Uni Stuff > Physics
umep physics thread
kamil9876:
Can an object roll down a frictionless surface? Think about the contact point and what happens there in rolling motion.
evaporade:
yes, it can appear to be rolling if it is already spinning.
kamil9876:
I'm talking about the specific case of placing a cylinder on a frictionless incline, keeping it stationary, and letting it go.
/0:
Oh ok... I guess this is just a conceptual thing I will need to play around with.
I think it's funny how when you actually analyse the torques and hence acceleration of the cylinder you normally take the pivot to be the contact point of the cylinder, so friction (which acts from the pivot) can be ignored and gravity (acting at distance R) is the one to focus on... and yet, friction is necessary for the gravity torque to do its job!
Hmm and if the cylinder on a frictional ramp is already rolling, then since there are no external torques it must keep rolling according to conservation of angular momentum, but its angular velocity stays constant... that is very interesting.
thanks kamil and evaporade :)
kamil9876:
dcc's way is also good: You can take the pivot as going through the centre of the cylinder and so the Gravity torque is 0, the Normal Torque is zero (it's parralel to radius vector(opposite direction)) but frictional torque is the only one. Hence friction is neccesary.
Wasting a lot of my time thinking about this, I think it's easier to just take the centre as pivot rather than checking if the contact point is really instantaenously stationary. The latter approach is probably easier to imagine in polygonal objects rolling down (a circle is a kind of polygon though lol)
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