Question

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.

Answer 1

Tension just means force in the string. If you hang a 100 pound block from a rope, the rope has a "tension" of 100 pounds.

Use Newtons's second law, f = ma

Total mass of the two blocks = 6 + 9 = 15.0 kg

The two blocks together are accelerating due to a horizontal force of 30 N. How much are they accelerating?
Use f = ma.
30 = 15 x a
a = 2m/s^2

Now you can find tension (force) in the string pulling the 6.0 kg block:
f = ma
f = 2 x 6 = 12.0 N

Hope this helps