Kinematics of a Falling Object: Calculating Velocity

[deaninator]

Homework Statement

Sarah shoots a toy dart vertically downward at 10.4 m/s at Mark standing on the ground below. Calculate the dart's velocity after 1.3 seconds.

Homework Equations

V=Vo + at
X=1/2(Vo -V)T
X=volt + 1/2at2
V2=Vo2 + 2ax

(If a 2 is after a letter then it means squared)
X=displacement
A=Acceleration
T=Tome
Vo=Initial Velocity
V=Velocity

The Attempt at a Solution

Vf = (10.4 m/s) + (-9.81 m/s^2)(1.3 sec) = 23.15 m/s

That answer is wrong.

Thanks.

 

[PhanthomJay]

If you choose the downward y-axis as positive, then every variable acting in that direction...velocity, acceleration, and displacement... should also be positive.

If you choose the downward y-axis as negative, then every variable acting in that direction...velocity, acceleration, and displacement... should also be negative.

Just be consistent.