I begin this problem by drawing a picture and, as I'm drawing, explaining, "This is a picture of Indiana Jones, standing in front of an open doorway. Directly in front of him, on the other side of the doorway is a blazing furnace. Behind him are two of the temple bouncers who have every intention of pushing him through the doorway and into the inferno."

I continue by explaining that Indie has no weapons with him, but he does (fortunately) have a stack of different grades of sandpaper with him. During the few seconds it takes the henchmen to reach him, he needs to calculate which sandpaper to affix to the bottoms of his shoes, so the henchmen will be unable to push him into the furnace.

The information available to him is shown below:

• Each henchman is capable of pushing with a force of 80 pounds.
• His own weight is 200 pounds.
• The coefficients of static friction between the floor and each grade of paper are given in the chart below.

Indie's reasoning is as follows: He wants to use the sandpaper with the lowest coefficient of friction that will work, thus saving the better sandpaper for another day. Using the information given, we draw a force diagram. The combined force of the henchmen is 160 pounds. We agree that Indie wants a bit of a safety margin, so he wants to have a high enough coefficient of friction to counteract a push of 165 pounds. Thus, we do the following:

Thus, he needs sandpaper which is rated for at least 0.825. The 120 grit sandpaper is what he wants.

At some point during the problem we may discuss the fact that even though they won't get him to slide, they will push him hard enough to knock him over, unless he gets down on his hands and knees, and affixes the sandpaper to the points where he is in contact with the floor.

I finish the problem off by saying, "Indie glues the sandpaper to the bottom of his shoes, and stands there confidently waiting for the henchmen to arrive and push him. It never crosses his mind that they might just pick him up and toss him in."