Question

two blocks on a level frictionless surface have mass 3 kg and 4 kg. a string pulls the 3 kg block to the left with tension n, a second string pulls the 4 kg block to the right with tension n. the two blocks are connected together by a third string in the middle as shown. 1)what is the net force on the 4 kg mass?

Answer 1

The tension on each end string = n

a = F / m = 0 given zero m = (3 + 4) kg N for no acceleration of the combination

The force on the middle string is also n because no acceleration is involved

For any one block to be accelerated there must be a net force on that block, in which case the force exerted by the string (in the middle) is not the same as the force exerted by the end strings

F = M a applies to any one block or combination of blocks

T the net tension on the middle string is the same on both ends because a net force on a mass of zero would produce infinite acceleration