Finding velocity and average velocity for objects thrown from a bridge

[Faiza]

***free Falling Objects ***

*FREE FALLING OBJECTS*

Q. A boy stands at the edge of a bridge 20.0 m above a river
and throws a stone straight down with a speed of 12.0 m/s.
He throws another pebble straight upward with the same
speed so that it misses the edge of the bridge on the way
back down and falls into the river. For each stone find (a)
the velocity as it reaches the water and (b) the average
velocity while it is in flight.

Note: Ignore the affects of air resistance.
there's SO MANY FORMULAS FOR VELOCITY WHICH ONE AM I SUPPOSE TO USE? PLEASE HELP

 

[Dark_Adonis]

*FREE FALLING OBJECTS*

Q. A boy stands at the edge of a bridge 20.0 m above a river
and throws a stone straight down with a speed of 12.0 m/s.
He throws another pebble straight upward with the same
speed so that it misses the edge of the bridge on the way
back down and falls into the river. For each stone find (a)
the velocity as it reaches the water and (b) the average
velocity while it is in flight.

Note: Ignore the affects of air resistance.
THERES SO MANY FORMULAS FOR VELOCITY WHICH ONE AM I SUPPOSE TO USE? PLEASE HELP

Alright what quantities do we have?
a, x, v_i... What are we solving for? v_f for part (a)
t for part (b)
Part (a)
ok, what relates a, x, v_i and v_f?
or perhaps I should say v_i^2 and v_f^2?... Hope that's not too obvious

ok part (b)
What relates a, t, x, and v_i?
I think you know the answer to this... hint it involves t^2 and t
Also what is the average velocity?
Wouldn't that be change in position over change in time?

 

[Faiza]

Okay so for part a, V_xf^2 = V_xi^2 + 2a_x (X_f-X_i) is the kinematic equation that gives the velocity as a function of position, so a=9.80m/s^2 and Vi=12.0 m/s and x= 20 m for both pebbles?