7th Grade Physical Science Model: Trebuchet

Physical Science Content: Conservation of Energy

Objective: Students in pairs will investigate and apply the principle of Conservation of Energy by building and working with a Trebuchet.

State Standards: South Dakota 7th Grade Physical Science Standards number 5: Students will apply the Law of Conservation of Matter and Energy to investigate and understand changes in matter.

Other standards that could be covered using a Trebuchet:

South Dakota 7th Grade Physical Science Standards:

Students will:

  • apply the Law of Conservation of Matter and Energy to investigate and understand changes in matter. (Standard 5)
  • investigate and describe scientific principles and technological applications of work, force, and motion. (Standard 8)
  • describe mechanical advantage in relation to work, force, and motion. (Standard 9)
  • identify Newton’s Laws of Motion. (Standard 11)

Cost:

Material: / Cost per unit: / Cost per Trebuchet: / Total Cost of Class of 20:
¼ in Round Wooden Dowel / 91.44 cm for $0.29 / 27.5 cm for about $0.09 / 275 cm for about $0.88
½ in Square Wooden Dowel / 91.44 cm for $1.00 / 53.5 cm for about $0.59 / 535 cm for about $5.85
Wooden Popsicle Sticks / 1000 for 1.27 / 76 for about $0.10 / 760 for about $0.97
Wood Craft Glue[1] / 1 bottle for $1.87 / 1 bottle for $1.87 / 10 bottles for $18.70
Screws / 100 for $2.50 / 4 for about $0.10 / 40 for about $1.00
Washers / 100 for $3.00 / 27 for about $0.81 / 270 for about $8.10
Nuts / 50 for $1.25 / 1 for about $0.03 / 10 for about $0.25
Gobstoppers / 100 for $2.00 / 2 for about $0.04 / 20 for about $0.40
Totals: / $3.36 / $36.15

Popsicle Trebuchet

Student Materials (Students should work in pairs):

  • 70 wooden popsicle sticks
  • 2 square dowel pieces 11cm long
  • 1 round wooden dowel 16cm long
  • 4 long black screws
  • 27 metal washers
  • 1 nut
  • 1 round wooden dowel 4cm long
  • 1 round wooden dowel 7.5cm long
  • 1 pre drilled square wooden dowel with 5 holes 31.5 cm long
  • Carpenter’s wood Glue
  • 1 piece of Light Weight Fabric
  • 1 needle and thread
  • 2 pieces of heavy string 18cm long each
  • 2 Gobstoppers
  • 2 pre-drilled top pieces
  • Ruler with the metric system
  • 2 scissors

Extra Materials for Teacher Only:

  • Electric drill
  • Drill bits
  • 4 handheld electric screw drivers
  • 2 handheld screw drivers
  • Small Handsaw or electric saw (not needed for class, but need to prepare parts)
  • Area to use to test Trebuchets such outside in open area or in open area of the class
  • Meter Sticks and Tape Measures for testing Trebuchets
  • 5 small hammers

Before Class Prepare:

Cut one for each Pair of students:

  • 16 cm long round wooden dowels
  • 4 cm long round wooden dowels
  • 7.5 cm long round wooden dowels
  • 31.5 cm long square wooden dowels

Cut two for each Pair of Students:

  • 11 cm long square wooden dowels

Drill large enough for the round dowels to fit into:

  • Sandwich three popsicle sticks together in the formation used to create the vertical arm found in step 2, drill hole at the end where the single stick is above the rest, cut off excess stick at the top (students will need two each pair of students)
  • At one end of the 31.5 cm long square wooden dowel drill a hole in the end (See figure 1)
  • At the other non-drilled end of the 31.5 cm long square wooden dowel drill another hole about a half a centimeter in from the end (See red hole in figure 2)
  • Drill three holes the first one about 8 cm from the end of the 31.5 cm long square wooden dowel. Then drill two more holes 1 cm apart from each other going away from the red hole (See Figure 2).
  • Drill two holes at the ends of each 11 cm long square wooden dowel (See figure 1).

How to make a Popsicle Trebuchet:

Step 1: Gather all materials.

Step 2: Take three Popsicle sticks and sandwich them together leaving a Popsicle stick width between the sticks using the carpenter’s wood glue. It should look similar Figure 3 when completed. Repeat to create another.

Step 3:Take five Popsicle sticks and sandwich them together using the carpenter’s wood glue, leaving a Popsicle width gap in between the sticks. It should look similar to Figure 4 when completed. Repeat to create another.

Step 4: Using two Popsicle sticks fill in the blankholes created in Step 3 and glue them in place. See red arrows in Figure 4 for placement of sticks. Do this for both sets of sticks. Cut off the excess Popsicle sticks.

Step 5: Glue together pre-drilled top pieces onto the sticks you created in Step 2. Make sure to glue them as straight as possible. Do this by inserting the top piece tip of the stick to the base of the other to created one long stick.

