Whether the pebble is moving upward or downward, the only force acting on it is due to gravitational acceleration which always acts towards down.
F = m × a
= 0.05 × 10 = 0.5 N
When the pebble is thrown at 45° angle there are two components of force, i.e. horizontal and vertical components. But in all these cases the net effect of horizontal component will be zero.