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Direct Measurement Video of Einstein riding the Graviton

Peter Bohacek and Matthew Vonk
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Summary

This Direct-Measurement Video gives students an opportunity to apply Newtonian mechanics to a model of an amusement park ride. An Einstein "action figure" (doll) is pinned against a vertical wall on a rotating platform. As the platform slows its rotation, Einstein slips down the vertical surface. Students can make measurements and calculations to determine the minimum speed that will keep Einstein from sliding, and calculate the coefficient of static friction between Einstein and the wall. In addition, students can develop an experiment that will let them determine the coefficient of sliding friction as Einstein slides down the vertical surface.

Learning Goals

Students will:

Context for Use

This activity is intended for introductory physics students who have had some experience with Direct-Measurement Videos and with using Newton's laws to analyze circular motion. For an introduction to using Direct-Measurement Videos for circular motion, see Measuring the Velocity of a Roller Coaster activity.

This activity is also intended for students to learn experimental design. This can be assigned as a homework problem, or as an in-class activity for a whiteboard problem solving session.

Description and Teaching Materials

There are two parts to this activity. First, students use the video to answer the question, "What is the coefficient of static friction between Einstein and the vertical wall?" The worksheet shown here is a highly scaffold-ed set of instructions. Ideally, students will need far less instruction. For the second part, ask students to develop a method to determine the coefficient of kinetic friction. Specifically, ask students to say what calculations they would do, and what measured values they wold need from the video. For example, if they say that they would need to know the mass of the Einstein action figure, they should show how that would be included in their calculations for the coefficient of friction. One value the students' methods may require is the vertical distance Einstein slides down the vertical surface. Based on the video, students can use the value 5.8 cm. More detailed instructions are included here:

Einstein on the Graviton video page contains all the available file types for this video.

Teaching Notes and Tips

Often students prefer when instructors provide explicit step-by-step instructions. But one of the intended advantages of Direct-Measurement Video is to encourage students to drive the investigation themselves. If students are stuck, make a small suggestion, preferably in the form of a question, such as, "What is providing the force causing the centripetal acceleration?", or "Did you use a force diagram to show the vertical forces?"

Assessment

Here are two routes for assessment. Students could be asked to solve a similar problem published by the College that was used on an AP Physics test in 1984. Although this question is not currently published on the College Board website, it is widely available by searching "1984 AP Physics Mech C".

References and Resources

The Physics Classroom Tutorial: Circles
Pretty Good Physics Circular Motion