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Contructing a projectile launcher and free falling target

Peter Bohacek, Henry Sibley High School
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This material is replicated on a number of sites as part of the SERC Pedagogic Service Project

Summary

This demonstration is an implementation of a classic physics demonstration usually called "the monkey and the hunter." The information posted here is a description of how we constructed a projectile launcher and a target that begins to fall at the same time the projectile is launched. The completed project is a spectacular demonstration of the concepts of projectile motion.

This demonstration can be used as part of an interactive lecture.

Learning Goals

Although there are many ways to introduce the concepts of projectile motion, few capture student attention as well as this classic experiment. The construction of this project is within the grasp of any motivated, reasonably experienced teacher. Once constructed, is will perform reliably time after time. Also, this design can be adapted and modified. For example, the triggering circuit can be use on a simpler launcher, such as a blow-dart gun.

The completed project can be used to teach the concepts of two-dimensional motion as part of a lecture demonstration. Students can be asked to predict the outcome of various experiments. Students can also collect data that can be used to calculate the launch velocity of the projectile.

Context for Use

This demonstration is intended for use in an introductory physics course, college or high school.

Description and Teaching Materials

Materials included in this activity are:
  • A description of the construction of this demonstration: Description of Construction of Pneumatic Cannon and Free Falling Target (Microsoft Word 4MB Aug15 10)
  • A sample worksheet that can be used as an interactive lesson about projectile motion: Worksheet for interactive lesson using projectile launcher with free falling target ( 35kB Aug15 10)
  • A video showing the demonstration in action

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Teaching Notes and Tips

An instructor can do several demonstrations with this apparatus:
  • Aim the cannon horizontally so that the projectile hits a wall. Students can measure the vertical distance the tennis ball drops while airborne. Vary the launch velocity by increasing the pressure in the reservoir, and observe the how launch velocity affect the vertical distance the ball drops during its flight. Students also measure the horizontal distance to the target. Using these two measurements, students can use the equations of motion for objects moving with constant acceleration to find the launch velocity.
  • Aim the cannon horizontally at the free falling target. Students can discuss how the projectile should be aimed so it will hit the free falling target.
  • Aim the cannon at an upward angle at a target suspended from the ceiling. Students can discuss and predict how this situation differs from the precious one.
In each case, these demonstrations can be done qualitatively (asking students to predict or explain the outcome of various situations), or quantitative (students collect measurements and determine outcomes numerically).

Assessment

Every introductory physics text includes conceptual and numeric questions that are intended to assess student understanding of projectile motion. This demonstration is intended to be a tool to increase student performance on these types of questions. Alternatively, students could be assessed by an alternative method, such as being asked for a written explanation of how this demonstration helped them understand projectile motion.

References and Resources