Aladin

Thank you of all very very much.I am very grateful to you.

I understand it thank you but I want more explanation of sign convension in coordinate system.

Problem : Two blocks of masses 10 kg and 20 kg are connected by a string that passes over a pulley as shown in the figure. Neglecting friction between surfaces, find acceleration of the blocks and tension in the string (consider g=10m/s2 ). Solution : The blocks are connected by a taut string. Hence, their accelerations are same. Let us assume directions of accelerations as shown in the figure. Also, let the magnitude of accelerations be “a”. According to Newton's Law.F=ma ∑Fy=T−10g=10ay The problem is that. in above equation why we took T as positive and mg as negative? On the other hand in ∑Fy=20g−T=20ay i.e for "20Kg" we took T negative and mg positive. I will be thankful to you if you solve my problem of signs.