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Question: What is the difference between a centripetal and a centrifugal pseudoforce?

(Posted by: slkslk on 2009-03-12 12:34:11)

How do you know which one to use when? i am having a hard time distinguishing between the two.

Posted by: (Ω)Mistress Bekki on 2009-03-12, 13:13:08

An object going in a circle is accelerating inward. So it requires a force to push it in. We call this a centripetal force. Now lets talk about pseudoforces in general. Let's say you are in an accelerating reference frame. Suppose you have no net forces acting on you, so you are not accelerating in an inertial frame. But with respect to your reference frame (which is accelerating around you), you accelerate in the opposite direction. So when you hit the brakes in your car, you are thrown forward. When you hit the gas, you are pushed backwards. And when your car goes in a circle, since your reference frame is accelerating centripetally, you are pushed outward. This outward push is called the centrifugal force. We call it a pseudoforce because it doesn't exist to an observer in an inertial reference frame--only to an observer in an accelerating reference frame. Lots of times (like when you are in a spinning vehicle or ride or planet), it makes a lot of sense to take your measurements with respect to the rotating reference frame. So you need to take into account pseudoforces like the centrifugal or coriolis forces. While you could always work in an inertial reference frame and "always use centripetal ", that is not generally the easiest, clearest way to work. If you are in a car, you are going to take measurements relative to the car--not some point outside. If you are on the earth, you are probably going to take measurements relative to a point on the ground--not a fixed point in space.

Posted by: Spiritual Paradigm Shift on 2009-03-12, 12:36:36

Always use centripetal

Posted by: radarsweep47 on 2009-03-12, 12:55:30

In order for an inert object to be made to move in a circle, an external force must be applied....because circular motion is accelerated motion and still very much abides by Newton's Law f = ma...it is just that in this case, circular motion, the accelerating force is always at right angles to the instantaneous line of motion...always remember that acceleration is the time rate of change of velocity as a VECTOR not just a scalar quantity and this is why circular motion is still accelerated motion even thought the scalar speed remains constant about the center...make sense? ok, well the force necessary to compel a body to move in a circle is always directed inward toward the center of that circle, therefore it is rightfully referred to as CENTRIPETAL force. The INERTIAL REACTION to this radial inward pointing force (remember action = reaction) is regarded as a reactive or "pseudoforce " and is directed OUTWARD...therefore rightfully called centrifugal (fleeing the center) even though it is in reality merely the INERTIAL reaction of a physical body that wants to move in a straight line to being forced to move in a circle. Make it clearer....the distinction?