# Centre of mass

1. Aug 6, 2006

### chandran

centre of mass

There is a mass at a position vector r1 from origin. like wise there are different masses at different r from the origin. the
centre of mass is the distance in which the entire mass can be concentrated at some distance R which will give the same EFFECT as that of the original
system.

What is the effect we are talking about?

What is centre of gravity.

2. Aug 7, 2006

### Universal

The "effect" would be related the gravitational force. When using Newton's Law of Universal Gravitation $$F = G\frac{m_1 m_2}{r_{12}^2}$$ to calculate the force between two objects you calculate the distance between the two objects $$r_{12}$$ as the distance between the center of masses of the two objects, no matter what shape the two objects posses.

For example, when calculating the gravitational force between the Earth and moon you do the calculation by regarding the entire masses of the Earth and moon as concentrated to two single points located at the center of masses of the Earth and moon. Here the original system is the real Earth-moon system and the "concentrated" system would be considering the Earth and moon as two points with the same masses as the Earth and moon concentrated at the center of mass of the two objects and the "effect" is the gravitational force, ie. it would be the same in both situations. I hope that makes sense.

The center of gravity and center of mass is basically the same thing. Its the average location of all the mass inside an object. You can read more about at http://en.wikipedia.org/wiki/Center_of_mass .

3. Aug 7, 2006

### neutrino

You can simplify matters by assuming that all mass of an object is contained at that point (centre of mass - CM) and you can evaluate the translatory motion of that object considering it as a point particle. This is what you generally do when solving problems in elementary mechanics, such as a car slowing down, a block sliding on a ramp, etc.

The centre of gravity(CG) is the nothing but the CM in a uniform gravitational field.

4. Aug 9, 2006

To Universal & Neutrino. Would your answer also apply to the three quarks found within the proton ? If not, how would one calculate the center of mass for these quarks--or is this even a valid question for a thread on Classical Physics ?

5. Aug 10, 2006