- 18 Jun '06 17:10

If P(t) = at^2+bt+c, and b = a negative constant, then V(t) = 2at + b.*Originally posted by Gastel***A ball rolls down a chain that is held at two points P1 and P2. The ball is already in motion, but there is a friction u1 constant for the complete path.**

What is the velocity function of the ball? - 18 Jun '06 20:00

I don't believe any combination of b, a or c could ever make that the proper function for P(t). Of course, I'm an electrical engineer not a mechanical one, but my memory of this problem is that it is not quadratic.*Originally posted by Ramiri15***If P(t) = at^2+bt+c, and b = a negative constant, then V(t) = 2at + b.**

By the way, this is just to break the monotony of "What are the factors of x^2 + 3x + 2?" questions. - 18 Jun '06 22:15

True, there is no way the function can be described as a quadratic because eventually, due to the friction, the ball would reach a constant velocity.*Originally posted by Gastel***I don't believe any combination of b, a or c could ever make that the proper function for P(t). Of course, I'm an electrical engineer not a mechanical one, but my memory of this problem is that it is not quadratic.**

By the way, this is just to break the monotony of "What are the factors of x^2 + 3x + 2?" questions. - 19 Jun '06 03:32 / 1 edit

yes, i see it as an exponential rise-to-max type scenario, much like terminal velocity. but the probelm takes on vector-type difficulties early in the piece, since gravity is pulling the ball down, and the chain is pulling the ball sideways, with the chains force dependant on the angle between the line P1-P2 and the vertical. the combination of these will give the downward spped without friction. the friction u1 can be thought of as acting on the final sum of gravity and angle, so that should be easier to put in. i'm not fantastic at doing vector math...*Originally posted by Ramiri15***True, there is no way the function can be described as a quadratic because eventually, due to the friction, the ball would reach a constant velocity.**

Edit: ah ha, it seems galileo considered this probelm, and found for a ball rolling down a slope, the velocity at the end of the slope is v= sqrt((10g-h)/7), where g= accleration due to gravity, h= overall height dropped by ball. should give a start point anyway. - 19 Jun '06 11:02

Thank goodness you have Galileo as a friend! Did he consider (in the solution you povided) that the weight of the ball is deforming the chain as it moves? Also, show the steps behind the calculation.*Originally posted by xcomradex***yes, i see it as an exponential rise-to-max type scenario, much like terminal velocity. but the probelm takes on vector-type difficulties early in the piece, since gravity is pulling the ball down, and the chain is pulling the ball sideways, with the chains force dependant on the angle between the line P1-P2 and the vertical. the combination of these will ...[text shortened]... ccleration due to gravity, h= overall height dropped by ball. should give a start point anyway.**

Finally, I was certain the function was a hyper-cosine function or "chain function". - 19 Jun '06 21:50 / 1 edit

no, his equation was for a ball on a solid surface. i missed the subtle point that the ball deforms the chain as it moves. that makes it a bit more interesting. and i think you are right, it could make a chain function.*Originally posted by Gastel***Thank goodness you have Galileo as a friend! Did he consider (in the solution you povided) that the weight of the ball is deforming the chain as it moves? Also, show the steps behind the calculation.**

Finally, I was certain the function was a hyper-cosine function or "chain function".

edit: i see above a "-" has crept in, the actual function: v=sqrt(10gh/7) ... (makes a lot more sense that way) - 21 Jun '06 01:07The force of friction is a function of the normal force and the coefficient of friction. The coefficient of friction is constant, and the normal force has a maximum at mg where m is the mass of the ball and g is the force due to gravity. This is never infinite but can approach zero. The effect of the deflection of the chain can be interpreted in various ways but one thought is to have an additional reactive force as the chain is deformed, but remember that this is counterbalanced by the fact that the chain wants to straighten itself out. If the mass of the chain is much greater than the mass of the ball, then this is eliminated.
- 21 Jun '06 01:20

I just said it could be called infinite because when the chain is parallel to the ground, movement of the ball is impossible so it would have the effect of inifinite friction, it might make for a simplification of the function.*Originally posted by Gastel***The force of friction is a function of the normal force and the coefficient of friction. The coefficient of friction is constant, and the normal force has a maximum at mg where m is the mass of the ball and g is the force due to gravity. This is never infinite but can approach zero. The effect of the deflection of the chain can be interpreted in various wa ...[text shortened]... t. If the mass of the chain is much greater than the mass of the ball, then this is eliminated.** - 23 Jun '06 00:34 / 1 edit

That would be a case of the normal force being parallel to gravitational force, leading to a net force of 0 on the ball and no movement.*Originally posted by sonhouse***I just said it could be called infinite because when the chain is parallel to the ground, movement of the ball is impossible so it would have the effect of inifinite friction, it might make for a simplification of the function.**