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

[1irishman]

Homework Statement

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?

Homework Equations

d=vit+1/2at^2

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
the answer in book says a=-11.1m/s i don't understand, please help?

 

[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.

 

[1irishman]

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

 

[kuruman]

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

 

[1irishman]

v=d/t right?

 

[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?

 

[1irishman]

a=vf-vi/t right?

 

[kuruman]

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