PIGS AND PUMPKINS
Background info .:.
Problem .:.
Hypothesis .:. Variables
.:. Method .:.
Materials .:.
Results .:.Video
.:. Conclusion
.:. Bibliography .:. Go Up
Background
Information:
The
primary feature of a compound bow is the set of pulleys which both magnify the
force applied to the string and provide an advantage known as "let
off."3 A standard longbow requires
nearly as much force to hold it fully drawn as it does to draw it back. In a
compound bow, once the string is about 50-80% drawn, the pulleys absorb the
force with which the string is drawn; when fully drawn, little force is needed
to hold the string back.2 The cables and pulleys make
compound bows easier for the archer to use, but the real secret to its power is
in the composite materials from which it's constructed. The central part of the
bow is usually made of aluminium and the limbs are made of very strong but
somewhat flexible composite materials.1
More
traditional bows, such as standard and recurve bows, store the energy within
the actual frames and strings of the bows and require the same force with which
it was pulled back to keep it in that position. The central parts of the bows are
generally made of simple wood, and they are commonly coated with a clear finish
and a layer of fibreglass on the limbs in order to increase the bow’s integrity.
The average recurve bow is 48-70 inches long, and most of these types of bows
possess a draw weight between 15 and 25 lbs.1
Problem:
When measuring the force curves of a standard, recurve,
and compound bow, how does the design of each bow impact its force curve?
Hypothesis:
When drawing the standard and recurve bows, their
force curves will increase directly proportional to their draw distances.
However, due to the pulley system in the compound bow, the force curve will
experience a significant decline when the pulleys activate, meaning that the
bow will necessitate a lesser amount of force to remain drawn.
Variables:
Independent: Draw
Length
Dependent: Force
Control: Type of bow
with respect to each set of data
Method:
In
order to measure the force curves of both the standard and recurve bows, we
will hang each bows from wooden supports with a meter stick placed perpendicular
to the bows' drawstrings, which will be used to measure the draw length of each
trial. Upon mounting the bows, we will proceed to hang masses from their draw
strings, starting with 1 kg and increasing by 1/2 kg for each data point over a
14 kg interval. With each new mass, we will record the resulting distance the
drawstring is stretched. Since the masses will be hung vertically, the forces
will be calculated by multiplying each mass by the acceleration of gravity
(9.81 ms-2). Above is a photo of the actual procedure being executed
using a recurve bow with a 1 kg weight being placed on the drawstring.
For
the compound bow, we will need to alter our procedure due to its unique draw
system. Using an apparatus constructed out of metal bars and clamps, we will
vertically mount the bow, and a meter stick will be placed parallel to the
ground and adjacent to the bow as a means of measuring its draw length. We will
use an electronic force meter to measure the force at any draw distance;
however, due to a limitation on the capacity of the force meter, a pulley
system must be used in order to divide the force being allowed to reach the
force meter by 7. This will enable us to collect a sufficient amount of data
that will be comparable to the other bows. Using the pulley system, we will
manipulate the draw length of the bow and record the force that coincides with
each point. The photo to the upper left shows the compound bow mounted to
the metal apparatus using duct tape with the meter stick placed perpendicular
to the drawstring. The second photo on the right illustrates the pulley system
used to draw the bow; this is connected to the force meter, which is shown on
the left of the photo.
Data File : Text
.:. Excel
Conclusion:
Our
results suggest that the designs of both the recurve bow and the compound bow
alter the force curve in a manner which significantly reduces the strain on the
user drawing the bow when compared to the standard bow, which, to some degree,
acts as a control for this comparison. This effect is optimized close to full
draw, which is particularly noticeable in the force curve of the compound bow.
The purpose of this is likely to allow users to aim more accurately and sustain
a full draw longer without sacrificing much, if any, power.
It
appears that our hypothesis is quite accurate for both the standard bow and
compound bow, but inaccurate for the recurve bow. Instead of following a
directly proportional pattern, the recurve bow's force curve begins with a
concave-down curve and, after an inflection point at a draw distance of about
25 cm, ends with a concave-up curve. This is a result of its altered design and
enables the user to exert less force than he/she would have had to with a bow
that has a force curve similar to the standard bow. As we predicted, the
standard bow nearly follows a direct proportion of 3.25 N/cm. It does, to a
very limited extent, appear to have a curve to it not dissimilar in form to
that of the recurve bow. This is likely standard for any bow (with the exception
of compound bows) and due to its small magnitude, its significance is minimal.
As we predicted, the compound bow's force curve significantly dropped as the
pulleys activated. During the period they went into action (between 18 cm and
32 cm), the force required to hold the drawstring dropped by about 45 N (from 115
N to 70 N). After this point, the force continued to rise again, as should have
been expected.
In the
procedure for the recurve and standard bows, areas in which errors could have
been derived from were minimal. The masses were assumed to have been manufactured
precisely enough to make it acceptable to neglect any possible uncertainty.
Similarly, the uncertainty pertaining to the distance of each trial’s draw
length was seemingly insignificant; granted, human error still impacted the
data (such as in the standard bow data at 30 cm), but such errors are difficult
to measure, and they have little to no effect on the general outlook of the
data.
The
procedure for the compound bow contained a few facets that may have contributed
errors to the data. For instance, the force meter that was used to measure the
force during each trial fluctuated sporadically during the experiment; it is
estimated that the force deviated as much as 7 N from the recorded data (1 N on
the actual meter, but 7 N after accounting for the pulley system).
Additionally, there were times when the pulley system that was used to draw the
compound bow lacked enough integrity to stabilize the bow, and this may have
further contributed to the uncertainty pertaining to the force recordings.
In
order to improve the lab, one fundamental aspect that could make the data more
precise would be to develop a more refined means of reading the distance of the
draw length during each bows’ trials. For the compound bow’s procedure, resolving
the issue pertaining to the force meter’s instability would yield better
results. Also, using a force meter that would be able to support the entire
force of the bow would make it unnecessary to use the pulley system, which
would eliminate another source of error.
BIBLIOGRAPHY:
1 Jeffrey, Graeme. "Basic
Bow Types." Centenary Archers Club. 1999-2008. Centenary Archers Club Inc.
October 29, 2009. <http://www.centenaryarchers.gil.com.au/basic.htm>.
2 Nicholson, Joseph. “How Does
a Compound Bow Work?” eHow.
October 29, 2009. <http://www.ehow.com/how-does_4568731_compound-bow-work.html>.
3 Walterscheit, Jen. “Selecting
the Right Compound Bow for You.” 2008. Articlesbase. November 24, 2009. <http://www.articlesbase.com/sports-and-fitness-articles/selecting-the-right-compound-bow-for-you-498937.html>.
Related Websites
http://www.imagemet.com/WebHelp/spip.htm#force_menu.htm Taught us how
to analyze force curves
http://mathewsinc.com/
Showed us different types of bows
http://www.hallmark.com/webapp/wcs/stores/servlet/SearchResultsView?Ntt=bows&gnav_go.x=0&gnav_go.y=0&Nty=1&storeId=10001&catalogId=10051&N=35&Ntk=all_fields&Ntx=mode%2Bmatchallpartial&RPP=12&SBQ=yes
Taught us a lot about "bows" (and happiness)
http://www.maconlysource.com/Gallery/images/apples.jpg Great picture
reference
http://www.centenaryarchers.gil.com.au/basic.htm
MORE BOWS
http://www.ehow.com/how-does_4568731_compound-bow-work.html
How compound bows work
http://www.articlesbase.com/sports-and-fitness-articles/selecting-the-right-compound-bow-for-you-498937.html
More on compound bows
