Solving 2D Kinematics Problem: Distance OP in m

[shlh]

I have a problem with this question.
An arrow is shot horizontally towards point O, which is at a distance of 20.0 m. It hits point P (right below point O) 0.10 s later. What is the distance OP (in metres)?
What do i need to find? How would i know the time taken for the arrow to reach o?

 

[mikeph]

The distance to the target is a red herring; this is a question of kinematics in 1D: the vertical dimension.

You know the arrow has been in free fall for 0.10s, and when something is in free-fall it accelerates downwards at 9.8 m/s^2. Since the arrow was shot horizontally, it's initial downward velocity is zero, and you can use

x = 0.5*a*t^2 + v*t

to find your distance.