Explaining physics

Question

A girl twirls a key chain in a circle if she twirls it faster, she finds that she holds the chain tighter. Explain why this is necessary.

 

I think it's that she has to hold the chain tighter because there is more velocity and force. 

 

Is this right? Explain further using physics terms

Answer 1

Chantelle,

You're exactly on the right track; she has to grip it tighter in order to exert a frictional force sufficient to keep the chain from slipping through her fingers. Now, you know that frictional force opposes whatever tendency the chain has to slip: in other words, her fingers are (passively) pulling the chain inwards to provide centripetal force to the chain. Each piece of the chain is following its own circular path; to change the momentum of a mass requires a force; to keep a mass moving in a circle at constant speed requires a centripetal force of constant magnitude, in a direction at right angles to that circular path at that moment.

So -- a = v^2/r = .omega.^2 * r

F(centripetal) = m * a

F(friction) == F(centripetal)

What may be tricky here, is adding up all the pieces of chain, each with its particular circle. That math depends on exactly how the chain is being swung -- in a conical arc, or oriented around a horizontal axis, and so on.

Hope this allows you to start calculating!

Answer 2

A girl twirls a key chain in a circle if she twirls it faster, she finds that she holds the chain tighter. Explain why this is necessary.

I think it's that she has to hold the chain tighter because there is more velocity and force.

Is this right? Explain further using physics terms

========================

 

You are pretty much right:)

 

Actually, she's creating the centripetal force (F) in order make the chain move by a circle.

If R is the radius of that circle, and V is the speed of movement,

[this can be analyzed in details, but we need the sketch of this activity; consider V as an effective speed]

then the force is,

   F=mv^2/R   , with m being the mass of chain.

 

R directed into the center of "twirling", and, it's provided by the "girl" [say, her fingers ...].

So, the faster she rotates the chain, the bigger force is needed to secure the circular motion ...

Even, you may see, that the force strongly depends on V - it's quadratic dependence.

 

message if questions;

good luck:)