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# Ping Pong Ball Launcher Research and Design

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## sarah wong

on 17 January 2014

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#### Transcript of Ping Pong Ball Launcher Research and Design

Objective
To construct a mechanism to launch a ping pong ball into a garbage bin 2, 4, 6, and 8 metres away from the launcher, with restrictions:
launcher must be placed on the floor
there must be a base for the launcher
size restriction of the launcher (size of photocopier box)
Ping pong balls are stiff, springy, air-filled spheres made of celluloids, a class of plastics. Its material and light-weight allows air resistance and its spin to have significant effects on its motion when airborne. Air resistance is the result of the projectile's leading surface colliding with air molecules, and it is affected by the velocity of the object and its cross-sectional area. Specifically, a higher velocity and greater cross-sectional area results in greater air resistance. Additionally, ping pong balls also bounce quite high when they strike a surface.

Since the launcher will launch the ping pong ball so that it launches with a parabolic trajectory, the physics behind projectile motion comes into play. In a projectile, the only force is gravity, thus resulting in a downward acceleration. Gravity does not act in the horizontal direction, so the ping pong ball will theoretically travel at a constant horizontal velocity according to the law of inertia.
Physics of Projectile Motion
Traits of Ping Pong Balls
Collectively, traits of ping pong balls and the nature of projectile motion will have important effects on the launcher that must be considered to ensure that the launcher fulfills the expectations successfully.
Because of the ping pong ball's light weight, it is very prone to missing the target as its flight in the air is easily affected by air resistance, drag, and if the ping pong ball spins.
Therefore, a lower initial velocity of the launch and a way to keep the ping pong ball stable when it is being launched will reduce the chance of these factors affecting its trajectory.

Additionally, a ping pong ball's tendency to bounce must be considered when it is launched, for a greater vertical distance will mean that the force of gravity will cause the ball to accelerate to a higher velocity in the downward direction. The higher the velocity, the more potential energy the ping pong ball acquires, thus the greater the force of the impact of the ball with the target bin. Furthermore, the hard surface of the target bin does not absorb much energy from the impact since the plastic is not very elastic and springy, which in addition to the hard and light-weight celluloid of the ping pong ball, means a large fraction of the potential energy of the ball upon impact will convert back into kinetic energy, resulting in the ball bouncing, perhaps out of the target bin!
If this occurs, a solution would be to minimize the vertical distance the ball travels in the parabolic trajectory by lengthening the arm of the launcher. However, the vertical distance must be, at the very minimum, slightly higher than the height of the target bin so it can actually go in to the bin, thus this will prove to be a challenge when figuring out the ideal angle of the launch and the length of the launching arm.

Ping Pong Ball Launcher
considerations for ping pong balls as the object being launched
types of catapults - which is best?
the variable: target distance - additional features to allow for adjustments of the launcher to aim for different targets
proposed design

Topics Researched and Discussed
Ping Pong Balls
Types of Catapults
Considering the restrictions of the assignment and the far distances the ping pong ball must be launched, a strong catapult would be suitable to complete the task, especially since there are a wide variety that can be made with readily available materials.

Ballista
The ballista is one of the first Medieval catapults that utilized torsion, a type of elasticity, stored in twisted ropes and sometimes bending wood to launch the projectile. It has a distinct crossbow-like design. Since this is an earlier model of catapults in history, there are better designs that were developed later that are more durable when made on a smaller scale, like for this assignment, that also happen to be a lot easier to build.
Trebuchet
The trebuchet was constructed after the ballista, using a weighted beam to swing a projectile located in a sling at the opposite end of the beam. Unfortunately, for the purpose of this assignment, a trebuchet may not be the best design to use. The sling may not provide the most accurate launch to get the ping pong ball in the target; additionally, the trebuchet must be adjustable to account for the different distances of the four targets, and the exact mass of the counter weight to provide enough power to launch the projectile 8 metres may be hard to achieve exactly with inexpensive, readily available materials. Overall, there are other catapult designs that are much easier to build and less complicated, such as the mangonel.
Mangonel
The mangonel was constructed after the trebuchet and it can be a very effective catapult for this assignment. It is torsion powered with many ropes twisted in a figure-8 formation around one end of the beam, with the projectile in a bucket at the other end to be launched.
Mousetrap Catapult
A catapult that uses the trapping mechanism of a mousetrap is a common and very effective design for a catapult to build at home. The mousetrap uses the distortion of the metal spring to trap mice, but this elastic force is definitely strong enough to power a catapult to launch a ping pong ball 8 metres using the mechanism of the mouse trap, removing the holding bar and catch pad.
This video briefly shows exactly how to turn a mousetrap into a simple catapult; obviously, a more substantial design would have to be constructed so that the launching distance can be adjusted to hit each target. Additionally, since rat traps tend to be larger, it would probably be able to provide more power to launch the ping pong ball even farther, so this can be used as well.

