# Proyectile Motion. Find angle!

alfredo24pr

## Homework Statement

A catapult on a cliff launches a large round rock towards a ship on the ocean below. The rock leaves the catapult from a height H of 34.0 m above sea level, directed at an angle θ above the horizontal with an unknown speed v0.

The projectile remains in flight for 6.00 seconds and travels a horizontal distance D of 149.0 m. Assuming that air friction can be neglected, calculate the value of the angle θ.

## Homework Equations

y=y0 + v0yt - 0.5gt^2

tan θ = y/x

## The Attempt at a Solution

I used y=y0 + v0yt - 0.5gt^2

y= -34 + 0 + 0.5 (9.8) (6^2)
y= -34 + 4.9(36)
y= -34 + 176.4
y= 142.4m

For x, the distance is given: 149 m

The angle:
tan θ = y/x
tan θ = 142.4/149
θ = 43.7

Is that correct? LONCAPA does not accept my answer, so if it is right I will assume that it is the degrees unit.

## Answers and Replies

Homework Helper
In the first equation in your solution, voy is not equal to zero.

alfredo24pr
how do I find it?

Homework Helper
I used y=y0 + v0yt - 0.5gt^2

The above equation becomes
y=y0 + v0*sin(theta)*t - 0.5gt^2
Find voy*sin(theta). From second equation, you know vo*cos(theta). Now proceed.

Homework Helper
I think this is a very difficult question. You need to use two s-formulas:

s_x = ...
s_y = ...

For time t = 6, you get two equations in v_0 and theta.

alfredo24pr
Please post some calculations, because I have tried different ways and still have not found the correct answer.

Homework Helper
y=y0 + v0yt - 0.5gt^2

y= -34 + vo*cos(theta)*6 + 0.5 (9.8) (6^2)

Find cos(theta)

alfredo24pr
y=y0 + v0yt - 0.5gt^2

y= -34 + vo*cos(theta)*6 + 0.5 (9.8) (6^2)

Find cos(theta)

now i have to unknows..y and the angle. Now what?

Homework Helper
now i have to unknows..y and the angle. Now what?

When the rock reacxhes the ocean, displacement is zero. Hence y = o.

alfredo24pr
When the rock reacxhes the ocean, displacement is zero. Hence y = o.

and what is the angle? I am missing the vo and thetha.

Homework Helper
0 = -34 + vo*cos(theta)*6 + 0.5 (9.8) (6^2)
Find vo*cos(theta)
x = vo*sin(theta)*t.
Substitute the values of x and t. Find vo*sin(theta).
From these two values find vo and theta.

Homework Helper
I think rl.bhat has made it perfectly clear what to do. You've been helped, Alfredo.

alfredo24pr
my answer was correct, it was the lon capa that did not accept my units. Thanks anyway :)