The ComPADRE Collections

Introduction to Work and Energy: The Hopper Popper Surprise

Steve Shropshire, Idaho State University
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This material is replicated on a number of sites as part of the SERC Pedagogic Service Project


This activity is designed to provide qualitative understanding of the Work-Energy Theorem. Students are expected to have read introductory material regarding the theorem, and are tested on this with a short online quiz prior to class. After a brief discussion a "warm-up" demonstration is conducted with student participation. A question is then posed regarding the height a "Hopper Popper" will reach if launched from a thumb instead of a hard flat surface. After initial responses are presented, discussion groups are formed to achieve consensus and provide justification of conclusions. This is followed by a confirming demonstration with surprising results.

Learning Goals

Students should gain conceptual understanding of the Work-Energy Theorem, especially on how the energy transfered depends on the distance over which the force is applied.

Context for Use

Educational level: Introductory Physics, all levels.
Setting: High school or college, all class sizes. This in an Interactive Lecture Demonstration classroom activity.
Time required: 15 to 20 minutes.
Special equipment: Marshmallow blow guns, overhead or lcd projector, audience response system, Hopper Poppers.
Pre-requisite knowledge: Forces, Newton's Laws, work, kinetic and potential energy, and introductory reading on the Work-Energy Theorem.

Description and Teaching Materials

Prior to instruction students are expected to read introductory material in their text on the Work-Energy Theorem. After this reading and at least one hour prior to class, students should be encouraged to take an online quiz on the theorem and simple applications based on the reading assignment. The lesson should start with a short introduction to the Work-Energy Theorem. The level and depth of this introduction should be determined by the results of the online reading quiz and the level of the class.

If students do not demonstrate understanding on the reading quiz that positive work will increase the kinetic energy of an object, additional time must be spent on the definition of work. Examples drawn from earlier discussions on Newton's Second Law, recast in terms of work and energy will help. Specifically, emphasize that positive work results in positive acceleration and an increase in speed, and thus kinetic energy, and that negative work results in negative acceleration (deceleration) and a decrease in speed and kinetic energy.

If students do not demonstrate understanding that an increase in the distance over which a force is applied will increase the change in kinetic energy or speed of an object, the Marshmallow Blow Gun demonstration and/or additional Newton's Second Law examples recast in terms of work and energy will be helpful.

If students show significant grasp of the Work-Energy Theorm, go straight to the Hopper Popper Surprise interactive demonstration. If there is some confusion, the Hill and Haystack concept test question and discussion prior to the demo may help. For either, pose the question as a peer instruction activity, preferably with an audience response system. Have each student answer individually, then divide the class into discussion groups to arrive at a consensus. Have each group present their answer and reasoning. Follow this with a second polling of the question and discussion prior to actually doing the demonstration.

Teaching Notes and Tips

For the Marshmallow Blow Gun demo, an additional pipe of the same length as the longest one already used but bent into one or more circular loops is useful to link to previous discussions on friction and centripital force. The centripital force on the marshmallow in the curved tube increases the contact force and thus the friction, so it will not shoot as far..

After the Hopper Popper Surprise demo, a launch of the popper from hard surfaces both concave and convex will help students understand the behavior of the popper, as will attempting to launch it from your fist with a quarter-sized opening below the center of the popper.

A follow-up lab using fan carts or may be useful.


A more numerical version of the Hill and Haystack concept test question works well as an exam question.

References and Resources

More work and energy demonstrations can be found by searching for "work and energy" in the PIRA Demonstration Bibliography.