"I have a problem," I told my Physics students. "When I'm playing the guitar, my hands get a little slippery, and the pick will sometimes slide out of my hand." One of my students, who also plays the guitar, nodded in agreement. "So," I continued, "you all are going to help me select a pick based on which has the highest coefficient of friction with the table surface." Note: I don't make "which pick is best for me?" part of the official purpose statement of the lab; see the last paragraph of this article for an explanation.

I pulled out a stack of six guitar picks for each pair of lab partners. They were the same brand (Jim Dunlop) - I had picked up variety packs that contained several different styles and materials. "I'm not going to ask you to calculate the coefficients of static and kinetic friction between each of these picks and the table. But...I do want you to rank them in order from lowest to highest coefficients of friction."

This is a kind of lab I really enjoy doing with the students; I don't provide them with any supplies, and I don't supply them with any instruction. It becomes their job to, with the materials they have on hand, identify a method for answering the question I asked. They usually spend some time talking with their lab partner about how they're going to complete the task. I wander around the room listening to them, and asking them questions to help steer their thinking if they're stuck, or if they're heading in a direction I think is going to be unproductive.

There are often multiple solutions to the problem, which I'll outline below.

Static Friction

• Tip the table up at one end until a pick starts to slide. Measure the angle of inclination and compare it with other picks' slide-ngle.
• Put all the picks in a line and tip the table slowly until one after another they begin moving. Record the order in which they slid.

Kinetic Friction

• Create a tournament, pitting one pick against another to see which pick takes the longest to reach the end of the table/inclined plane.
• Use a timer to measure each pick's time to the end of the incline.
• Put all the picks in a line and hold them all in place with an object such as a ruler. Tip the table past the angle where all picks are expected to slide, and then lift the ruler to release the picks simultaneously.

Students should be encouraged to do multiple trials; however, students will then need to identify a way to compile these trials into a single report. Averaging each pick's ranks, and then ordering those averages from least to greatest is a good way to find out which pick is "best" when it comes to high friction.

In addition to working on a table surface, have the students do the same experiment using other surfaces, such as sandpaper. Having the students do this is important, because students will observe that just because a pick has the highest coefficient of friction with the table doesn't mean it will have the highest coefficient of friction with a sheet of sandpaper. Some students will be surprised by this outcome, This leads to a very important discussion about the fact that they can't make a confident conclusion about which pick is best for me, since they haven't determined how their coefficients of friction with my skin rank from lowest to highest.