Falling Objects Problem for Calculus

[Soccer07]

Homework Statement

The multiflash photograph in Figure A shows two balls falling from rest. The vertical rulers are marked in centimeters. Use the equation s=490t^2 (the free-fall equation for s in centimeters and t in seconds) to answer the following questions:
a) How long did it take the balls to fall the first 160 cm? What was their average velocity for the period?

Homework Equations

The Attempt at a Solution

160cm=490t^2
square root of (160/490)
My question is how do you find the average velocity? Do you take the derivative of the first equation which would be 490t and plug 160 cm in again?

 

[Strants]

Firstly, the derivative of $$s(t) = 490t^{2}$$ will not be 490t.

Secondly, if you want to find the average of a linear function over an interval, wouldn't it be best to know the endpoints of the function on that interval? Remember that velocity is a function of time, not distance.

EDIT: Actually, I take that back. You really don't even need to find the derivative. You can just use the equation distance = rate * time, seeing as you just solved for time.