Large-Scale Projectile Motion Demonstration

Project Overview

The previous projectile motion demonstration for the classroom was small, imprecise, and overall temperamental. The general task presented was to upscale the system so that it was easily observable in an auditorium-sized classroom full of 100+ students and so that the user could accurately hit the target every trial.

"The Monkey and the Hunter" Problem

In many high school and college classrooms, a common problem known as the "monkey and the hunter" is used to explain projectile motion. The instructor usually starts by asking "if I want to hit a monkey (target) as it is falling from a tree, where should I aim right before it drops?" The students quickly discover that aiming at the target while it is still in the tree is the correct answer because both the projectile and the target (monkey) will experience the same acceleration due to gravity once the monkey lets go of the branch. This means they will fall at the same rate, regardless of their horizontal velocities. Since gravity affects both the projectile and the monkey equally, aiming directly at the monkey’s initial position ensures that their paths will intersect as they fall. The key insight is that the vertical displacement of both objects relative to their starting points will be identical, causing the projectile to strike the monkey mid-fall.

This was the goal of this demonstration, and its execution was successful.

Image credits: https://media.wired.com/photos/593341914cd5ce6f96c0cc47/master/w_1600%2Cc_limit/monkeyshot3.png

Design Highlights

Final Assembly

Above, the final launcher assembly can be seen. Labels describe essential parts of the design

View from User

The launcher is on its mount, pointed at the target hanging 42.43 feet away
(30 feet horizontal and vertical)

IMG_2222.MOV

Successful Test Video

The first successful test was recorded in this video above

The system works as follows:

The launcher contains a laser pointer which is used to aim directly at the target. Once aimed, the user can shoot the tennis ball from the launcher. The tennis ball passes through a photogate mounted in front of the launcher, which releases the electromagnet that is suspending the target in the air. This allows for the projectile and the target to be released at exactly the same time, as per the problem. Following the instructions as described in the problem description, the target is hit by the projectile as it falls, proving the system works as desired.

Requirements

The system shall be designed to be easily observable and used in an auditorium-sized classroom
- The system shall be designed for a 30 foot long table and 30 foot tall target mount
The set-up and procedure for use shall be simple and easily replicable - the target must be hit 95% of the time
The mount to the NERF tennis ball launcher shall be non-invasive; the launcher shall not be modified to be used
Launcher shall be usable from both the fixed position on the lab table and anywhere the user may take it in the classroom (free aiming)

Design Progression

Below, the design iteration and progression can be seen. Often, changes would be made after receiving feedback from various professors and instructors that would be the users of the system. Other changes were made when something in the system failed. Descriptions of each of the designs and changes can be found below.

Design 1: The Clamp

This design was the first attempt at making the mount non-invasive, and allowing for a quick replacement of the launcher if needed.

Releasing the clamp would allow for the launcher to easily slide out of the mount. The replacement launcher would be placed back in the clamp and mount, where the clamp would be tightened. The mount was fixed to an aluminum plate, which also supported the photogate.

While this gave some new ideas on how to make the mount non-invasive, this design was clunky and unrefined. It also increased the system's weight by at least twice what just the launcher weighted.

Design 2: 3D Printed Brackets

Assembled with Brackets (Side)

Assembled with Brackets (Top)

Screw with Rubber Block

The clamp idea was maintained but executed in a new way - adding screws with rubber blocks allowed for the user to tighten and loosen the brackets onto the gun as desired. Additionally, the addition of the brackets allowed for the pivot point to be easily mountable to the stand for the launcher. Finally, the acrylic and PLA brackets drastically reduced the weight of the system.

However, these brackets posed a problem; the launcher was often crooked within the brackets because of how much free-space there is between the launcher and the brackets.

Design 3: The Full Back Bracket & Aiming

Using similar concepts from the previous design, the main change was the back bracket with the top. This helped prevent the launcher from being crooked, directly addressing the problem from design 3.

The main issue was that the launcher still was not fixed in place as well as it could be.

An additional part that was added in this design is the laser pointer, seen on the right front side of the launcher. This allowed for the user to aim at the target. The mount for the laser pointer was inspired by view-finders on telescopes, allowing for more precise aiming. The laser mount design can be found below.

This was the penultimate design for the launcher. The final design can be found at the top of this page.

Laser Pointer Mount

As mentioned above, the laser pointer was included because it allowed for the user to aim at the target. The mount for the laser pointer was inspired by view-finders on telescopes, allowing for more precise aiming. The laser pointer is fixed in two miniature tube-like structures, each which contains 3 thumb screws. This allows for the user to adjust the direction of the laser, thus allowing for the aiming of the gun to be calibrated.

Below the first image, an image from a telescope can be seen - this is where the direct inspiration came from for this design. Because the laser pointer is offset from the actual barrel of the launcher, pointing the laser straight would not be accurate. Therefore, this design allows for that correction.

Photo credit: https://thumbs.dreamstime.com/b/viewfinder-telescope-refracting-device-instrument-land-lunar-planetary-observation-distant-object-mag-finderscope-188125606.jpg

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