# Conservation of energy of while rolling down a hill

This material is replicated on a number of sites as part of the SERC Pedagogic Service Project

#### Summary

When studying conservation of energy, we often use the example of an object sliding or rolling down a ramp. Here is a real-life example that we can use to see whether mechanical energy really is conserved when objects roll down hills. Students analyze video clips of children rolling down a driveway on roller blades and various bikes. Using the techniques described in the activity called Measuring velocity of objects using video clips, students can determine the velocity of the kids rolling at the bottom of the hill. The video clips also include the height of the hill. Students can use this information to determine the loss of mechanical energy, and look for a relationship between the type of vehicle used to roll down the hill and the amount of mechanical energy lost to friction.

## Learning Goals

Students will learn to apply the concept of conservation of energy to a real-life situation. As a result, the intention of this activity is that students will:

• Learn to extract data from video clips to find quantitative information.
• Use quantitative information they glean in combination with the concept of conservation of mechanical energy to determine quantitatively whether mechanical energy is conserved.
• Be able to form conclusions about what types of vehicles roll with the least loss of mechanical energy.

## Context for Use

This activity is intended for use in an introductory physics course, high school or college level. Specifically, it is intended for use while students study the conservation of mechanical energy. It is intended as an alternative to traditional textbook word-problems.

## Description and Teaching Materials

Video Clips

These are very short video clips that show a boy rolling past a ladder at the bottom of the hill. When stepping through the video one frame at a time (using Quicktime player, for example), students can count the number of frames it takes for the boy to roll past the ladder to determine the velocity. The instructions below show how this can be used to explore whether mechanical energy is conserved as the boy rolls down the hill, and how much mechanical energy is lost to friction.

Boy rolling down a hill on rollerblades. Use the Quicktime version of this video for the actual activity.
This short clip shows a boy rolling past a ladder at the bottom of a hill. When viewed in Quicktime, students can advance the video frame-by-frame to determine the boy's speed.

Each of the three Quicktime video files below shows a boy rolling past a ladder positioned at the bottom of the hill. Each video tells the height of the hill, and the length of an object moving with the boy. With this information, students can determine the velocity of the boy as he rolls past the bottom of the hill.

Note that the videos to the right are not for use in this activity since most Flash players do not allow viewers to step through the video frame by frame. Use the three Quicktime video files linked here.

Boy rolling down a hill on a bicycle. Use the Quicktime version of this video for the actual activity.

Student Handout and Instructions

student/teacher instructions for conservation of energy of a bike rolling down a driveway (Microsoft Word 683kB Sep18 10) These instructions describe how students can use the data gleaned from the video to find the percent of mechanical energy that is lost to friction. The instructions are very detailed and include sample questions and sample results. Teachers may choose to modify these instructions so that they include less detail, making this activity more inquiry-based.

## Teaching Notes and Tips

This is part of a three activity series in which students learn to analyze video clips and use the data as the basis for quantitative work. It is beneficial for students to have already completed the activity Measuring velocity of objects using video clips.

Students need computers equipped with a video player, such as Quicktime player that allows them to view the video frame-by-frame, so that they can determine the speed of the boy at the bottom of the hill. The data collection can be an in-class activity, with each student doing their own work on their own computer, or, a teacher led activity with the videos projected on a screen in front of the class. Once students have determined the number of frames for the boy to pass one of the ladder posts, the rest of the activity can be done without computers.

See a complete description of how to teach with video clips, including pedagogic strategies and a collection of sample videos.

## Assessment

Teachers can use several methods to assess student understanding after this activity. Since this activity is intended to increase students' understanding of how to apply the concept of conservation of mechanical energy, teachers can use a traditional assessment such as, questions from the Conservation of Energy chapter of any text book. Alternatively, an activity similar to this one can be used as an assessment, such as Conservation of energy of a roller coaster.