# Does the Weight of a Pinewood Derby Car Matter

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```					   Does the Weight of a
Pinewood Derby Car Matter?

John Cox 5th
November 26, 2007
Problem

I am investigating: does weight affect how fast a pinewood derby car goes? I
chose this experiment because I am interested in pinewood derby. Pinewood derby is an
event where you build your own miniature car out of a block pinewood and race it.
Gravity is the only force that causes the car to speed down the ramp.

Background Information about Pinewood Derby Car Racing
Pinewood derby was first thought of by Donald Murphy when he wanted a project
that he could do with his 10 year old son. He presented the idea to Cub Scout Pack
280C of Manhattan Beach, California. The first pinewood derby car was held in 1953 in
the Scout House in Manhattan Beach. The following year, the Boy Scouts of America
1
adopted the idea for Cub Scout packs nationwide.

A pinewood derby car is built from an official Boy Scouts of America (BSA)
Pinewood Derby Car Kit as shown in Figure 1. The kit contains a block of pinewood,
four plastic wheels and four nails that serve as axles. This is intended to be a
parent/child project to build the car for the Pinewood Derby Race. Trophies are
typically given for the fastest cars and special awards are sometimes given for the best
looking cars. The official rules state that the car can weigh a maximum of 5 ounces
2
(141.7 grams).

Figure 1: An Official BSA Pinewood Derby Car Kit
3
Figure 2 shows four pinewood derby cars built from the official BSA kit. These
cars loaded in the starting gate and are ready to race! The race starts when the starting
pin is dropped. This is usually controlled by one of the parents.

1 David Meade, Pinewood Derby Speed Secrets, 2006, p. viii
2 www.wikipedia.com, Pinecar Derby, visited November 10, 2007
3 http://home.simplyweb.net/bosworth/track_plans.htm

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Figure 2: Four Pinewood Derby Cars Ready to Race!
Pinewood derby tracks are usually around 30 to 45 feet (9.14 to 13.72m) in length
and have approximately a 4-foot (1.22m) drop at the beginning of the track to accelerate
4
the cars down the track. A picture of an actual track is shown in Figure 3.

Figure 3: A Pinewood Derby Race in Progress
It appears that almost everyone involved in racing pinewood derby cars believes
that the weight of a pinewood derby car is THE MOST IMPORTANT factor to consider
when building a winning pinewood derby car. Here are some quotes from books and
Internet resources that claim how important weight is in building a winning pinewood
derby car:

The heavier your car is, the faster it will go. 5

4 Ibid
5 David Meade, Pinewood Derby Speed Secrets, 2006, p. 89

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Weights are one of the biggest factors in a building a winner. 6

One thing to remember is that, if the limit on weight is 5 oz., you don't want your
car to weigh ANY less than that, because if your racer is even 1/10th of an ounce below
that of your competitor, you will almost assuredly get beaten no matter what else you
do. 7

Rule 1: The heavier your car is, the faster it will go! 8

Best speed is achieved at the maximum allowable weight. 9

The more weight that is in (or within) the car, the faster it will go! 10

We will add additional supplemental weight at the weigh-in to achieve the full 5.0
ounces desired. 11

Because the Derby Car is gravity-powered, you need to make sure that it weighs
as much as allowed by law. 12

If you use less than the allowed 5 ounces, you are shorting yourself potential
energy. This would be bad. 13

I will assume you already know how crucial it is to get your car as close to the 5-
ounce weight limit as possible. 14

Purpose
The purpose of my science fair project is to evaluate if weight has an effect on a
pinewood derby car's speed/velocity.

6    Pinecar How-to Book, 1995, p. 13
7    Michael Stewart, How to Win Pinewood Derby.com, 2006, p. 4
8    David Meade, Ultimate Speed Secrets: How to Win a Pinewood Car Derby, 2006, p.31
9    Randy Davis, Maximum Velocity!, 2005, p. 9
10   Thomas L. Pedigo, How to Win a Pinewood Car Derby, 2005, p. 11
11   Darryl Huntley, Dominant by Design 2006, p. 81
12   Official Scoutorama.com Pinewood Derby Handbook, 2003, p. 20
13   The Pinewood Professor Presents: Pinewood Derby Speed Secrets, 2004, p. 2
14   T. Dean White, Extremer Pinewood Racing: Advanced Technique and Design Volume II, 2005, p. 4

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Hypothesis
Based on the information found in books and Internet articles that I discussed
earlier in this report, my hypothesis is the heavier a pinewood derby car is, the faster it
will go down the track.

