Solve Throw Up Problem: 15m, +12m/s & -12m/s

[nic0le]

Homework Statement

Two rocks are thrown off the edge of a cliff that is 15m above the ground. The first rock is thrown upward, at a velocity of +12m/s. The second is thrown downward, at a velocity of -12.0m/s. Ignore air resistance. Determine how long it takes the first rock to hit the ground and at what velocity it hits. Determine how long it takes the second rock to hit the ground and at what velocity it hits.

Homework Equations

The Attempt at a Solution

15=12t+.5(9.8)t^2

To figure out time I tried v/g=t (12/9.8) but it was coming out saying the answer was wrong.

 

[learningphysics]

15=12t+.5(9.8)t^2

Is that for the first rock or the second?

 

[nic0le]

That was for the first rock.

 

[cristo]

Check your sign convention. Take upwards to be positive and downwards negative. Then the velocity is positive, which you have correct. What is the net displacement-- up or down (i.e. should the 12m be positive or negative)? In which direction is the acceleration due to gravity (g) acting? If you get these correct, you should have the right solution.