A particle slides up a line of greatest slope of a smooth plane inclined at an angle \(\alpha^\circ\) to the horizontal. The particle passes through the points \(A\) and \(B\) with speeds 2.5 m s\(^{-1}\) and 1.5 m s\(^{-1}\) respectively. The distance \(AB\) is 4 m (see diagram). Find

1. the deceleration of the particle,
2. the value of \(\alpha\).