A thin wheel can stay upright on its rim for a considerable length of time when rolled with a considerable velocity, while it falls from its upright position at the slightest disturbance, when stationary. Explain.

Explanation

When the thin wheel is rolling upright, it possesses some angular momentum in the horizontal direction along the axis of the wheel. As angular momentum is conserved in magnitude as well as direction (in the absence of external torque) the wheel cannot fall from its upright position. However, when external torque due to friction reduces angular velocity of the wheel to zero, (i.e. when the wheel becomes stationary) it falls from its upright position at the slightest disturbance, on account of moment of force due to its weight.