Forces and Newton's first law

The idea

Newton's first law: if the resultant force on a particle is zero, it stays at rest or moves with constant velocity. Draw every force: weight $mg$ down, normal reaction $R$ from a surface, tension, thrust, friction and driving forces.

1. Draw the particle and every force acting on it.
2. Resolve in the directions that matter, usually up/down and left/right.
3. Zero acceleration means the forces in each direction balance.

Watch out Constant velocity is still equilibrium: the resultant force is zero.