Hi! Your answer would be "node"
Go and click to the invitation bar and you can find an option written as " search friends " . Then it's easy to find that unknown user if you're pretty fond with his/her username and DP ( display picture ).
You're going to use the constant acceleration motion equation for velocity and displacement:
(V)final²=(V)initial²+2a(Δx)
Given:
a=0.500m/s²
Δx=4.75m
(V)intial=0m
(V)final= UNKNOWN
(V)final= 2.179m/s