# Centripetal confusion

by Muon12
Tags: centripetal, confusion
 P: 34 This is a classical mechanics question requarding circular motion. I understand that centripetal force causes objects to accelerate toward the center of the circular path they travel in, but how does this apply to objects traveling on the outside of a circle? I have come to understand that if an object is travelling within a loop, (with enough v) it's natural tendency is to remain on the inner edge of that circle, since its velocity is directed tangentially, and the centripetal acceleration is directed toward the center of the circle. But my question is this: would that same object be pulled toward the center of the circle if it were travelling on the outside of the loop? Say for example, a car is driving over a hill that has a circular shape. If Fr=-mv^2/r (- in this case since its travelling on the outside), then would we consider the centripetal accel. to be inward, especially when, giving enough velocity, this car would take off and leave the circle (until gravity pulled it back down, that is)? Conceptually, how is the acceleration (ar) directed toward the hill's center when the centripetal force seems to have little effect on the car at this point?
 Mentor P: 7,318 Frankly it is rather difficult to find your question among the verbiage. To maintain a circular path a centripetal force is required. A car does not always stay on the outside of circular loop because there is NO centripetal force to keep it there. A centripetal force is not present the instant something is curved, the force must be provided by some mechanism. When the car is on the inside loop, the track provides the centripetal force, there is no corresponding force when the car is on the outside of the loop. So if gravity is not sufficient to provide the force the car will leave the track. Does that help?
Mentor
P: 41,438
 Originally posted by Muon12 This is a classical mechanics question requarding circular motion. I understand that centripetal force causes objects to accelerate toward the center of the circular path they travel in, but how does this apply to objects traveling on the outside of a circle?
In exactly the same way. If an object moves in a circle, then there must be a force pushing it towards the center.
 I have come to understand that if an object is travelling within a loop, (with enough v) it's natural tendency is to remain on the inner edge of that circle, since its velocity is directed tangentially, and the centripetal acceleration is directed toward the center of the circle.
Careful! The "natural" tendency for any object is to keep going in a straight line at a constant speed. The only reason it goes in a circle is because something is pushing it towards the center. The only way a car can go around a circular track is if a force exists to push it towards the center. The only external forces on the car are the road and it's weight. The road exerts two forces on the car: a friction force sideways and a "normal" force pushing straight out of the ground. The weight just pulls down. The only way a car can go in a circle (inside or outside a loop) is if these forces happen to point towards the center!
 But my question is this: would that same object be pulled toward the center of the circle if it were travelling on the outside of the loop? Say for example, a car is driving over a hill that has a circular shape.
Gravity pulls it down (which is toward the center)! But that force is fixed; go too fast and it won't be enough to hold you to the ground.
 If Fr=-mv^2/r (- in this case since its travelling on the outside), then would we consider the centripetal accel. to be inward, especially when, giving enough velocity, this car would take off and leave the circle (until gravity pulled it back down, that is)?
The acceleration is just a description of the motion. If it goes over a hill, then there is some "centripetal" acceleration. Think of acceleration as the effect of some force, which is the cause. No force, no acceleration, centripetal or otherwise.
 Conceptually, how is the acceleration (ar) directed toward the hill's center when the centripetal force seems to have little effect on the car at this point?
The only "centripetal" force in this case is gravity: the weight of the car. Sure it has an effect---it pulls the car down! But if you go too fast, it won't be enough to make you go in a tight enough circle (centripetal)---you'll keep going straight: into the air! Of course, once the car is in the air, the tires can't push the ground any longer, so the car falls just like a tossed ball would.

 Sci Advisor P: 2,510 Centripetal confusion Within the Event Horizon of a Black Hole, centrifugal force works in the opposite dsrection from common experience. This means that orbitting at a high rate of speed around the center generates a force pushing one toward the center, rather than away. Does this mean that within the EH, a centrifugal force becomes centripedal, or is it still considered centrifugal force with a reversed vector value or something like that? Just a matter of terminology, but I'd be interested in knowing the answer.
 Mentor P: 7,318 Lurch, That is the first I have heard of that phenomena. Does it connect to the concept of closed geodesics, which I believe occur inside the EH. I am sure that once you have crossed the EH you have entered the singularity, therefore you must be very careful about application of normal laws of physics.
 Math Emeritus Sci Advisor Thanks PF Gold P: 39,497 Circles do NOT HAVE "inner" and "outer" edges. Centripetal force and acceleration applies to any object moving in a circle. It is not a question of "inside" or "outside".