1. The problem statement, all variables and given/known data 2. Relevant equations F = ma 3. The attempt at a solution I'm completely unsure as to what the horizontal reaction force on m1 actually is. Any horizontal reaction force coming from m3 being constrained to be in between the two sides of m1 as shown will not cause m1 to move which is clear from the diagram. There is also no friction between m1 and m2 therefore I have no idea what reaction force is actually causing m1 to move. Thanks!