Calculating Time Interval for Ball to Strike Ground | Ball Velocity Question

[I am a Newbie]

Homework Statement

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?

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?

 

[Hootenanny]

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?

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?

 

[I am a Newbie]

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

 

[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