# Homework Help: Centripetal vs centrifugal acceleration

1. Dec 8, 2005

### dnt

i dont quite understand the difference between the two. i know centripetal means towards the middle and centrifugal goes out but i cant quite relate that to the problems.

for example, if you are twirling a ball on a string above your head in a circle, the centripetal acceleration points towards the middle and acts on the ball? but where is the centripetal acceleration? how does that figure into it?

are they just opposite forces?

in a related problem, its dealing with those spinning carnival rides where the floor drops from below you but you dont fall due to centripetal forces.

now the question says the coefficient of friction is .30 and the radius is 2.5 m, whatis the minimum rotational speed so you dont fall...this is what i have so far:

looking at a person on the ride, you have gravity (mg) down and friction upward. to make sure he doesnt fall you set the two forces equal to each other. now the friction is equal to the 0.30 times the normal force, which i believe in this case is the force due to centripetal acceleration (pushing the guy against the wall)? is that right (or is it centrifugal?) that equals mv^2/r.

so if i set mg = mv^2/r and the m's cancel out, i can solve for v. then i can convert that into a rotational speed? is my work correct?

thanks.

2. Dec 8, 2005

### Galileo

The centripetal force is the force responsible for the maintainance of circular motion. So it's an actual force acting on the object and of course directed towards the center (centripetal=centre-seeking)

From the point of view of an object in circular motion, it seems as though there is a force which wants to pull it away from the center. This is called the centrifugal force. It is a ficticious force, so in reality not really a force at all. When we place ourselves in a noninertial frame there are all kinds of ficticious forces. Like on earth, because it spins it is not an inertial frame. We experience a centrifugal force, but also coriolis forces etc.

You know when a car accelerates you get pushed back into your seat? That's also a ficticious force. Nothing is pushing you backwards, rather, the seat is pushing you forward. The apparant force is ofcourse equal and opposite the real one. So also is the centrifugal force equal and opposite the actual centripetal force.

3. Dec 8, 2005

### dnt

thanks - did i set up my problem correctly?

4. Dec 8, 2005

### Fermat

Friction = mu*Normal Reaction
and
Friction = mg
so
mg = mu*mv²/r

Other than that, your analysis of what's happening is absolutely correct