Physics

[meal ticket]

What is the difference between center of mass and center of gravity? Is a (boat + dog)'s center of mass or center of gravity changing when the (boat + dog) is in the middle of a placid lake, with the dog moving from one end of the boat to the other end?

[Paddy The Cat]

i dont really know the difference between center of mass and center of gravity, but I am a Physics genius (heh kinda)

Gravity is the force acting upon mass-it exists everywhere. On earth it is appx. -9.8 everywhere (9.8 is its magnitude, - implying direction).
Mass is a scalor quantity-mass is the amount of matter that makes up any given substance. Gravity acts upon the matter.

So my best guess would be center of gravity is where the pull (in our case the pull of the earth) of gravity is centered on an object. If you crouch, it gets lower, if you lay down it moves, etc.

So i'd venture that center of mass i just the center of the mass lol... idk i guess a constant location? Unless one gains/looses mass...


can anyone verify this =p


^ i know the center of gravity moves, not sure about the center of mass...

[Frigidus]

They're the same thing. I should know, having checked the font of all knowledge, Wikipedia. Although there is apparently a military concept called "Center of Gravity" that refers to the source of an army's strength.

[PLAYER57832]

So my best guess would be center of gravity is where the pull (in our case the pull of the earth) of gravity is centered on an object. If you crouch, it gets lower, if you lay down it moves, etc.

So i'd venture that center of mass i just the center of the mass lol... idk i guess a constant location? Unless one gains/looses mass...

That is pretty much what I was taught, too.

Gravity is in reference to another object (generally very large, eg Earth)

Mass is in reference to just the one object.

Its akin to the differance between weight and mass.

The question is a bit of a trick. The dog's mass and even, you could argue, its center of gravity stay roughly the same (as long as it remains upright anyway). The BOAT's center of mass AND center of gravity shift as the dog shifts.

If the dog were to lay down or "tilt" (say, lift a leg), then its center of gravity would change. Its center of mass would not.

[oVo]

My logic says the center of mass is the focal point (locaton) of the largest accumulation of weight in a structural form and the center of gravity is the proximity of that weight to the ground to establish physical balance. Stability is increased as the center of mass moves closer to the ground which creates a low center of gravity.

[AAFitz]

As a point of further interest in definitions: The terms gravitation and gravity are mostly interchangeable in everyday use, but a distinction may be made in scientific usage.

"Gravitation" is a general term for the attraction that bodies with mass have to one another,
"gravity" refers specifically to the net force bodies such as the Earth have on objects in their vicinity, including the effect of the Earth's rotation.

[a.sub]

actually this is what they are according the physics

Center of gravity is the point at which the sum of all vector of pull towards an object (ie all the vectors in your body that are pulled towards the earth) equals a vector that is perpendicular to the tangent line of your position relative to the object. So basically if you are standing on the earth, the center of gravity is the point where if you add all the vectors you get one giant vector that points to the center of gravity of the larger mass, the earth. if you lean to the side too far, the vector that connects yoru center of gravity and the earth are no longer above your base and you fall.

The center of mass is the point where if you take all the points in the object and add their Distances from given point times the masses (so it would be D1*M1 + D2*M2 ... Dn*Mn) you get zero.

no compare these two, if the pull on all the atoms by gravity is the same, then we can conclude that the center of gravity and the center of mass are the same. Why? because the vector mentioned with center of gravity is determined by F=ma, where F is the vector, m is the mass and a is the accel of gravity. At the center of mass, if the pull is constant, then all vectors around it will cancel out in ONLY direction, in the x direction. Now we have one giant vector pointing in the Y direction protruding from the center of mass, which is teh definition of the center of gravity. BUT in a given object the pull is not constant, because gravitational pull decreases with distance the pull from your head is slightly different from the pull on your toes. So the center of mass and the center of gravity are different only because gravity is not constant through out an object

[Mylittlepuddykat]

The best way to understand the difference between the centre of mass and the centre of gravity is to think of a really really tall building. If you assume that the building is of uniform mass, then the centre of mass is going to be directly in the middle - halfway up the building. However as gravity gets less the further from earth you get there will be a stronger force acting on the bottom of the building than at the top meaning the centre of gravity will be lower.

The same thing applies to anything although for most things the difference between centre of gravity and centre of mass is so minimal that it can be ignored.

[PLAYER57832]

The best way to understand the difference between the centre of mass and the centre of gravity is to think of a really really tall building. If you assume that the building is of uniform mass, then the centre of mass is going to be directly in the middle - halfway up the building. However as gravity gets less the further from earth you get there will be a stronger force acting on the bottom of the building than at the top meaning the centre of gravity will be lower.

The same thing applies to anything although for most things the difference between centre of gravity and centre of mass is so minimal that it can be ignored.

Which means that if a dog is running back and forth, roughly perpendicular to the Earth, then the center of gravity of the dog will remain roughly the same. However, the center of gravity of the boat will change every time the dog moves from its place in the boat AND most times the dog shifts its position, as well. The exception to the latter would be if the dog is able to keep all its weight on the same point(s) of the boat, despite shifting position.

The dog's center of gravity shifts only as it shifts its position relative to the center of the Earth. (i.e. tilts,puts weight on fewer legs, etc.)

[HungrySomali]

They're the same thing. I should know, having checked the font of all knowledge, Wikipedia. Although there is apparently a military concept called "Center of Gravity" that refers to the source of an army's strength.

There is also a military term "center mass" referring to where you should aim when shooting at someone.

[KoolBak]

Theres another military term used whilst being shot AT - its:

THIS (EXPLETIVE DELETED) SUCKS!!!!!!!!!!!!"

Sorry Paddy....even though you are a self professed genius, I think ol A Sub had the best definition here.......

[a.sub]

Theres another military term used whilst being shot AT - its:

THIS (EXPLETIVE DELETED) SUCKS!!!!!!!!!!!!"

Sorry Paddy....even though you are a self professed genius, I think ol A Sub had the best definition here.......

thx lol i just hope it was simple enough to understand

[Rocketry]

Well, you wont fool the children of the revolution.

Rocket.