Procedure
1. Build/set up a ramp that does not go to the ground. Gather balls of different mass,
size, and material. I need to investigate the height and angle of the ramp in order
to approximate a pinewood derby track. I will use balls instead of actual
pinewood derby cars because there are lots of variables in an actual pinewood
derby car [ex: where the weights are placed, the wheels are a little bit different
than each other (i.e., there are different molds used to make the wheels), etc.] and
I only wish to study the effect of changing the weight.

2. Put the balls at the beginning of the ramp and let go for each ball used.

3. See, measure, and record the distance the balls went, and repeat the experiment
with the balls of different weight at least ten times. Record the results inside the
logbook.

4. Compare the results from step 3 to determine if weight affects the speed of the
balls. Record the results inside the logbook.

Results
I purchased aluminum right angle approximately 1 meter long at Home Depot. I ordered
steel, delrin, polypropylene, glass, acrylic, and nylon balls of 19 and 25 mm diameters
from Science Kit & Boreal Laboratories. I also ordered steel balls with 16 and 38 mm
diameters from the same company. I measured the mass using an electronic scale and
found the data shown in Table 1. A picture of the balls used is shown in Figure 4.

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Table 1: Properties of Balls Used in Experiment

Material        Diameter (mm)   Mass (g)
Acrylic                    19            4.2
Acrylic                    25             10
Delrin                     19            4.9
Delrin                     25           11.8
Glass                      19            8.5
Glass                      25           21.8
Nylon                      19            3.9
Nylon                      25            9.8
Polypropylene              19            3.1
Polypropylene              25            7.2
Steel                      16           16.4
Steel                      19           28.2
Steel                      25           67.4
Steel                      38          225.4

Figure 4: Balls Used in Experiment (front to back: Acrylic, Delrin, Glass, Nylon, Polypropylene, Steel)

At first, I used a straight piece of aluminum angle to create a ramp to roll the balls
down. However, I found out that the balls bounced off the track and gave different
results every time they rolled down the track. I found that I had to put a bend in the
track at the bottom to put an end to the uncontrollable results. That one bend at the
bottom helped a lot with getting repeatable results. The resulting experimental apparatus

Page 6 of 11
is shown in Figure 5. The ball is released from the top of the ramp. After it rolls down
the ramp, it flies off the end of the ramp and curves downward toward the paper on the
stool. I marked the location where every ball hit on the paper. I measured the distance
from the end of the ramp to the spot where the ball hit the paper.

Figure 5: Experimental Apparatus with a Polypropylene Ball Flying off the End of the Ramp

Table 2 shows the results of the experiment. As you can see all the balls traveled
the same distance regardless of their mass, diameter, or material. This is also shown in
Figure 6.
Table 2: Experiment Results
Distance
Material        Diameter (mm)   Mass (g)       Traveled (mm)
Acrylic                    19            4.2               42
Acrylic                    25             10               42
Delrin                     19            4.9               42
Delrin                     25           11.8               42
Glass                      19            8.5               42
Glass                      25           21.8               42
Nylon                      19            3.9               42
Nylon                      25            9.8               42
Polypropylene              19            3.1               42
Polypropylene              25            7.2               42
Steel                      16           16.4               42
Steel                      19           28.2               42
Steel                      25           67.4               42
Steel                      38          225.4               42

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Distance Traveled vs Ball Mass
45
40
35
30
25
20
15
10
5
0
0             50             100             150     200    250
Ball Mass (g)

