Homework Help: What is the average velocity of the object as it falls to the ground?

1. Nov 21, 2009

1irishman

1. The problem statement, all variables and given/known data
An object is dropped from a height of 25m above the ground. What is the average velocity of the object as it falls to the ground?

2. Relevant equations
d=vit+1/2at^2

3. The attempt at a solution
d=-25
a=-9.8
vi=0

-25=-4.9t^2
t=2.3s
vavg=displacement/time
-25/2.3=-10.8m/s

2. Nov 21, 2009

kuruman

Use the 2.3 s time of flight to find the velocity just before the object hits the ground. Half of that (because the starting velocity is zero) is the average velocity.

3. Nov 21, 2009

1irishman

I still don't understand, how does that get plugged into the formula? What formula do I use?
Thank you

4. Nov 21, 2009

kuruman

Use the kinematic equation that gives you the velocity of an object at any time t.

5. Nov 21, 2009

1irishman

v=d/t right?

6. Nov 21, 2009

kuruman

Nope. That equation is valid only when the acceleration is zero. Here the acceleration is g down. What equation do you know that relates the initial and final velocity to acceleration and time?

7. Nov 21, 2009

1irishman

a=vf-vi/t right?

8. Nov 21, 2009

kuruman

That's the one. You know everything except vf.