1. The problem statement, all variables and given/known data The acceleration of a particle moving only on a horizontal xy plane is given by a = 3ti + 4tj, where a is in meters per second squared and t is in seconds. At t = 0, the position vector r = (20.0 m)i + (40.0 m)j locates the particle, which then has the velocity vector v = (5.00m/s)i + (2.00m/s)j. At t = 4.00s, what are (a) its position vector in unit-vector notation and (b) the angle between its direction of travel and the positive direction of the x axis? 2. Relevant equations x = x0 + vox * t + 1/2 * ax * t2 y = y0 + voy * t + 1/2 * ay * t2 3. The attempt at a solution So, since ax = 3t and ay = 4t, I got the following values for the position vector: (136m)i + (176m)j The problem is that the values aren't the solution given by the book (which is (72m)i + (90.7m)j ). I keep reading but nothing comes to my mind besides the fact that probably I did something wrong with the acceleration.