Three blocks with masses $m, 3m$ and $5m$ are connected by strings. After an upward force $F$ is app

Question

Three blocks with masses $m, 3m$ and $5m$ are connected by strings. After an upward force $F$ is applied on the block $m$, the masses move upward at a constant speed $v$. What is the net force on the block of mass $3m$?

Professor PC Thomas Classes · Answer

Since all blocks are moving with a constant speed $v$, their acceleration is zero. According to Newton's second law, net force $F_{net} = ma = m(0) = 0$. So the net force on the $3m$ block is zero.