# Homework Help: Projectile motion tennis ball problem

1. Sep 30, 2007

### mxer101

1. The problem statement, all variables and given/known data
Tennis Ball hit at a vertical speed of 10 m/s & Horizontal speed of 30 m/s. How long will the ball remain in the air? How far will the ball travel horizontally?

2. Relevant equations
I am not sure which equation to use? The angle that the ball was hit is not given, would I try all angles? 30,37,45,57,60? I know that 45 would give the max distance

3. The attempt at a solution
How far: 60m
How long: 2s

Thanks!

2. Sep 30, 2007

### learningphysics

To find how long the ball is in the air, use the equation:

d = v1*t + (1/2)at^2. apply this equation for vertical displacement...

3. Sep 30, 2007

### Vijay Bhatnagar

As vertical and horizontal components of velocities are given, angle is not required. At the highest point the velocity will be zero (v = 0). Initial vertical velocity = u = 10 m/s. Acceleration a = - 9.8 m/s^2 (-ve because acceleration due to gravity is always downwards i.e. opposite of the +ve upwards displacement) Apply v = u + at to get t, the time to reach the highest point. Double that to get the time during which the ball is in air (T).

As the horizontal velocity remains constant throughout, determine horizontal displacement in time T (calculated above) using s = ut