Physics
posted by Sierra .
Consider the motion of the rock in the figure below. What is the minimum speed the rock can have without the string becoming "slack"? (The rock is traveling in vertical circle. Assume that m = 1.6 kg and r = 0.48 m.)

What is the minimum speed the rock can have without the string becoming "slack"? (The rock is traveling in vertical circle. Assume that m = 1.6 kg and r = 0.48 m.)

At the top,
mv^2/r=mg
v= sqrt(rg) 
2.2m/s

Could someone explain how to do this?

the equation is A(c)= v^(2)/r
solve for v, which is v=[A(c) x r]^(1/2)
A(c)=9.81, r=radius=.48m, mass is irrelevant.
solve for v: v= [9.81 x .48]^(1/2)
v= [4.7088]^(1/2)
v= 2.169 
Mars has two moons, Phobos and Deimos. It is known that the larger moon, Phobos, has an orbital radius of 9.4 106 m and a mass of 1.1 1016 kg. Find its orbital period.