Figure 6: Graph of Experimental Data

Analysis of Results

Aristotle believed and taught that the heavier an object is, the faster it falls when it
is dropped. He must not have tried the experiment, because he was not correct.
Everyone believed him for many centuries. Then, Galileo checked if Aristotle was
correct or not. Galileo dropped two objects with different masses off the Leaning Tower
of Pisa and found that they hit the ground at the same time. 15
It is always important to know that the difference between mass and weight. Mass
is the amount of matter in an object. Weight is the measure of the force of gravity acting
on an object. Mass is measured in grams (g) and weight from a physics perspective is
measured in newtons (N). So, for this experiment, we are using mass, but many people
confuse this with weight.
In preparing for the experiment, I spoke with a physicist about my science fair
project. He showed me that I could apply math to the problem to prove the results of my
experiment were correct. One of the fundamental laws of physics is that energy cannot

15 www.wikipedia.com, Galileo Galilei, visited November 10, 2007

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be created or destroyed. In this experiment, there are two types of energy to consider.
One is potential energy and the other is kinetic energy. Potential energy is when the ball
is at the top of the ramp, but is not in motion. When the ball is released at the top of the
ramp, it is in motion because it transfers the potential energy into kinetic energy. There
are two types of kinetic energy -- translational and rotational. Translational kinetic
energy is when the ball is going in straight line, while rotational kinetic energy is when
the ball is rolling around its center. Thus, the rolling ball has both translational and
rotational kinetic energy.16
If we turn this description of our experiment into math, we need a few variables:
m=mass
g=gravity
h=height
v=velocity or speed
From physics, we find the mathematical definitions of energy:
Potential energy = mgh
Kinetic energy = translational energy + rotational energy
Kinetic energy = ½mv2 + 1/5mr2(v/r)2
Since energy cannot be created or destroyed, the potential energy of the ball at the
top of the ramp must equal the kinetic energy at the bottom of the ramp. So, we find
that:

16 Dr. Martin Kykta, personal interview, October 25, 2007. Dr. Kykta holds a Ph.D. in Physics from the University of
Texas.

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Potential energy at top of ramp = Kinetic energy at bottom of ramp
⎛v ⎞
2
1      1
mgh = mv 2 + mr 2 ⎜ ⎟
2      5     ⎝ r⎠
1      1 mr 2v 2
mgh = mv 2 +
2      5 r2
1      1
mgh = mv 2 + mv 2
2      5
5       2
mgh = mv 2 + mv 2
10      10
7
mgh = mv 2
10
1    7    1
mgh = mv 2
m 10       m
7
gh = v 2
10
10      10 7
gh = • v 2
7       7 10
10
gh = v 2
7
10
v 2 = gh
7

We cannot control gravity so height is the only thing that matters for determining the
speed or velocity of the ball when it reaches the bottom of the ramp. The racetrack sets
the height that a pinewood derby car moves down, so this does not change when
pinewood derby cars are racing. In other words, the weight of the ball does NOT
determine the speed of a ball. 17

Conclusion
This science fair experiment has shown that my original hypothesis is not correct.
In other words, the weight of a pinewood derby car is not likely to be the most important
factor in which car wins the race. I approximated a pinewood derby car with balls of
many different materials, sizes, and mass and found that the speed of the ball was the
same at the bottom of a ramp. Other factors such as friction or car design must be the
determining factors for who wins a race. A suggestion for future scientific work is to
explore the effect that the location of the weight in a car, friction, and car design have in
affecting the speed of a pinewood derby car.

17
Ibid

Page 10 of 11
Bibliography
Randy Davis, Maximum Velocity!, 2005

http://home.simplyweb.net/bosworth/track_plans.htm

Darryl Huntley, Dominant by Design, 2006

Dr. Martin Kykta, personal interview, October 25, 2007. Dr. Kykta holds a Ph.D. in
Physics from the University of Texas.

David Meade, Pinewood Derby Speed Secrets, 2006

David Meade, Ultimate Speed Secrets: How to Win a Pinewood Car Derby, 2006

Official Scoutorama.com Pinewood Derby Handbook, 2003

Thomas L. Pedigo, How to Win a Pinewood Car Derby, 2005

Pinecar How-to Book, 1995

The Pinewood Professor Presents: Pinewood Derby Speed Secrets, 2004

Michael Stewart, How to Win Pinewood Derby.com, 2006

T. Dean White, Extremer Pinewood Racing: Advanced Technique and Design Volume II,
2005

www.wikipedia.com, Galileo Galilei, visited November 10, 2007

www.wikipedia.com, Pinecar Derby, visited November 10, 2007

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