# Golf Ball Trebuchet

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This is a trebuchet we made for our Concept Physics class. Our trebuchet threw a golf ball a distance of over 29 yards! We were the only all girls group in our class and we built the best trebuchet. We did a little bit of research and set out to construct our own and here is what we created!

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## Step 1: Materials

7 - 4 ft long 2x4"s
2 - 2 ft long 2x4"s
4 - 3 ft long 2x4"s with the ends cut at 45 degree angles
4 - swivel chair wheels
1 - 2 1/2' re-bar
an eye hook
4 ft of string
a sack

## Step 2: Building the Frame

1) Make a rectangle out of two of the 4 ft long 2x4's and the two 2 ft long 2x4's.

2) Take two more 4 ft long 2x4's and center them so that they lie flat in the center of the 2 ft long 2x4's. This will create the platform that the golf ball will slide along before being released.

3) Attach the swivel chair wheels to the bottom of the four corners of the frame. The wheels not only help with transportation but also allow more force behind the throw of the trebuchet.

## Step 3: Setting Up the Arm

1) Center two of the three remaining 4ft long 2x4's on each of the 4 ft long sides of the frame (not on the slide platform). Attach the boards vertically.

2) For extra support attach the four 3 ft long 2x4's to each side of both of the vertical 2x4's.

3) Drill one hole,  large enough for the re-bar to fit through, on each of the vertical boards, one foot from the top.

4) Drill a hole in the last unused 4 ft long 2x4. Make sure this hole is large enough for the re-bar to fit through, too. This will become the arm.

5) Place the remaining 4 ft long 2x4 between the two vertical boards, line up the three holes, and place the re-bar in.

6) In our design, we held the arm in place with zip ties, but I'm sure there are better methods you could think of that would cause less friction if you were to re-create this trebuchet.

## Step 4: Adding Weight and Sling

1) Attach the eye hook to the short end of the arm so it sticks out of the top of the board.

2) For our weight we used a laundry detergent bucket and just put the handle through they eye hook to attach it. That way we could easily vary our weight. The bucket also works well because the free moving handle allows the bucket to work with the law of gravity longer.

3) For the sling, cut the 4 ft of string into two 2 ft pieces. Attach one end of each piece to opposite sides of the sack. Attach a metal ring to the other end of one of the strings.

4) On the long end of the arm hammer in a nail at about a 45 degree angle into the end. This is where the string will release. We wrapped our nail in electrical tape so that the head of the nail wouldn't interfere with the release or the string.

5) Tie the string that is not attached to the metal ring to the top or the long end of the arm.

## Step 5: Launch!

1) Add weight in the bucket. The ratio we found that works the best is 133:1, for weight in the bucket to weight of the thing you are launching.

2) Pull all the way back on the long end of the arm.

3) Load the sling with what you want to launch - in our case, a golf ball. Make sure the the sling is pulled all the way back so the inertia of the throw will keep the ball in the sack. Attach the hanging side of the sling with the ring on the nail.

4) RELEASE! (and get out of the way of the arm ;) )

## Step 6: Results

Our trebuchet still needed a little tinkering on the launch day. I feel our data is not exactly representative of the potential of our trebuchet - if we'd had the time we would have adjusted the launch and point of release. Also I think we would add more to the weight to increase the velocity of the throw. But, that being said, I am still quite impressed with what it did throw. Our trebuchet threw a golf ball 29.3 yards and the ball had a peak acceleration of 292 m/s^2. Our trebuchet also threw the ball the farthest out of the rest of the class for trebuchets. Overall, this was a really fun machine to build. I think it worked better than anyone in my group thought it would and it was actually quite satisfying seeing a golf ball flung at high speeds for the first time, too :)

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## 19 Discussions

Where was the accelerometer mounted? Where could I get one? Congrats on your project. I think you did very well. I was surprised to read the not-so-nice commentary.

In step 2 you state that ..

"The wheels not only help with transportation but also allow more force behind the throw of the trebuchet."

what is the basis for the assumption that the wheels add to the force?

the only way I see that being possible is if you are rolling the trebuchet downhill at the time of launch, and even then the added momentum would be negligible unless it is an insanely steep hill.

6 replies

Wheels on the trebuchet let the weight at the end of the arm fall along a straighter path than the arc it would fall along without wheels, which pulls the trebuchet forward at the end of the swing, making it more efficient.

Upon reading Lorddrake's description of the Trebuchet rolling down hill I couldn't help but chuckle at the imagery it created in my mind. However the visual rocks and makes sense as to why the wheels would work! However I think I might see a flaw in this via their design. They stated they used swivel chair wheels which do not have a strong reputation for gliding in a straight line or doing so without resistance (aka "swiveling"). I think this type of wheel would throw the machine off if it were to have a strong enough counterweight. Perhaps the machine could rest on rails to travel from the front to the back in the swing and a spring attached to return it's momentum forward and again, increase the efficiency without losing any energy through the swiveling wheels. It would also negate sway to the left or right during firing which would increase accuracy substantially. But this was a cool siege machine! I build a small tabletop size one that I'll be making another version of and posting a build soon!

hi did you post your tabletop design you built? Would love to get a few tips and ideas - have to build one for physics project, there is a restriction on height and area of base - have built it but the range is not enough. I need a range of at least 100 feet, only getting half of that at the moment - height can only be 2 foot, and area of base restricted to 140 square inches

what type of wood did you use for it??? I am trying to build it for a school project so I need to know what type of wood it was made of.

cheers

The instructions for making this trebuchet were vague and often incorrect. The three-foot long 2x4s are too long and don't fit within the frame on the bottom. The supporting beams in the photo also dont look three foot long. Two and a half-feet or two feet would be more appropriate for the supports. The order for doing things is also really inconvenient. It makes a lot more sense to drill the holes and to add the nail/eye-hook to each end of the arm before attaching the arm to the other two four-foot long 2x4s with the rebar. Also, attach the supporting pieces of wood to the four-foot long 2x4s before attaching them to the base. The string lengths specified are also too short (and also much shorter than the picture shows). Finally, there are no instructions for how to create the sling. We basically made something up and our trebuchet didn't shoot.

Seriously nice job I'm a freshman in high school I have seen many but yours was the most professional and clean cut

I recently made a trebuchet for a robotics course to chuck a 30lb pumpkin. Using a 150lb trashcan my team only got the pumpkin 12 meters. Of course we made ours out of PVC held together with pneumatic-tubing!

2 replies

Your design is the reason for lack of range.
Instead of the smack against the stop bar, you should use a sling on a rail. The sling will give you the extra force and speed. Your system will shortly destroy itself because of the violent stop at the end of the shot. Properly built a trebuchet is a powerful and quiet weapon.

As for Kitman's responses... I don't think he was being overly rude, just frank. He's right, we should not be giving our children a rosy ride. Bullying isn't necessary, but the real world is rough. Your employer or the scientific community will not give you a pass because of your age or handicap. Accuracy counts. There is a lot of jealousy and frankness out there. Also people want their mention if you use their data. Don't ever forget to include your sources.

I think you guys did a great job! Question though... What did you use for a sack, and how large was it? Another thing: How far along the arm did you drill the hole for the rebar?