Maximum Speed of a Whirling Rock on a String - Solving a Physics Problem

[mattmannmf]

A 630 gram rock is whirled on the end of a string 49 cm long which will break under a tension of 20 N.

a) What is the highest speed the rock can reach before the string breaks? (Neglect gravity.)
V=M/S

T- mg= ma

T-mg= m(v^2/r) solve for v

i get 2.188 m/s (which is wrong)

20-(630*9.8)= (630)(v^2/.49)

I converted 49 cm into .49 meters

help?

 

[w3390]

I don't know why you have the mg term in there. I think all you need is the T=mv^2/r and solve for v.

 

[mattmannmf]

20= (630)*( (v^2)/ .49)

v= .125 m/s... that was wrong when i plugged it in

 

[mattmannmf]

nvm, i got it. conversion problem thanks!