Interactions, FBDs, and systems

Before this sitting

Can you already do this?

Two quick checks on “you will be able to…”. Right on both and this sitting becomes mixed practice, not a lecture. Wrong is useful — we keep the full sitting.

1. A 2 kg and a 6 kg glider push off via a compressed spring on an air track. While the spring force on the 2 kg is 12 N left, the force on the 6 kg is…

   A4 N right, so a is equalB12 N rightC12 N left, so they cancelD0, springs don't obey the third law

2. A 50 kg crate is at rest on a scale in a stationary elevator. g = 9.8. The scale reading is 490 N. This number is…

   AThe crate's massBThe normal force, here equal to the weight mgCA third-law partner of mg sitting on the same FBD and canceling it so a can be zero for a different reasonDTension