Two blocks, joined by a string, have masses of 6.0 kg and 9.0 kg. They rest on a frictionless horizontal surface. A 2nd string, attached only to the 9 kg block, has horizontal force = 30 N applied to it. Both blocks accelerate. Find the tension in the string between the blocks.

Explanation:

30 N force is pulling total mass of 15 kg , so acceleration in the system of masses

= 30 / 15

= 2 m / s²

Let us now consider forces acting on 9 kg . 30 N is pulling it in forward direction . Tension T in the string attached to it is pulling it in reverse direction

so net force on it

30 - T

Applying Newton's law of motion on it

30 - T = mass x acceleration

30 - T = 9  x 2

30 - 18 = T

T = 12 N

f is prop to m1xm2 double m1 double force