How will your roly poly move?
3
Axles and shafts
If a wheel is connected to a rod so that the wheel can spin freely while the rod is fixed, the correct term for the rod is "axle".
If a wheel is connected to a rod so that the wheel is fixed and can only turn if the rod turns, the correct term for the rod is "shaft".
In the roly poly, the body of the roly poly is really a shaft.
It is easy to get these terms confused and to some extent a highly pedantic approach is not necessary. What is important is that pupils understand that there is a design decision to be made when fixing a wheel to a rod. Is it best for the wheel to spin freely on a fixed rod or is it better for the rod and wheel to turn together. Different constructions are needed to meet these different requirements as shown in the drawings below.
