1. Find the force exerted by the corner on the wheel in Example 12-4, just as the wheel lifts off the surface.

2. As a farewell prank on their alma mater, Sharika and Chico decide to liberate thousands of marbles in the hallway during final exams. They place a 2-m × 1- m × 1-m box on a hinged board, as in Figure, and fill it with marbles. When the building is perfectly silent, they slowly lift one end of the plank, increasing θ, the angle of the incline. If the coefficient of static friction is large enough to prevent the box from slipping, at what angle will the box tip? (Assume that the marbles stay in the box until it tips over.)

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1m 1m 2 m