Why Is My Approach to Solving Physics Homework Incorrect?

[Garen]

Homework Statement

Homework Equations

The Attempt at a Solution

The way I understood is it is by taking ma and setting it equal to mv^2/r and solving for v. With this I got answer choice number 9, sqrt(glsinTHETA). But it's wrong. Can anyone direct me in the right direction to solving this deceptively easy problem? Thanks in advance.

 

[Hootenanny]

Homework Statement

Homework Equations

The Attempt at a Solution

The way I understood is it is by taking ma and setting it equal to mv^2/r and solving for v. With this I got answer choice number 9, sqrt(glsinTHETA). But it's wrong. Can anyone direct me in the right direction to solving this deceptively easy problem? Thanks in advance.

You're on the right lines. However, you incorrectly assume that the tension in the string is mg.

 

[Garen]

You're on the right lines. However, you incorrectly assume that the tension in the string is mg.

So you're saying that I set mv^2/r equal g/cosTHETA ?

 

[Hootenanny]

So you're saying that I set mv^2/r equal g/cosTHETA ?

No. Start by writing down two equations: one for the requirement of the sum of the forces in the vertical plane, and one for the sum of the forces in the horizontal plane. In otherwords, apply Newton's second law separately to the vertical and radial planes.