To find the acceleration of the particle, we use the equation of motion:

\(v = u + at\)

where \(v = 4.5 \text{ m s}^{-1}\), \(u = 1.5 \text{ m s}^{-1}\), and \(t = 1.2 \text{ s}\).

Substituting the values, we have:

\(4.5 = 1.5 + 1.2a\)

Solving for \(a\), we get:

\(a = \frac{4.5 - 1.5}{1.2} = 2.5 \text{ m s}^{-2}\)

To find the angle \(\alpha\), we use the equation:

\(mg \sin \alpha = ma\)

\(g \sin \alpha = a\)

\(\sin \alpha = \frac{a}{g} = \frac{2.5}{9.8}\)

\(\alpha = \sin^{-1}\left(\frac{2.5}{9.8}\right) \approx 14.5^\circ\)