Circular Motion (finding speed)

[lindsey0307]

Homework Statement

A ball of mass 0.23 kg is attached securely to a string, then whirled at a constant speed in a vertical radius of 0.75 m. Calculate the minimum speed of the ball at the top of the path if it is to follow a complete circle.

Homework Equations

F = mv(squared)/R

The Attempt at a Solution

I first solved for Fc when the ball is at the top of the circle with F=mv(squared)/R - mg and it equals 1.7 N. I then rearranged the equation to solve for V using V = the square root of FR/m and got the answer 2.35 m/s. My sheet says the answer is 2.7, which is pretty close to what I got, but I may have done something wrong. Any thoughts?

 

[The1337gamer]

This is what i think, right so the forces acting on the mass are gravity and the tension of the string, it's best to draw a diagram.
T = tension.
Defining positive direction to be downwards.At the top of the circle: F = mg + T

As you say F= (mv^2)/r for circular motion. So equate these.

Now can you figure out how to find the minimum speed?

 

[lindsey0307]

Yes, I've figured it out now, thanks!