Step 6: Glue the base of the sticks you created in Step 5 to the base the gap you of the stick created in Step 3. It should create a shape that looks like an upside-down T. Make sure that the two pieces are square and create a 90-degree angle where they join. Repeat to create an identical side. See Figure 5.

Step 7: Glue another Popsicle stick over the joint that you just created on each piece. Center the Popsicle stick as best as you can, make sure that the bottom of your of your T that you created is level because this will become your base. Look at the red rectangle in Figure 5 to see where to glue it. Do this to both sides of each T and repeat for the other T.

Step 8:Now you are going to add some supporting Popsicles. Glue one of the ends of the Popsicle stick to on the left side of the stick you glued on in Step 7. Do not glue it onto the stick but just before it. Glue the other end of the stick about 2.5 cm from the top of the lower stick on the T shape. See Figure 6 to get a better idea of what it should look like. Again, make sure that the stick is flush with the bottom of the T, since it is going to be the base of the Trebuchet. Repeat to the other T Shape, then flip over your T shape and do the same to the other side.

Step 9:Now you have to build another longer brace to support the top part of your T shape. You will do this using four Popsicle sticks per brace. Cover one stick with glue and place two of the sticks on top of the one stick, centering them as best as possible. Then cover the remaining stick with glue and glue to the other side. See Figure 7 to get a better idea of what it should look like. Repeat this step to create a total of 4 large braces.

Step 10: Now it is time to install the large braces. Glue one end of the brace to the far left end of the base. Glue the other end of the Popsicle stick about 3cm from the top of the upright. See figure 8 to get a better idea of what it should look like. Repeat this process to the other T then flip it over doing the same thing to the other side.

Step 11:Now that you have completed your bases, it is time to fill in the missing areas to install more braces. Cut Popsicle sticks to size to fill in areas where the braces are located. See red pieces in figure 9 to see where to place the fillers. There should be four additional pieces on each side of the braced T shape. Do not cover any of the ends of the braces, but get the pieces as close to the braces as possible.

Step 12:Now that you have filled in the gaps, glue an additional full Popsicle stick over the areas that you just glued to give them additional support. See blue outlines rectangles in Figure 9 to get a better idea of where to place the additional supports. There should be a total of four full-sized Popsicle sticks that will be added to each braced T shape.

Step 13:Now that you have completed your two sides. Go to your teacher and ask her to drill a hole at the end of your braces and ask to use an electric screwdriver.

Step 14: Now that you have holes in your bases, connect them together using your four screws, have your partner help you hold the square dowels in straight and use the electric screwdriver to carefully screw in the screws. Connect the two braces together so they look identical when you look at them. You do not want them to be the mirror image of one another. When you are finished with the screwdriver return it to the teacher.

Step 15: Now you have completed your base of your Trebuchet, it is time to complete the arm. Use the hammer to carefully pound the dowels into the arm. Use the shortest dowel to pound into the very end of the arm, it should stick straight out from the arm and should be very secure, if it is loose, remove the dowel and put in a drop of glue.

Step 16: Now take the middle-sized dowel and pound it carefully into the single hole at the end of the arm. Make sure that the dowel does not come all the way through the arm, but is flush to the bottom of the arm.

Step 17: Stack the washers on the dowel that you added in Step 16 and top with the nut by carefully screwing it onto the top of the dowel.

Step 18: Now that you have your arm, you must create a pouch for your Trebuchet to throw from. Cut a 7cm by 7cm square of fabric.

Step 19: Fold fabric square into a triangle. Measure 3 cm on the folded edge of the triangle and fold corner of fabric in and stitch carefully with the needle and threat about half way up the folded edge that you just created. Repeat on the other side of the fabric square.

Step 20: Cut a small slit into each corner of non-sewed side of the pouch that was created. Carefully tie one of the 18 cm’s of string to each slit. Tie the other end of the strings over the end of the arm that doesn’t have the washers on it, making separate loops for each string.

Step 21: Carefully push your remaining round dowel through the top of your Trebuchet base using the holes at the top, before sticking the dowel through the other side, side your arm onto the dowel using one of the three holes. Have your partner hold onto the arm while you insert the other side of the dowel through the other hole in the base.

Step 22: Have your partner hold the end with the washers, making sure the washer are on top of the arm while you putt the loops on the pouch over the end of the dowel and insert your gobstopper.

Step 23: Go to the designated throwing area and test out your Trebuchet. To use it, push the pouch underneath and push the arm with the weights in the air down and then let go and stand back. Watch how far your Trebuchet throws the gobstopper.

Assignment (Please write these on a separate sheet of paper):

  1. Hypothesize how far your Trebuchet will throw the gobstopper.
  2. Test your Trebuchet at least 3 times observing what the Trebuchet is doing while it is throwing. Measure the distance of either the pouch or the gobstopper or both from the Trebuchet. Write your observations and your test data.
  3. Answer these questions:
  4. What causes the Trebuchet to throw the gobstopper?
  5. Where does the energy go when the Trebuchet throws the gobstopper?
  6. What principles could a Trebuchet represent?

[1] Students will not use all of the glue, if project is done multiple times, glue will be a one-time cost.