Need help on a simple question

[Cmac22]

Hi. i just started physics this year so im not too deep into it yet. just had a question about center of mass. we were doing a centripetal force lab, spinning small weights on a string over our heads and varying the mass. so my question is:

 

As your spinning the weights overhead, is the center of mass simply the midpoint between one end of the weights and the other end on the string (all the weights are the same). Or would the center of mass be at the end of the string. this is probly a stupid question but im just making sure.

[Edtharan]

this is probly a stupid question but im just making sure.

The only stupid questions are the ones asked to annoy others, so this is not a stupid question at all.

 

Ok, lets think about the term "Centre of Mass". Actually, it reall shouold be called the centre of the mass distribution, or the Average centre of mass. You are thinking of the geometrical centre.

 

Think of the centre of mass like the balance point. If you were to place the whole system on a needle point, at which spot would the whole thing balance? This would be the centre of mass.

 

Imagine a seesaw with an adult on the end and a child on the other end. To balance the seesaw we would need to put the support close to the adult. This would also be the centre of mass of the adult + child + seesaw system.

 

Actually it is a bit more complex than being just like this seesaw. The seesaw is only looking at the centre of mass in just 1 dimension (the one between the adult and the child along the length of the seesaw. To find the true centre of mass you have to do so in 3 dimensions. But it is no harder than the seesaw example, it is just that you have to do it 3 times instead of just once.

 

If you are swinging a weight on a piece of string around your head, then the centre of mass of that system (You + weight + string) would be quite close to you.

 

When the Olymipcs roll around, have a look a the Hammer throwing. These Hammers weigh quite a bit and as the Thrower swings it around, they must swing it around the centre of mass (otherwise either them or the hammer or both would go spiralling off). Because the Hammer is so heavy (massive) it centeew of mass lies a littel bit outside the Throwers body and you can see them actually moving around theis centre of mass.

 

I hope this helps.

[swansont]

If you are swinging a weight on a piece of string around your head, then the centre of mass of that system (You + weight + string) would be quite close to you.

 

However, if you are just looking at the ball + string, it will be near or inside the ball. Which COM you want depends on what problem you are solving.

 

The location of the center of mass is [math]\frac{1}{M} \Sigma m_i r_i[/math]

 

that is, sum up the mass*location of each object and divide by the total mass

[Maqboole]

interesting

[stingray78]

You can define in a continium the center of mass as Integral(r*dm)/Total Mass

or as a Sum like swansot said before. Center of mass is the point where you han hold an object with a needle without falling (Theoretically of course),.

[Kristafar]

hello, you question is interesting, ..i think about it ...and if i understand: the teoritically understands ... the Total mass and the center of the mass it - can be absolutley the same mass and value...but the another question is, should this mass are static or with eclipse... ectually every mass on our Planet are in some procent full with eclips becouse our sun sistem all the time travel in the univerce... and more.. or few..the invisible energia cross all matter ...well we know that the sun sistem travel with definite speed in the universe ...and i think there are no static mass or "object"

[jinsan]

i would think that centre of mass is just in the ball as wen an object has centripetal force acting on it it also has centriFUGAL force to balance it out. But I could be wrong

[stingray78]

It depends on which particles you consider as part of your system. The easiest way is to consider the mass as the system with its center of mass. And then you consider your hand appling centripetal force to the mass through the string.

 

Hope it it helps