Discussion on: Problem 001-mj | Method of Joints

This is because the Summation of Forces along Y-axis is equal to

+Rd-50kn+Ra = 0 (assuming Reaction to the Hinge support is positive.)
Rd = 53.33 (Substituting Rd to the equation.)

+53.33 - 50 + Ra = 0
Ra = -3.33 (Negative means you have the opposite assumption to the reaction.)

this is why the hinge support has a downward force instead of upward.

Because weight goes down.