Let us assume that the skier is falling in the - y direction

The Lift force \(\displaystyle{F}_{{{l}{\quad\text{if}\quad}{t}}}={1800}{N}\)

There are two forces acting on him, 1) the Lift forceacting upwards

2) his wight acting downwards

The Net Force \(\displaystyle\sum{F}_{{{y}}}={m}{g}-{F}_{{{l}{\quad\text{if}\quad}{t}}}\)

According to Newton's second law, \(\displaystyle\sum{F}_{{{y}}}={m}{a}_{{{y}}}\)

Or \(\displaystyle{a}_{{{y}}}=\frac{{{m}{g}-{F}_{{{l}{\quad\text{if}\quad}{t}}}}}{{m}}\)

\(\displaystyle=\frac{{{m}{g}-{1800}}}{{m}}\)

Or \(\displaystyle{a}_{{{y}}}={g}-\frac{{1800}}{{m}}\)

b) If the vertical lift is reduced to 1200N, \(\displaystyle{a}_{{{y}}}={g}-\frac{{1200}}{{m}}\)