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A particle moves in 2D with a vector velocity which at time t is ' v = (-0.4ti + 5j) m/s '.

At time t=0 the particle is at (3,2)

d) Find the time at which it crosses the y-axis

e) Show that the acceleration vector is perpendicular to the position vector at time t= square root of 15 seconds

I have calculated the acceleration vector to be '-0.4i', the position vector is 'r = (3-0.2t^2)i + (5t + 2)j', and the speed at time t = 10 seconds is 3 m/s. But, now I have no idea what to do for the next 2 parts, they're only worth 4 marks so it can't be too difficult I'm just not sure what to do.

Please could someone give me an idea of what to do here as it is the last part of my work and would be greatly appreciated! Any help would be amazing!