When a wheel is ________, its rotation will hold it in place on an axle due to the air moving on its sides.

• A. Smooth
• B. Rough
• C. Curved
• D. Flat

Answer: (C)

The shape being tested is about how air flows around a rotating wheel. When a wheel has curved sides, air is guided smoothly along those curves as the wheel spins. That curvature helps create a thin layer of moving air between the wheel and its axle, acting like a tiny air bearing. This air film reduces direct contact and can provide a stabilizing cushion that helps keep the wheel centered on the axle as it rotates. If the wheel were smooth, rough, or flat, there wouldn’t be the same steady air film to form on the sides, so the stabilizing effect would be weaker or absent. So the curved shape best enables the air movement on the sides to hold the wheel in place on the axle.