# Homework Help: Centripetal and Centrifugal Forces

1. Mar 23, 2005

### chunkymonkey

In class, I was told that the "centrifugal force" is merely an effect, and I can dig that. However, isn't the supposed "centripetal force" in actuality also merely an effect?

2. Mar 23, 2005

### Shockwave

A quick search on google.com uncovers 437,000 various web pages to your question. Please look at least one over to clear up any confustion that you may have.

3. Mar 23, 2005

### whozum

Thats mean, he came here for help.

Centrifugal force isnt a real force, it does not play any role in circular motion.

The centripetal force is THE force of circular motion, it is the force that is of just the right magnitude (mv^2/r) to keep you going in a circle at a given speed.

4. Mar 23, 2005

### marlon

The theory of General Relativity was born in 1915 and its father is Albert Einstein. The basic postulate is that there is no preferable frame of reference what so ever. So basically the concept of an inertial observer is banned and every observer needs to be treated equally.

For example when an inertial observer looks at a spring that is making a circular orbit around him. He will write down the law of Hooke in order to describe the spring. But a non-inertial observer, moving along with the spring, will have extra terms in this equation that have nothing to do with the spring but with the fact that he is NOT inertial.

These extra terms can come from the fact that this frame of reference itself, is making a circular motion, for example, so there is a constant centripetal force that works onto this reference frame. Though the non-inertial observer moving along with the spring, and being submitted to the spring force; will write ma' = -kx. This means that the non-inertial observer is basically hanging like a block on the spring so the spring works on the observer.

But this is incorrect because there is also the centripetal force mv²/R. this force is a socalled pseudo-force for the non-inertial observer and he must write : ma' = -kx –mv²/R. This is correct because the inertial observer would have written ma = mv²/R = -kx. This is the second law of Newton for a spring in circular motion. Basically what we see here is a manifestation relativity, meaning that equations of motions are different when we change our point of view. It really depends on how we look at things. Basically a' =a -a_0 where a_0 is the acceleration of the non-inertial frame with respect to the inertial frame.

This equation states that the non-inertial force or acceleration (a') is equal to the inertial force or acceleration (a) without this influence of the motion of the non-inertial frame (-a_0). The inertial force is here -kx and it denotes the correct physical laws...

read my journal (page 8, 'the string theory part 1' entry) for more indept info

regards
marlon

5. Mar 24, 2005

### chunkymonkey

Well, I’d like to first say that it’s interesting to see how people response when one’s language is less erudite than one typically uses in speech. The intention of utilizing slang was more to mask my question from my science teacher who frequently replies to homework questions. Nonetheless, my question did go beyond the rather elementary concept of circular motion resulting from an inward-pointing force as opposed to an outward-facing one. In actuality, I am not a buffoon as Shockwave foolishly assumed, and I appreciate Whozum’s defense of the question; it is funny how an incoming MIT physics major can be misinterpreted. I would like to thank Marlon for his/her intelligent response; it was exactly what I was looking for.

6. Mar 24, 2005

### futb0l

The centrifugal effect = Inertia. Think about that.

7. Mar 24, 2005

### Sirus

A more fruitful search may be the one in PF. This exact topic has been discussed in depth extensively on the forum. Search around for additional information if you like, and post any follow-up questions you may have.