(a) The acceleration due to gravity acts downwards throughout the upward motion of the pebble, thus the magnitude of the force on the pebble is
0.5 N = F = mg = 0.05 kg x 10 ms-2
The force is in a downward direction.
(b) The magnitude of the force will be equal to 0.5 N during the downward motion, and the force will act downwards.
(c) The pebble’s velocity will have both horizontal and vertical components if it is thrown at a 45° angle to the horizontal direction. The vertical component of velocity will be 0 at its greatest point, while the horizontal component will remain constant throughout the pebble’s journey. The force applied will be unaffected by this component.