How to Solve Vertical Acceleration Problems

[Iceclover]

Problem:
a rock is thrown vertically with a velocity of 20m/s from the edge of a bridge 42m above the river. how long does the rock stay in the air.

Equation:
d=vt+1/2at^2

Atempt at solution:
I've actually had a couple of attempts and i came up with a bunch of crazy things. I just need a hint, or to know if I am using the correct formula.

 

[cristo]

Iceclover, please post in the homework forums in future.

Your equation for this question is correct. Please show your work, and we will be able to help.

 

[AngeloG]

A few good helpful hints is to do this first; label what you have and what you don't have and what you're trying to find. Then look at the equations you have; use algebra to isolate a variable and these types of problems will come a lot more easily.

Using Y's since this is acting in the Y direction, not the X direction. Also, since it's acting in the Y direction there's a force at work here =). Think about it.

Y (final position):
Yo (initial position):
Vy (final velocity):
Vyo (initial velocity):
a (acceleration):
t (time):

Hint: Your starting position will usually be 0 m. Unless told otherwise through the problem. Also, you can use a string of equations to solve for things =). Such as isolate the t, and then use that t for another equation =).