Internal tension needs the right system choice
- 01
The tempting model
Trying to find an internal force by writing ΣF for the whole system.
- 02
Why it feels right
Treating everything as one object usually simplifies problems.
- 03
Where it breaks · how to catch it
You need a connecting force but chose a system where it's internal and cancels.
- 04
The correct model
Internal forces cancel for the combined system. To get tension or a contact force, isolate one body so that force becomes external.
- 05
Repair · see it, then test it
worked example Blocks A (2 kg) and B (3 kg) in contact, pushed by 10 N on A. Find the contact force.
Tempting move
Use ΣF on both together — but the contact force cancels.
Correct move
a = 10/5 = 2 m/s²; isolate B: contact force = 3·2 = 6 N.
Test the repair NL-06 · transferinclude NMake a fresh prediction before referring to the worked example.