Two blocks joined by a string have masses of 6 and 9 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. a. 18 N b. 28 N c. 12 N d. 15 N

12N

Explanation:

Suppose the string mass is negligible, the total mass of the 2 block system is 6 + 9 = 15 kg

So the acceleration of the system when subjected to 30N force is

a = F / M = 30 / 15 = 2 m/s2

So both blocks would have the same acceleration, however, the force acting on the 6kg block would have a magnitude of

f = am = 2 * 6 = 12N

This is the tension in the string between the blocks

double the distance = divide strength by 4