A particle is moving on the x-axis. At time t=0 the particle is at the point where x=5. The velocity of the particle at time t seconds is (6t-t^2)m/s. Find the acceleration of the particle when t=4. I found this to be -2, so i assumed it is 2 metres per second squared deceleration, ...however the book says it is 2ms^(-2) directed towards the origin, i.e. the particle travels in the opposite direction. I am confused, surely for it to change direction the particle has to come momentarily to rest, this does not happen until t=6?? Have i missed something??