Intro — Want a fun weekend DIY that actually teaches physics? Below you’ll get a simple, safe, step-by-step plan to build a tennis-ball catapult, plus materials, safety tips, tuning tricks, and expected ranges. It’s written for beginners and teachers — no weird jargon, just do-able instructions.
? Quick Highlights
- Build a small, safe tennis-ball catapult using plywood, a dowel, and elastic (7–10 tools).
- Start with a hinged-arm design (best for beginners) — it’s stable and easy to tune.
- Keep launches aimed at an open field and never at people — safety is priority (school project rules).
- Use projectile basics (angle ≈ 35–45° often gives good range) but expect air drag to change results.
- Typical ranges (approx): small popsicle/bench catapults ~10–30 ft, medium PVC/wood builds ~30–100 ft, large backyard rigs 60–200+ ft (depends on power).
- NASA and Science Buddies provide classroom-ready lessons and math if you want to measure speed/angle scientifically.
Table of Contents
- What design to pick (quick comparison)
- Materials & tools (for a beginner hinged-arm catapult)
- Step-by-step build: Hinged-arm catapult (measurements included)
- How to tune angle, tension, and measure range (simple physics)
- Safety checklist and rules for school projects
- Expected range table & testing notes (approximate)
- Quick troubleshooting & upgrades
- FAQ (6–8 helpful questions)
What design to pick (quick comparison)
Pick a design that matches your skill level and safety needs:
- Hinged-arm catapult — Best for beginners: simple pivot, elastic band power, easy to control. Good for classrooms and backyard demos.
- Small trebuchet (counterweight) — A bit more complex but smooth power delivery; great for demonstrating potential energy conversion.
- High-power PVC sling or cannon — Can reach long ranges but may be unsafe for kids without strict supervision — avoid for school projects.
Materials & tools (for a beginner hinged-arm catapult)
Materials (approx. cost: $15–$60 depending on quality)
- 1 sheet ¾” plywood (12″ × 18″) or two 1×6 boards for base
- 1 hardwood throwing arm (¾” × 18–24″ pine or hardwood)
- 2 side supports (¾” plywood triangles or 2×4 pieces)
- 1 hardwood dowel or metal bolt for pivot (½”–¾”, length ≈ 6″)
- Elastic power: heavy-duty rubber tubing, surgical tubing, or thick bungee cord (choose safer elastic rated for stretching).
- Small cup or sling to hold the tennis ball (plastic cup or carved wood)
- Screws, washers, nuts, and wood glue
Tools
- Drill + drill bits, screwdriver, saw (hand or jigsaw), measuring tape, pencil, sandpaper, safety goggles.
Note: Science-fair versions often use popsicle sticks or small kits; NASA/Science Buddies recommend starting with low-power materials for classroom safety.
? Step-by-step build: Hinged-arm catapult (beginner-friendly)
These measurements are a starting point — you can scale up later.
Parts to cut
- Base: plywood 12″ × 18″
- Two side supports (isosceles triangles): height 10″, base 6″ (cut two)
- Throwing arm: ¾” × 18″ wood strip (drill pivot hole 2″ from one end)
- Pivot dowel: ½”–¾” dowel or bolt, with washer on each side
Assembly steps
- Mark and cut side supports; sand edges smooth.
- Attach side supports to base, 9″ apart, leaving center gap for arm rotation. Secure with screws; reinforce with glue.
- Drill a ½” hole through both side supports at 3″ from top (center for pivot).
- Drill matching hole in throwing arm 2″ from the short end (this short end is the pivot side).
- Insert dowel/bolt through side supports and arm; secure with washers and nut so arm spins freely but without wobble.
- Attach a small cup or cradle to the long end of the arm to hold the tennis ball. Use screws and test fit.
- Attach elastic bands from the short pivot-side of the arm to the base (or anchor under base) so when you pull the long end down, elastic stretches. Use multiple bands for more power; keep bands at safe, non-surprising tension.
- Sand all edges, check fasteners, and make a safety stop (a block on base) to prevent over-rotation.
Quick build tips: keep the pivot tight but smooth, and start with just one elastic band — add more only after safe test launches.
