Trebuchets

by David Spackman and Bryan O'Halloran

 

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Background

Statement of Problem

Review of Literature

Hypothesis

Method

Results

Discussion

Conclusion

Links

 

Background:  top

            What is a trebuchet? The trebuchet was the most amazing weapon of its time. They had only one use, destruction.  What a coincidence that a large destructive weapon works because of physics, and better yet, testable properties.  A trebuchet is a simple machine in concept but they can be very tricky to fine tune into perfection.  The most basic example of a trebuchet is a teeter-totter with a big kid on one side and a small kid on the other.  The big kid’s side will fall causing the small kid to rise.  In trebuchet mechanics though, the counterweight is usually close to 100 times the weight of the projectile.  Also, the projectile sits in a sling in order to get more throw power when it is released.  A ball on the end of a long string travels faster then one on a short string if they are both rotated at one RPM. The same property helps the trebuchet get the projectile moving faster, hence, more distance. 

 

Statement of Problem:   top

            The objective of this project is to find any relationships between the sling length, and hang time and distance of the projectile.

 

Review of Literature:  top

            Our project involves moving parts, so it is obvious that we should start with Newton’s second law.  It states “whenever one object exerts a force on a second object, the second exerts an equal and opposite force on the first” (Giancoli).  A trebuchet is like a basic lever, with the counterweight providing the work, and the projectile providing the load (ripcord.ws...).  The trebuchet has five basic parts: the frame, the counterweight, the beam, the sling, and the projectile (geocities.com....). Mainly, we will be manipulating only the sling length, because the other variables like counterweight mass seem like common sense.  The sling length properties should be interesting to test.

 

Hypothesis:   top

            I believe that as the sling gets longer, the projectile will spend more time in the air, and will travel farther.  The “sling length” is described as the distance from the release pin to the bottom of the “basket” of the sling.  Distance will be measured from the base of the trebuchet to where the projectile first hits the ground.  We will be using a golf ball for our projectile.  Hang time will be measured with a stopwatch and will start when the ball leaves the basket and stop when the ball hits the ground.

 

Method:  top

            Here is the basic structure by level of our trebuchet (not including the sling and arm.

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After the completion of our trebuchet, we got ready to gather data. It started by us finding a place to set the trebuchet because we didn’t have any clue as to how far it was going to launch. Once our location was selected, all the necessary utensils were gathered brought. First, we set the trebuchet in place made sure it would not move once the data collecting had begun. Then we measured out points in twenty five foot intervals using a tape measure and extremely large screws.

            When recording our data, Spackman was set in charge of preparing the trebuchet (a.k.a. reattaching the sling and cocking back the arm) and then launching the projectile. Bryan was in charge of recording the time it was in the air and the total distance it traveled. This made it easy and quick to take data which was convenient because it was below freezing outside. Below is a diagram of the final setup with the “X” symbolizing the trebuchet.

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Results:   top

All of our graphs are set on another page. Fortunately, our results came out wonderfully. As you will see in the attached graphs, all of our data formed a smooth curve. Very few times did the balls go crazy or the trebuchet not launch correctly. Since there were very few flaws as far as throwing the projectiles or keeping the trebuchet in place, all the balls landed with in five feet to the left or right of the tape measure. We feel that the wind may have played a small role in placing the balls a few inches from where they should have landed. We also feel that the sling getting wet and slightly heavier may have also altered our results. The balls traveled farther at the beginning of the experiment then at the end.

 

Discussion:   top

Our hypothesis was only half way correct. Above is states that we think the ball will travel farther as the sling length increases. But for our experiment, this is only true until the sling length is as long as the length from the axle to the pin. See diagram below.

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At sling length 40 the distance that the projectile traveled is the greatest. In a close second is the sling length 36 where the projectile travels an average of 68 feet or 816 inches. We determined that the sling length 39 would have been the best due to the length between the axle and the pin.

 

 

 

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Data: Text .:. Excel

 

As you can see, the time that the projectile is in the air never exceeds 4 seconds, but it reaches lengths of over eighty feet.

 

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Conclusion:   top

Our original purpose in this investigation was to determine if a trebuchets sling length affects its launch distance and hang time.  From the data we collected, we can conclude that sling length does affect the performance of the trebuchet in a few ways.  First, the projectile goes much further when the sling hangs close to the axel.  The ball goes less far when the sling length is at extremes from this measurement.  Hang time has a different story though.  With hang time the peak was at about a 30 inches sling, the only explanation for this is the angle of launch.  With the 28 and 32 inch slings the ball released at a very high angle while still staying in the sling long enough to get a good push.  The ball didn’t go the farthest but it did stay in the air the longest.  If we were to do this again, there would be some changes in order to get better and more accurate results.  It would be a good idea to try more sling lengths with a smaller interval in between them.  This would make the graphs more accurate and give a larger domain to them as well.  Overall we believe this experiment to be fairly accurate and a lot of fun.

 

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