Angular Acceleration: Min Speed for Cord to Not Slack

[notsam]

Homework Statement

Hello every one :) I do NOT understand rotational speed ect. SO
A ball of mass 22.8 g is attached to a cord of
length 0.739 m and rotates in a vertical circle.
What is the minimum speed the ball must
have at the top of the circle so the cord does
not become slack? The acceleration of gravity
is 9.8 m/s2 .
Answer in units of m/s.

Homework Equations

F= ma, F=mv^2/r

The Attempt at a Solution

Ok So this seems fairly simple, I'm pretty sure that I am supposed to set ma=mv^2/r? Is that correct?

 

[cupid.callin]

Ok So this seems fairly simple, I'm pretty sure that I am supposed to set ma=mv^2/r? Is that correct?

Yes but using only this will not work

Try energy conservation

kinetic energy at lower most point provides potential energy to reach top + kinetic energy at top.
This gives you 1 eqn

Now use mv²/r for speed at top point ... which is provided by weight and tension
and for minimum speed, tension = 0​

 

[notsam]

Thank you, I never thought about using the conservation of energy to work it out. Then using each end of the "radius" as a refrense point for 0 meters. That's smart!