How to tune angle, tension, and measure range (simple physics)
You can treat the ball as a projectile. The classic result (no air resistance) is maximum range happens near 45°, but real balls feel drag and spin so optimal angle is often a little lower for tennis balls. NASA classroom activities use the standard projectile equations to measure initial speed and angle; Science Buddies recommends measuring range experimentally and adjusting.
Simple tuning checklist
- Angle: start at about 35–45° for longest distance.
- Tension: add elastic bands one at a time; more bands = more speed but more stress.
- Release height: raising the release point a bit increases range.
- Aim: always aim across an open field; check wind.
- To measure initial velocity: measure range and angle, then use projectile equations or classroom worksheets (NASA provides simple calculators and lesson PDFs).
Safety checklist and rules for school projects
Safety is everything. Follow these rules before any launch:
- Wear safety goggles while testing and when near the launcher.
- Never aim at people, animals, windows, or fragile objects.
- Test at an open field (school yard, football field) with adult supervision. NASA classroom guides stress open-area testing.
- Keep fingers away from the pivot and elastic during cocking — pinch injuries are common in catapult builds.
- Use low-power elastic for kids projects (popsicle-stick or small rubber-band versions). Reserve heavy tubing for adult-built backyard rigs only.
- If used in a school fair, check the event rules — many fairs prohibit devices that can cause injury; science-fair pages give explicit safety notes.
? Expected range table & testing notes (approximate)
These are approximate ranges — actual results depend on arm length, elastic strength, angle, and wind.
| Build type | Typical materials | Approx range (typical) | Notes / source |
|---|---|---|---|
| Tiny classroom catapult | Popsicle sticks, rubber bands | 10–30 ft | Good for controlled indoor demos. |
| Beginner hinged-arm | Plywood, dowel, 1–3 elastic bands | 30–100 ft | Easy to tune for school projects. |
| Large backyard rig | 2×4s, heavy tubing, longer arm | 60–200+ ft | Powerful — use outdoors only and adult-built. Examples show >60 ft easily. |
Quick troubleshooting & upgrades
- Problem: ball wobbles or falls off — Fix: deeper cup or a small elastic sling to hold ball.
- Problem: no power — Fix: add more bands or rinse and replace old elastics.
- Want more range — Upgrade to longer arm, stronger (but safe) elastic, and fine-tune angle.
- Want more accuracy — Add a simple sight (aligned notch) and consistent release notch on the arm.
- Want a lesson — Use NASA or Science Buddies printable worksheets to measure speed, angle, and plot trajectory.
FAQ — quick expandable-style answers
▶ Q1 — Is building a tennis-ball catapult safe for kids?
A1 — Yes if you use low-power materials (rubber bands, popsicle sticks), supervise, wear goggles, and follow school-safety rules. High-power rigs should be adult-built and used outdoors only.
▶ Q2 — How far will my tennis ball go on my first try?
A2 — For a basic hinged-arm home build expect 30–60 ft with modest elastic; tiny classroom versions will do 10–30 ft. Results vary with design and power.
▶ Q3 — Can I use PVC and surgical tubing as the elastic?
A3 — Yes — surgical tubing and PVC-based builds are common, but they can be powerful. Use caution: secure all joints and test gently.
▶ Q4 — What angle gives maximum range?
A4 — The theoretical maximum without air resistance is 45°, but because tennis balls feel drag, many builders find 35–45° works best in practice. NASA classroom materials explain how to measure and calculate exact values.
▶ Q5 — Can I do this as a science fair project?
A5 — Absolutely. Science Buddies and NASA offer project templates that turn a catapult into a repeatable experiment (measure launch speed, angle vs. range, etc.). Follow their safety protocols.
▶ Q6 — Should I worry about air drag or spin?
A6 — Yes — drag and ball spin affect range and accuracy. For beginner experiments you can ignore drag roughly, but for more accurate results use NASA/University resources to include drag in calculations.
▶ Q7 — What’s the difference between a trebuchet and a catapult?
A7 — A trebuchet uses a counterweight to swing an arm (gravitational potential energy), while a catapult usually stores elastic energy (rubber bands, springs). Both launch projectiles but feel and tune differently.
▶ Q8 — Any quick lesson ideas to pair with the build?
A8 — Measure ranges for different angles and plot them, calculate initial speed from range and angle using projectile formulas, or compare energy stored in elastic vs. kinetic energy. NASA and Science Buddies have classroom-ready lesson PDFs.
