is as stated, the system wont balance as there is clearly a larger torque on the side of the weights.

If you wish to find the point at which it remains horizontal, you could try the following.

The mass / unit length would be 2935/2000 = 1.4675 .

Let the balance point be at a distance d from the end with the weights.
which are assumed to be on the right side.

The counterclockwise torque provided by the free portion of the rod will be

1.4675 (2000 -d) g x (2000 -d) / 2.
weight x distance



Since the addititional weight is 405.1 gm it should be mass and not weight. We can take each mass to be thus 405.1/4 = 101.275.

The clockwise torque will be provided by 3 forces the weight of the rod to the right of the pivot, the pair of masses closer to the end, and the pair of masses closer to the balance point which are

1.4675 d (d/2)

2 x 101.275 ( d -3/2)

2 x 101.275 [ d - (3 + 3/2)].

Thus we can write

1.4675(2000 -d)g x (2000 -d)/2 = 1.4675 d (d/2) + 2 x 101.275 ( d -3/2)

+ 2 x 101.275 [ d - (3 + 3/2)].

Solve for d.
Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off
Forum Jump


All times are GMT -7. The time now is 02:08 PM.


Powered by vBulletin® Version 3.7.3
Copyright ©2000 - 2010, Jelsoft Enterprises Ltd.
SEO by vBSEO 3.2.0 ©2008, Crawlability, Inc.
©2009 Physics Help Forum

Physics Help Forum is a community of physics forums with an emphasis on physics help in all levels of physics.
Register to post your physics questions on the message board.