Motion along a Straight Line problem

[ledhead86]

Motion along a Straight Line problem !PLEASE HELP!

An alert hiker sees a boulder fall from the top of a distant cliff and notes that it takes 1.30 s for the boulder to fall the last third of the way to the ground. You may ignore air resistance.

I have calculated the last 1/3 of the distance to be 8.281 m by the formula y=(1/2)(a)(t^2) where a=gravity=9.8 meters per second.
However to find the total distance of the cliff, I can't just multiply the distance of the last third times 3 because the boulder is accelerating throughout the entire distance, so how do I find the totatl distance?

 

[whozum]

For some reaosn I'm stumped right now, but I can tell you that

I have calculated the last 1/3 of the distance to be 8.281 m by the formula y=(1/2)(a)(t^2) where a=gravity=9.8 meters per second.

doesn't work because

because the boulder is accelerating throughout the entire distance

applies to this part of the problem too.

 

[Fermat]

You only have distance moved and time to work with. And gravity of course.

Let u be the velocity at beginning of last third
Let v be the velocity when stricking the ground.

Calculate u in terms of distance fallen
Calculate v in terms of distance fallen

The distance moved has been used. Can you now relate u and v in terms of t ?