Being precise about measurable quantities
Teaching Guidance for 14-16
Wrong Track: The speed is 20. The acceleration is decreasing.
Right Lines: Charlie records Bob's velocity as +20 metre inverse second. The acceleration is negative, because the acceleration is reducing the changes in distance between Charlie and Bob in each second (the change in velocity is negative).
Taking care to make reports unambiguous
Thinking about the learning
The point of view to be taken is often not thought about, and the implicit decisions made in the initial description of the situation often impede deeper understanding.
This is particularly the case if such ambiguities are added to vague assertions about rates of change.
Thinking about the teaching
Choose an explicit recorder for the measurements (Alice, Bob or Charlie have served us well). Include this decision in the description of the motion. We'd also suggest taking the opportunity to spell out what's changing with time (e.g. velocity is rate of change of position).