Internal tension needs the right system choice
The mistake
Trying to find an internal force by writing ΣF for the whole system.
Why it feels right
Treating everything as one object usually simplifies problems.
How to catch it
You need a connecting force but chose a system where it's internal and cancels.
The first principle
Internal forces cancel for the combined system. To get tension or a contact force, isolate one body so that force becomes external.
worked mini-example
Blocks A (2 kg) and B (3 kg) in contact, pushed by 10 N on A. Find the contact force.
Common wrong move
Use ΣF on both together — but the contact force cancels.
Correct
a = 10/5 = 2 m/s²; isolate B: contact force = 3·2 = 6 N.
Test the repair NL-06 · transfer
include N
Make a fresh prediction before referring to the worked example.