physics
posted by squires .
A particle has ⃗r(0) = (4 m)j and ⃗v(0) = (2 m/s)i.
If its acceleration is constant and given by ⃗a = −(2 m/s2) (i +j), at what time t does the particle first cross the x axis?
Answer in units of s.
part 2. At what time t is the particle moving parallel to the y axis; that is, in the j direction?
Answer in units of s.

Let u = x direction velocity (i) and v = y direction (j) velocity
Xo = 0
Yo = 4
Uo = +2
Vo = 0
Ax = 2
Ay = 2
u = Uo + Ax t
u = 2  2 t
x = Xo + Uo t +(1/2) Ax t^2
x = 2 t  t^2
y = Yo + Vo t + (1/2) Ay t^2
y = 4  t^2
so
y = 0 when t = 2
u = 0 when t = 1 
Thank you very much.. i understand it...it crosses the xaxis when the y point is =o, and is parallel to the y axis when the vector in the x direction aka the slope is = to 0 =]