Two blocks of unequal mass are connected by a string and placed on a smooth horizontal surface. A force F is applied on the heavier block. The tension in the string is:

When two blocks of unequal mass are connected by a string and placed on a smooth horizontal surface (i.e., no friction), and a force F is applied to the heavier block , both blocks accelerate together as a system. Let: • M1 be the heavier block (force is applied here), • M2 be the lighter block, • F is the total force applied on M1, • a is the common acceleration, • T is the tension in the string. Tension in the string pulls the lighter block forward. So, for lighter block: Thus, the tension is only a part of F , used to accelerate the lighter block. It is less than F because the entire force F is used to accelerate both blocks, not just one.