How to Wrap the Rope for a Torsion-Powered Catapult
It can be very powerful and accurate since it is built with almost entirely wood (save for the ropes for torsion). As well, it is very easy to adjust the catapult to aim for the different targets by changing the catapult's arm length, and/or the proximity of the beam at the front of the catapult parallel to the ground (circled in the diagram to the left). These factors will be further explored later in this presentation.

Overall, the torsion-powered mangonel and the mousetrap catapult appear to be the most suitable to build for this assignment, as they allow for easy adjustments to hit targets at different distances, and can be made with readily available and inexpensive materials that will not degrade or lose effectiveness over a short period of time (i.e. rubber bands).
by Sarah Wong
Features to Adjust for Different Target Distances
This video shows what adjustments can be made to a torsion-powered catapult (and any catapult with a similar design) to launch at different horizontal and vertical lengths.
Noted timestamps in the video:
(1:24)
He mentions ways to strengthen the catapult structure
(1:51)
He explains that
building the crossbar closer to the back
of the catapult causes the launching arm to release the projectile earlier, which will give it a higher arc in the trajectory. This can also be mimicked by
building the crossbar so that it is adjustable
so the catapult can launch different distances (the lower the crossbar, the higher the trajectory and farther the horizontal distance)
(2:29)
He also states that the catapult can launch even further if the
so the projectile is launched from a higher altitude with respect to the ground. However, the time the launching arm stops (the height of the crossbar/proximity to the back of the catapult) must be adjusted.
(5:24)
He also lists
tightening the torsion ropes
as a way to improve the catapult.
Proposed Design: The Mangonel
Since mouse traps are so powerful, it would launch the ping pong ball at too fast of an initial velocity. As well, the spring may wear out after repeated use, and thus lose effectiveness. Therefore, torsion in twisted ropes is the easiest yet sufficiently powerful way to launch the ping pong balls.
*Diagram is not to scale. Actual dimensions are thus not included as they are subject to change. Screws are not included as well to maintain clarity of design.
The round bucket will decrease the amount of movement of the ping pong ball before being launched, therefore the ping pong ball will not spin as much in its trajectory.
(not visible in diagram) The cross bar is adjustable to change the angle that the launching arm stops at, changing the angle of the trajectory to aim for targets at different distances.
The ropes are twisted in a figure-8 structure (as described in the diagram and video provided under the "Types of Catapults" section. It will be twisted tightly to give the catapult even more power.
The knob at the top allows the launching arm to be pulled down for the launch without disturbing the ping pong ball itself.
wooden frame
plastic lip balm lid

eye hook

multi-strand string

Works Cited
http://en.wikipedia.org/wiki/Table_tennis
http://www.physicsclassroom.com/class/newtlaws/u2l3e.cfm
http://www.real-world-physics-problems.com/catapult-physics.html

Source: http://www.stormthecastle.com/catapult/images/ballista-drawing1.jpg
Source: http://www.stormthecastle.com/catapult/images/trebuchet-drawing1.jpg
Source: http://www.stormthecastle.com/catapult/images/mangonel-drawing1.jpg

Source: http://2.bp.blogspot.com/-MSo-bS4pteA/UhZMvrSFsVI/AAAAAAAABBI/z5TOQOB6s1A/s1600/catapult%2Bping%2Bpong%2Bball.jpg