How Do You Calculate the Time and Velocity of a Dropped Object in Physics?

[a.k]

Homework Statement

A man using a hang glider who is descending at a speed of 8 m/s drops his cell phone from an altitude of 30 m.
a. How long does it take the cell phone to reach the ground?
b. What is the velocity of the cell phone just before it hits the ground?

Homework Equations

V_f^2=V₀^2+2ay
t=v-v₀/a

The Attempt at a Solution

(-8m/s)^2+2(-9.8 m/s)(-30)

I want to make sure I am using negative velocity and accel due to descending speed...

 

[a.k]

64 m/s^2+(-19.6m/s)(-30m)
64+588
v_f²=652
sqrt(652)
25.53
Due to descending, it is -25.53 m/s

t=v_f-v_i/-9.8 m/s^2

=-17.3/-9.8=1.765
t=1.77 seconds

 

[PhanthomJay]

64 m/s^2+(-19.6m/s)(-30m)
64+588
v_f²=652
sqrt(652)
25.53
Due to descending, it is -25.53 m/s

t=v_f-v_i/-9.8 m/s^2

=-17.3/-9.8=1.765
t=1.77 seconds

those answers look good. You can avoid confusion with the minus sign by assuming down direction as positive, but what you did was fine, although you did have to have the insight that the sq root of 652 was the negative root