# Ball Velocity Question

1. Sep 23, 2007

### I am a Newbie

1. The problem statement, all variables and given/known data
A ball is thrown directly downward, with an initial speed of 7.75 m/s, from a height of 30.8 m. After what time interval does the ball strike the ground?

3. The attempt at a solution
The way I understand it is that since it is freefalling, it is moving at 9.8m/s towards that ground, thereby I neglected the 7.75m and did 30.8/9.8 = 3.14s..however it is wrong and the similar problem book is even numbered therefore cannot work through it backwards.. Any help?

2. Sep 24, 2007

### Hootenanny

Staff Emeritus
I'm afraid that is incorrect; the ball is accelerating at 9.81m/s2 which makes a big difference. I'm assuming that your doing a mechanics course, so which kinematic equations do you know?

3. Sep 24, 2007

### I am a Newbie

Actually just in a standard Calc based physics, thereby I have no equations

4. Sep 24, 2007

### stewartcs

Hint: The ball is moving at a constant acceleration.

You should have plenty of constant acceleration equations in a Calculus based Physics course to relate velocity, displacement, acceleration, and time.

If not, check out these...

http://hyperphysics.phy-astr.gsu.edu/hbase/mot.html

CS