Four objects are situated along the y axis as follows: a 2.00kg object is at 3.00m, a 3.00kg object is at 2.50m, a 2.50kg object is at the origin and a 4.00kg object is at -0. 500 m. Where is the center of mass of these objects?

Aparticle's position is given by x = 3.00 - 9.00t + 3t2, in which x is in meters and t is in seconds. (a) what is its velocity at t = 1 s? (b) is it moving in the positive or negative direction of x just then? (c) what is its speed just then? (d) is the speed increasing or decreasing just then? (try answering the next two questions without further calculation.) (e) is there ever an instant when the velocity is zero? if so, give the time t; if not, answer "0". (f) is there a time after t = 3 s when the particle is moving in the negative direction of x? if so, give the time t; if not, answer "0".