A metal ball tied to the end of a string 1.8 m long is whirled in a horizontal circle at 4 m/s. What is the mass of the ball if the tension on the string is 30 N?

What formula/s should I use?

To solve this problem, you can use the formula for the tension in circular motion:

Tension (T) = (mass (m) x velocity squared (v^2)) / radius (r)

In this case, the tension (T) is given as 30 N, the velocity (v) is given as 4 m/s, and the radius (r) is given as 1.8 m. We can rearrange the formula to solve for the mass (m):

Mass (m) = (Tension (T) x radius (r)) / velocity squared (v^2)

Therefore, you should use the formula:

Mass (m) = (30 N x 1.8 m) / (4 m/s)^2

Now you can substitute the given values into the formula to calculate the mass of the ball.