# Circular Motion Part II.

1. Feb 12, 2005

### Cyrus

I have a problem with circular motion that is bugging me. In my dynamics book, there is a rod with a collar on it that slides outwards as the rod rotates. From general physics, I understand this example in the case of something like say, a person standing on a frictionless floor inside a train car. They are moving forward, but as the train turns, the person continues to move forward, and so the wall runs into him, and he feels this fake force due to the turning. So it looks like hes been throw radially outward, relative to inside the car.

But in the case of this collar, its also moving forward, and also has inertia. But when this rod starts to turn away from straight line motion, the collar must start to turn too, because its physically around that bar. So in this case,
it cant be seen as moving in a striaght line as the bar curves. The same reasoning as the train cant work here. I tried to break it down, but had little sucess. It seems to me that as this bar turns, there is a little bit of slop between the colar and the shaft of the rod, otherwise it could never slide. So as the rod starts to turn, the collar wants to move forwards, but then the two opposite sides of the collar are going to make contact with the rod. The reason being that the collar is rigid, so as it trys to keep moving forwards and the bar starts to turn, the two opposing edges of the collar will come into contact with the rod, and will turn the collar with the rod. So how come it does not just sit there going along for the ride, but instead it flys off?

Thanks, Cyrus

2. Feb 12, 2005

### Crosson

To me, this analysis seems to suggest the collar would "fly off".

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3. Feb 13, 2005

### Staff: Mentor

For the collar to stay in place requires a radial force on the collar. (Since it would be in circular motion.) If the collar/rod surface is frictionless, then the rod cannot exert the radial force and the collar would fly off. (I'm agreeing with Crosson.)