Trajectory of a tennis ball 2?

Question

Follow up to a previous question.
http://answers.yahoo.com/question/index;_ylt=ApfILz5IggBhOltT4xreL7Dsy6IX?qid=20070817115919AAUZq7c

A tennis player serves the ball such that
1) the initial speed of the ball is 50 m/s
2) the ball is angled downward at an angle of depression θ from the horizontal
3) the ball is initially served from a height of 2 m above the ground

The net is 1 m high, and is 12 m away from the server. The ball must clear the net and land within 6 m from the net, ie the ball must hit the ground 12-18 m from the server.

It is NOT possible to do this without topspin. How much force of topspin is required to make the ball clear the net (x=12) and land at x=18? And what value of θ is required?

For simplicity consider topspin to result in a constant downward force.

Ignore air resistance, side spin, serving out wide etc. Only use the values given here.
g = 9.81 m/s^2

Answer 1

Since topspin is assumed to be a constant downward force this problem is equivalent to solving for the value of g for which the ball clears the net in the stated way. Use the general formula for parabolic trajectory in a gravity field and solve for the value of g that works. Then subtract our familiar value for g, 9.81 m/s^2, and that will give you the value of topspin required expressed in m/s^2 downward.

Answer 2

Write the equation of the trajectory versus initial speed and θ in a coordinate system say with origin at initial position of the ball. Obtain θ so that the path passes point (a,-2) where 12-1 at x=12 which for the range of a should give the acceptable range of theta

Answer 3

So I'm guessing this is not your typical projectile motion problem since there's this new stuff introduced called "topspin"