The can could slide off the dash, tip over, or stay put. Much depends on how the can is supported at the boundry. In this case normal force is a fraction of the weight of the can. Figure 4.2.3 illustrates analysis of the sliding part of the problem. Let us suppose that the dash has not been cleaned in a while and the coeffient of friction is 0.20.
Friction is what holds the can in place.
So, in this example, the can will move. There are many more types of boundry conditions than just a can sitting on a dashboard. How a boundry performs depends on its configuration. Figure 4.2.4 illustrates several types of boundries that commonly occur. Notice how the type of reactionary force developed depends on the type of boundry.