# Centrifuagal and centripital force

1. Sep 6, 2008

### firavia

-in balancing masses , why do we always say that we need to put a mass in a rotating disk that produce a centrifugal force that is equal to the unbalanced mass ?

-cant we say that a centripital force of a certain rotating mass is euqal to a centrifugal force of the same mass but in different direction ? , and if this is true dont we have an equilibrium at this case ? 2 opposite equal forces ??? , so if this is the case , why do we need to balance ?

-as I know , centrifugal force do not really exist , so can we say that there is only 1 true force that is acting on the mass which is the centripital force that need to be canceled by another mass ????

-why do we speak about a centrifugal balance and not about a centripital balance ...

2. Sep 6, 2008

3. Sep 6, 2008

### tiny-tim

Hi firavia!

I don't understand … what is "balancing masses" … can you explain this, please?

4. Sep 6, 2008

### atyy

If you sit in a centrifuge and get spun round, you will feel a force pushing you outwards. That is the centrifugal force and I doubt you will say it is not real since you can feel it.

The person standing outside the centrifuge will see that you are being spun round. Going round and round is not going in a straight line with constant velocity, so it is accelerated motion. From his point of view, there is a centripetal force that is spinning you.

The essential difference is the point of view - whether you are sitting in the centrifuge, or standing outside.

5. Sep 6, 2008

### firavia

-so atty there is only one force , that is seen relatevly ??

-tiny tim , balancing masses is when we have the center of gravity is different then the center of rotaion , so this would create a centrifugal force that is equal to MR.v/r(square).

well i would like to know if centrifugal is one force and centripital is another or both are 1 force that is seen in different direction based upon the origin , cause if they are 2 real existing forces , we wouldbe having a balance without adding another mass , it is confusing.

6. Sep 6, 2008

### Gear300

Centrifugal force is a perceived force, but not the actual physical force. Yet, that mainly matters depending on what frame you're observing from. A force is a force, whether it is an illusion or not. Gravity may just be an illusionary phenomenon we perceive as a force. All forces may just be illusions and may simply be phenomenon we perceive as such...we just might not be seeing it. That is why we take into account centrifugal force.

7. Sep 6, 2008

### firavia

you are right Gear 300 , but the main question remains : if centripital force and centrifugal force exist together with equal forces and opposite direction , cant we say that the rotating mass is balanced , due to the existance of the 2 equal opposite forces ?

8. Sep 6, 2008

### atyy

No there are two forces and which one you see is relative.

The guy standing outside the centrifuge says the centrifuge is exerting a centripetal force directed toward the centre.

The force you feel is outward, in the opposite direction, so it cannot be the same force. Looking at the equation a=v2/r, we see that the part of you that is nearer the center is accelerating more, and the part of you that is further from the center is accelerating less. The outward force you feel is due to different parts of you accelerating differently. (Or something like that, I don't think I have the details right - hopefully someone else can clarify this.)

9. Sep 6, 2008

### tiny-tim

Hi firavia!

(btw, it's "centripetal", with an "e", as in "petition", meaning "seeking the centre" )

(and it's mv2/r)

I still don't really understand what you mean.

Are you thinking of a force on someone sitting in a car when it goes in a circle to the left?

As seen by someone on the ground, the friction between him and the seat is to the left (centripetal), and it accelerates him to the left (centripetally).

As felt by the person in the car, the friction is also to the left, but in that frame there is an "inertial" force on everything, to the right (centrifugally), and these two forces balance, so that he remains stationary (in that frame).

The friction force is undoubedly real … he can feel it.

But he cannot feel the centrifugal force … if there was no friction, he would feel nothing … so in that sense it's not a real force.

10. Sep 6, 2008

### Gear300

There is a subjective element. When driving a car and taking a turn, you'll feel a centrifugal force, while the centripetal force may not be as noticeable. From this, you can say that you perceive the centripetal force with a centrifugal force. Lets say that a person was being swung around in a circle by a centripetal force that can act over a distance, such as gravity...but the person has no awareness of his or her surroundings...so what the person will perceive is a centrifugal force and instead think something is pulling him or her to the left or right (the person wouldn't even know that he/she is moving in a circle...or even moving with a perpendicular velocity...all the person will feel is a force pulling him/her in a seemingly constant direction). Altogether, centrifugal and centripetal forces exist in respect to one another, but do not balance each other out.

11. Sep 6, 2008

### firavia

thats all what I wanted to know Gear 300 , thank you , I am satisfied by the answer :" they do not balance each other " . thats why we need an opposite mass that make the balance .