Choosing objects to be examples
Teaching Guidance for 11-14
Keep it simple
When selecting things from the lived-in world to be about objects on which forces might act it's a good idea to select simple objects. In particular asking for
Teacher: … the forces on the human…
Teacher: … the forces on the bicycle…
requires quite sophisticated (and perhaps even implicit) modelling of the object before you can answer the question.
We think it's best to avoid things with internal structure that's essential to the forces that they exert.
It should be obviously easy to reduce the selected things to a small blob (technically a point mass – but that can come later (more in the SPT: Force and motion topic) without affecting its motion in any way. For cars being propelled by the action of their wheels on the ground, bicycles propelled by human riders and running people this is not the case.