I've worked through this problem a few different ways now and each one gives me different answers and I am looking for some guidance as to which is correct: Four blocks of mass m1=1kg, m2=2kg, m3=3kg (very original right?), and unknown mass m4 are on a frictionless surface (pic attached). Force Fl pulls in the left direction at 30N and Fr to the right at 50N. The tension between m2 and m3 is 36N. What is m4? As I worked through the problem I labeled all tensions as shown in the picture. I know T1=30N and T5=50N. My biggest problem arises from whether T3=36N or if 2*T3=36N. The first time I did this problem I used 2*T3-36N or T3=18N. Then using Newton's 2nd law I set up the equations T2-T1=m1*a, T3-T2=m2*a, and so forth. Using this method I got that a=4m/s2 to the right and m4=5kg. The second time I did this I used T3=36N and using the same equation setup as before I got a=2m/s2 to the left which seemed plausible, however using T5-T4=m4*a I ended up with m4=-10kg which seems not so plausible. Any suggestions?