Missing Mass: What is the Mass of Block 2 in a System of Three Blocks?

[Sherrylee]

Homework Statement

Three blocks are pulled along a frictionless surface. The masses of two of the blocks and the tensions in two of the cords are given. Find the mass of the third block (Block 2).

Block 1 ------- Block 2------Block 3 ------>
Block 1: m1 = 12 kg
Block 2: m2 = UNKNOWN
Block 3: m3 = 35 kg
Tension between Block 2 and Block 3: 111 N

Homework Equations

Newton's 2nd Law: F = ma

The Attempt at a Solution

111N = (m1+m2)a = m3a <-- is this true?

Block 2, m2 = 35-12? This seems to easy to be true.

 

[Stovebolt]

That does not seem correct. You stated that the tensions in two cords are known, but only give the tension between Block 2 and 3. What is the other known tension?

Once you have that, draw free body diagrams, it might help to clear things up.

 

[Redbelly98]

You said two cord tensions are given, but seem to have left one of them out here?

111N = (m1+m2)a = m3a <-- is this true?

111N = (m1+m2)a is correct.
But it is not equal to m3a ... there are two forces contributing to m3's motion: the 111N and ____ ?