Solving Projectile Motion Equations for Initial Velocity and Angle

[jumbo123]

Homework Statement

A ball is thrown horizontally with speed u from a height of 20m. It hits the ground with a speed 3u. Find the value of u, and the angle at which the ball strikes the ground.

Homework Equations

The equations of motion.

The Attempt at a Solution

I know that I must deal separately with the horizontal and vertical components of the motion.
I have worked out that the final vertical velocity will be 19.8m/s but I don't where to go next to find u.

 

[gneill]

Write an expression for the speed at which it hits the ground. Set it equal to the required condition.

 

[jumbo123]

I have realized that I now have a triangle where the x component is u, y component is 19.8 and the hypotenuse is 3u. So I could easily work out u and then the angle by using simple trig.

Is this correct?

 

[gneill]

Looks good! Perhaps consider using Pythagoras.