Newton's problems... well not his problems but his problems to the world- ehhhh you know what i mean >:(

First of all, The 4 problems are Equilibrium, Inclines, Pulleys and Trains(not the band, actual trains :))

- They all need assumptions
For Equilibrium- No Friction and a=0
For Inclines- positive direction of motion, a=0, no resistance
For Pulleys- No friction, no resistance, 2 FBDs
For Trains- 3 FBDs, No resistance, a=0 and constant, horizontal and positive motion

Example from my notes!-(sorry about it being sideways, its the only way to take the picture :( )
Equilibrium-

 Inclines-

 

Pulleys-

 Lastly, Trains-