# Homework Help: Tennis ball example

1. Jul 11, 2006

### dopey9

a tennis ball is hit from a height h above the ground with a speed V and at an angle alpha to the horizontal. it hits a wall at a horizontal distance d away . Air resistance is negligible

can any one find expressions for the height and velocity of the bal when it his the wall for me to compare?

thankz

2. Jul 11, 2006

### dt19

try resolving the velocity into horizontal and vertical components. then see what you can do with the suvat equations.

3. Jul 11, 2006

This is the case of simplified projectile motion, let me give you a crash course somewhat quickly.

If we assume gravity is the only acting force on the ball then the horizontal component of its velocity is a constant since the gravitational force is assumed to be entriely vertical. (downwards of course)

Thus Vx = Vo*cos(alpha)

Where Vo is the initial velocity and alpha is the launch angle measured TOWARDS the upward vertical FROM the horizontal in the direction of motion.

Vx is the x component of velocity

By applying some basic kinematics we find Vy = Voy*sin(alpha) - g*t

From the pythagorean theorem V = sqrt(Vx^2+Vy^2)

now the goal for you is to find the time at which the ball has traveled a distance d.

I'll give you a hint:

set D = Vo*cos(alpha)*t and solve for t.

Notice that the displacement kinematic equations given the assumptions are:

X = Vo*cos(alpha)*t
Y = Vo*sin(alpha)*t - 1/2*g*t^2

Assume you start at (x,y) = (0,0) and you will end at the point (D,h)