Max speed of roller coaster at bottom of drop

[pb23me]

Homework Statement

The top of the roller coaster is 77.7m above the earth. From this height what speed can be reached when it reaches the bottom of the drop? This problem must be solved using work and energy equations not kinematic equations..

Homework Equations

Wg=-(mgy_f-mgy_i)
Wnet=(1/2mv²_f-1/2mv²_i)

The Attempt at a Solution

I set the two equations equal to each other-assuming no friction-and tryed to solve for v_f. However i can't see how to come up with that if i don't know the initial velocity?

 

[rock.freak667]

Normally roller coasters stop at the top of a steep fall, so I would assume that its initial velocity is 0 m/s. Otherwise you do not have enough information to get the initial velocity.

 

[The CdePster]

Ha thriller i remember these questions man, I'm like oh no how on Earth can i do this when i don't know the mass?! But setting E=05mv^2 and E=mgh equal to each other sound like a grand plan if you want to get rid of those cheeky masses! Cuz don't forget they should equal each other at the bottom of the slope in this imaginary magical world where the drop is perfectly vertical and there are no energy losses whatsoever. And yes rock.freak starting with u=0 is grand also. You've probs already solved it by now eh? Gosh darnit i wanted to sound smart. :( ;)