Frames, leftover motion, and net force

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. You are in a car braking hard. Groceries slide forward off the seat. In the road frame…

   AA forward force struck the groceriesBThe groceries kept their leftover forward velocity; the car slowed under themCThe road is non-inertialDMass became negative

2. Forces on a 2 kg box: 9 N up, 9 N down, 6 N right. The acceleration is…

   A12 m/s², adding every sizeB3 m/s² rightC0, because up and down cancel so nothing can happenD9 m/s² up