What Height Will the Projectile Strike the Barrier?

[schyuler2]

Homework Statement

A projectile is thrown from the horizontal ground with a speed of 40 m/sec at an angle with the horizontal of 30 degrees. At what height will the projectile strike a high barrier located at a distance of 85m from the firing point?

Homework Equations

ok so: point A is located at the point in which the projectile is launched
point B is located at the point at which the projectile strikes the barrier
point is located at the bottom of the barrier, on the ground directly below the striking point
theta = 30 degrees

change in time from A to B= [VB - VA] / acceleration

dAB = distance from A to C * cos(theta)

dBCx = VB (change in time from B to C) + 0.5a*(change in time B to C)²

The Attempt at a Solution

change in time A to B = [0 - 40] / 9.8 = 4.08 sec

dAB = 85m * cos(30) = 22m

dBCx = 0 (4.08 sec) + 0.5 (9.8) (4.08)² = 245 m
^my teacher said that this wasn't right. i think i went wrong on the acceleration-i don't think i am calculating it right or using the right value.

 

[scoldham]

You're value for acceleration is fine in the y direction.

How did you derive your equation for time?