Potato Gun Physics
Students are told to make use of a Food Delivery Device (FDD) to deliver a potato to a
circular compound some 30 meters distant from firing point. Students should “calibrate”
their FDDs on the first day to determine affect of pressure, acceleration length, angle of
launch, and potato mass on ability to hit the target. After the first day of calibration the
“winds of fate” blow, and students are constrained on one of the firing parameters but
must make adjustments to hit the target none the less.
Potato Gun Assembly
Approximately 70 cm of 3 inch PVC tubing for pressure chamber
Schrader type tire valve, available at auto parts store (short version is better).
Cap for 3 inch PVC
1.5 meters approximately of 2” PVC tubing
10 cm piece of 2” PVC tubing Ball valve for 2 inch PVC
Reducer for 3” to 2” PVC
Cleaner and glue for PVC pipe.
1. Drill hole for valve in 3” PVC, about 15 cm from the end, per hole size
recommended by the valve manufacturer..
2. Feed the Schrader valve in from the inside, and carefully using pliers, pull it
through until the capture flange is in place.
3. Cut about 10 cm of 2” PVC pipe to go from reducer to the ball valve.
4. Without glue, assemble your gun, front to back and make sure all the parts fit.
The order is: End cap, pressure chamber (with Schrader valve), reducer, 10 cm of
2” PVC, ball valve, 1.5 meters of 2” barrel
5. Disassemble on a long table, and clean each joint with PVC cleaner (in a well
ventilated area). Then one at a time, put PVC solvent on the interior of each
fitting and press in the next piece. Working from end-to-end, assemble the FDD
one joint at a time. Ensure you hold the parts long enough to keep them straight
6. Dry for 24 hours before pressurizing or firing.
Cautionary and Helpful Notes:
? Take a piece of 2”PVC and using an electric sander (disc sander is ideal) sharpen
the end to make a potato cutter. This allows students to cut the potato and mass
it before putting it in the barrel.
? Load potatoes with the ball valve open, then, close the ball valve. Measure the
length down the tube over which the potato is accelerated for later force
? Have students fire first test at less than 15 pounds of pressure and at an angle of
greater than 65 degrees. This ensures potatoes are not fired too far.
? Never fire at higher than 60 pounds and don’t tell the students the rating on the
PVC. The 3” tubing is not rated for pressure at all anyway.
? The dirty little secret of the whole thing is that potato seal in the barrel, and valve
turning rate are the real keys to repeatable results.
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Student Rules for Firing
These rules are non-negotiable and part of the criteria for success. Failure to follow
these rules (there will be NO warnings), will result in immediate removal from the
activity and a call to the highest authorities
? Whether charged with air, or not, the FDD must NEVER be pointed at another
person, building, or object, EVEN IN JEST.
? The pressure end of the FDD must be on the ground when fired. No shoulder
fired, hand held, or other operation lifting the FDD off the ground is permitted.
? The FDD shall never be pressurized above 25 lbs/in2 unless given permission by
? A “firing’ warning shall be issued for all launches audible to a perimeter of 40
meters from the launch location.
? Loading of the potato shall be done “from the side”, never load the potato while
standing or being otherwise located in front of the FDD muzzle.
Typical Data Table for Data Collection
Trial Angle Pressure Acceleration Potato Distance Time
(degrees) (psi) Length Mass Δx Δt
l(m) (kg) (m) (s) ?a
Typical Data Table for Calculations
Trial actual model = VVAccel.-a Force (N) Max ?xi iV x2 Height (m) VV?tg?xxi= ma 22(m/s) Visin?cos?2l2cos?sin?a 2(m/s) 2g(m/s) (m/s)
Students calculate and compare model values to actual values and observe how good the
model is (It’s actually pretty good). They also realize that science is messy and
repeatable results can be challenging to obtain.
? Lab write up including calculations, and comparing model (with out air resistance) to
actual data (with air resistance). There is usually very good agreement regardless of
how “bad” the firing and repeatability.
? Provide some dummied up data and have students calculate velocity or force from
? Discuss Sources of Error in measurement, timing, assumptions
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Advanced Topics – Derivations of the Equations used Above
Derivation of Equations (in word form)
1. Since Velocity is displacement over time, the velocity in the horizontal (x)
direction must be equal to Δx/Δt.
2. Since Velocity is a vector, then we can calculate the velocity out of the end of the
gun by dividing by the cosΘ.
3. Again, since velocity is a vector we can find the velocity in the vertical direction
by multiplying the velocity out of the gun by the sinΘ.
4. Vi-model - This derivation is a little tedious and is shown below:
A. At the peak the Velocity in the vertical direction is zero. How can this help me? You
also know that this occurs when the time is ? of the total flight time. So, you have
moved ? of the distance. Looking at my 2D motion equations what can I do with this
information? Well, if you find the time of flight and the initial velocity you can use that
12information to find the height using ?y = V sin??t - g (?t) i22 22. 1. V = V sin?- 2g(?y) y,fiVertical parts
122. ?y = V sin??t - g (?t) i2
3. V = V sin?-g(?t) y,fi
4. V = V cos?= constant x,fiHorizontal parts 5. ?x = V cos??t i? Don B. Cameron – email@example.com 3 of 4
= V sin?-g(?t) and we know that V = 0 at the If I make use of equation 3 I see that Vy,fiy,f
Visin??tpeak. So we can solve this equation for ?t and get which is true ? way ?g1through the flight. We also know the ball has gone half the distance in this amount of
time. Using equation 5 you see that ?x = V cos??t or, half way through the flight that i
?x?t?x2means . So now we have two equations in terms of time and ??Vicos?12Vicos?
can find Vi.
?t?xVisin??tThe two equations are: and ??g112Vicos?
Setting these equal to each other you get:
Solving the second two equations for V I get. i
5. Finding Acceleration – “a”. Since the potato starts at rest, but leaves the gun at
2Vispeed Vi, then we can use the acceleration length in the equation . This 2la
equation is itself derived from the standard distance formula and the definition of
acceleration equation (change in velocity over change in time) and is not shown
6. Force – Since force is just mass times acceleration they could multiply the
acceleration by the mass (kg) to get force.
7. Maximum height can be modeled as follows. Either use ? of the actual time, and
put it in the height equation (and assume 9.81 for gravity) or put the velocity into
Visin??tthis equation: and use this Δt to calculate max y. Substituting ?g1
algebraically into the height equation, this can be done in one step using:
22Visin?. As you see this does not make use of time, so it is definitely a model. 2g
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