If two particles P and Q are connected as in the diagram below…
…the first thing to do is to draw all the forces acting on each particle.
The reactionis perpendicular to the surface.
The Force of gravity always acts down
The tension in the rope always acts towards the centre of the string.
The force of frictionacts against motion. We assume the particle on the slop above is moving up the slope.
It is good also to indicate the direction of acceleration, shown above with a couble headed arrow.