# Kinematics-acceleration in two dimensions

PEZenfuego

## Homework Statement

A tennis player standing 8.0 meters from the net hits a ball 1.5 meters above the ground toward her opponent. The ball leaves her racquet with a speed of 25.0m/s at an angle of 14.0 degrees above the horizontal. The net is 1.0 meters high. The baseline is 12 meters back from the net; a ball striking the ground beyond the baseline is out of play

## Homework Equations

x=V.costheta(t)
y=V.sintheta(t)-0.5g(t^2)

## The Attempt at a Solution

I attempted to solve for t using the quadratic equation in conjunction with the above formula for y. I ended up with two solutions, neither was extraneous from what I could tell. I plugged both in for x and ended up with two answers. One was slightly more than 8 (meaning that the ball landed in), the other was a little over 21 (meaning that the ball landed out). I am lost, which is it and how do I know?

Mentor
You need to include the initial height of the ball (where its launched) when you work with the vertical component of the position.

PEZenfuego
You need to include the initial height of the ball (where its launched) when you work with the vertical component of the position.

Right, and I did. I got the equation deltaY=V.sintheta(t)-0.5gt^2

Rearranged and solve the quadratic equation. I ended up with two answers. One was 0.89 and the other was 0.3437. How am I supposed to determine which is correct and which is not?

Mentor
Right, and I did. I got the equation deltaY=V.sintheta(t)-0.5gt^2

Rearranged and solve the quadratic equation. I ended up with two answers. One was 0.89 and the other was 0.3437. How am I supposed to determine which is correct and which is not?

I'll bet you've set your Δy to a positive value, right But doesn't it FALL from its initial position to hit the ground?

PEZenfuego
:facepalm: I appreciate the help. I remember running into this same problem in high school. Maybe after learning from the same mistake twice, I won't make it again. Fingers crossed.