Find RPM at which Lower Cord Goes Slack in Figure 5.72

[Niles]

Homework Statement

In http://session.masteringphysics.com/problemAsset/1007030/9/yf_Figure_5_72.jpg I have to find the number of revolutions per minute at which the lower cord goes slack.

The Attempt at a Solution

I just need to be sure: What they are asking me to find is the number of revolutions, so the lower cord is still tight, but it's the limit - so if the revolutions is a teeny weeny bit smaller, it goes slack?

If I have understood it correctly, is this the number of revolutions at which the block is only affected by the upper cord, and then the number of revolutions has be to bigger than that number? I hope you get me.

Thanks in advance.

 

[Doc Al]

Sounds to me like you understand what's being asked. At the point where the lower cord just barely goes slack, what is its tension?

 

[Niles]

it is zero? :-)

 

[Niles]

- I mean, the text says "... just goes slack", so it is slack and thereby zero?

 

[Doc Al]

it is zero? :-)

Exactly. So set up your equations, set that tension equal to zero, and turn the crank.