# Center of mass

1. Dec 9, 2008

### cashmoney805

Hey guys, does anyone know a good website teaching center of mass? I need to know how to find the center of mass of objects like spheres, cubes etc. and also do problems such as "a man is standing on a boat and moves from one side to the other, how far does the boat move." Stuff like that. Thanks so much!

2. Dec 9, 2008

### mgb_phys

It's really a very simple problem - just split the object into simple shapes and then add all the distances to the centre of each shape * the mass of the shape.

wiki has a good article http://en.wikipedia.org/wiki/Center_of_mass

3. Dec 9, 2008

### cashmoney805

I looked at Wikipedia before posting, and I understand the examples on there. I need more examples though that have solutions so I can practice.
Also, there aren't any examples like how far does a boat/person on the boat move if the boat/person moves.

4. Dec 9, 2008

### cashmoney805

I guess I should have included translation motion in the subject too

5. Dec 17, 2008

### minger

Yea, the boat/person moving definitely isn't center of mass; that problem relies on conversation of momentum.

6. Feb 1, 2009

### AROD

Hmmm how would you set up the following situation in conservation of momentum terms?

A young physicist weighing 80 kg enjoys the sunset together with his girl friend on a rowboat floating on a calm lake. The boat weighs 30 kg and does not move. The young couple swap their seats which are symmetrically located with respect to the boat's center of mass. The man observes the boat shifting by 40 cm with respect to a spatially fixed buoy. With that the physicist can tell his girl friend how much she weighs.

7. Feb 3, 2009

### Hobnob

I can't even understand the question. 'Shifting' how? Horizontally? Vertically? Are we talking about the boat getting a velocity? In that case, we need a time as well as a distance. If it's a vertical displacement, then we need the distances of the lovebirds from the centre of mass in order to know the moment. It all sounds a bit odd.