Physics Problem

[MisterPants]

There is a cone, point at the bottom, generated by a line an angle t from vertical. It is being rotated on its point. There is a block somewhere on the inside of the cone at height h. Coefficient of friction is u. Find the max velocity and the min velocity at which the block stays in place.

I can't understand how there could be a max velocity - the only forces acting are gravity, a normal force from the side of the cone to counter gravity and keep the thing going in a circle, and friction, a retardant. How can it do anything but slide down or stay in place?

 

[Rob9874]

max = 1/2t / (3hu)
min = thu

 

[TuxDave]

Originally posted by: MisterPants
There is a cone, point at the bottom, generated by a line an angle t from vertical. It is being rotated on its point. There is a block somewhere on the inside of the cone at height h. Coefficient of friction is u. Find the max velocity and the min velocity at which the block stays in place.

I can't understand how there could be a max velocity - the only forces acting are gravity, a normal force from the side of the cone to counter gravity and keep the thing going in a circle, and friction, a retardant. How can it do anything but slide down or stay in place?

Let's say the block is rotating REALLY REALLY fast. You need a large centripetal force to sustain that rotational velocity. If that centripetal force exceeds the force due to gravity, then it will slide up.

 

[MisterPants]

How can centripetal force being exterted by a wall, normal to the wall, cause an acceleration upward along the wall?

 

[Evadman]

Originally posted by: MisterPants
How can centripetal force being exterted by a wall, normal to the wall, cause an acceleration upward along the wall?

because centrpital is from center of rotation, but the force is acting on a wall that is not perpendicular to the force.

 

[MisterPants]

Centripetal force doesn't exist - it's a force required to make something go in a circle. Though you can resolve the normal force into the centripetal and a component that is directed above normal, it obviously has no component up along the wall. Am I missing something?