Tennis ball thrower that catapulted my team to the International stage

As part of the selection process to select the team to represent India at the FIRST Global Challenge, M/s STEM Education trust the organise of FGC in India launched a STEM challenge. The participating teams had the build a contraption to throw tennis ball the farthest without using using any external power source.

Your challenge is to design and build a device that can throw a tennis ball the farthest distance without using any external energy source. The ball-throwing-device can rely on principles of conservation of energy and must be built using only the materials specified below.

(1) Plastic pipe (such as PVC or electrical pipe) — max 2 inches in diameter, and 20 feet in total length. PVC Connectors (T and L shaped) may also be used, but for connecting the pipes only.
(2) Nuts and bolts (any size or quantity, as required)
(3) Newspaper (in any form, no limit on quantity)
(4) Cello tape or masking tape (max 5 rolls) and Glue (max 1 kg, any type)
(5) Candles and matchsticks (max 3 candles and 1 box of matchsticks)
(6) Thread (commonly available thread, non-elastic, max diameter 2mm)
NOTE: You cannot use any other material other than the 6 items above. You cannot use more than 20 feet of plastic pipe in total.

(1) The device must be capable of throwing a common tennis ball (such as this one). Other types of balls may not be used.
(2) The device should be capable of supporting itself independently without external support from any direction. It should not be bolted or fixed to the ground. The device should be portable and capable of quickly set up on any flat surface.
(3) After loading the ball on the device, a remote or automatic trigger is required to launch the ball. The only interface between you and the device is the trigger. The trigger should not add additional force to throw the ball. Any team that violates this rule will be disqualified. The trigger should be built using only the materials specified above. Be creative in building your trigger!
(4) The distance from the device to the point of first bounce of the ball on the ground will be considered as throwing distance
(5) Once the device has been set up to throw a ball, any forms of external energy (such as a human pushing or pulling it) cannot be supplied. Be creative in generating the energy required to throw the ball (hint: gravity is your friend).

Introduction to the challenge

In this challenge, we explored the principles of conservation of energy to develop an energy-efficient tennis ball launcher. By harnessing the potential energy available within the system and transforming it into kinetic energy, just by using nuts and bolts, T and L shape PVC connectors to connect the Plastic Pipes used in the module, using Newspaper in a bowl shape to hold the Tennis Ball, Cello Tape to wrap the ball holder so that we get more durability, Thread to connect the ball holder to the Swing Arm.

Based on the challenge we searched for information on the physics involved in the process of catapulting a ball and we found that the device to throw projectiles is called a Trebuchet and there are multiple variations in trebuchet designs based on various parameters

We initially planned to make a fixed-arm trebuchet and analysed the maximum distance it could travel on a virtual simulator.


We then found a design variation called the whipper trebuchet that further increases the throw distance

Design Process:

First, we were planning on bending the PVC pipes then attaching a ball to one end and then firing it. We realised however that this wouldn’t get us very far. So we started thinking of other designs.

Soon we came across the concept of Trebuchets. Trebuchets work on a lever principle and use a raised weight to generate potential energy which then gets converted to kinetic energy.

We started designing our first trebuchet on a piece of paper and came up with a rough design. We made modifications to this design as we learnt how to optimize our trebuchet, by adjusting the ratios of the lengths.

Working of the mechanism

Let’s understand how a normal trebuchet works, the weight is on one side and the ball is tied to a sling on the other side. The weight causes this side to lift, and the ball launches. This is how a typical trebuchet works.

Now our design is a whipper trebuchet so it’s a little special. In this design, the arm is a lot shorter and the weight is kept on top of the axle. In this design when released the arm falls in front due to the weight, spins around and then launches the ball at high speeds. This design is far more efficient as the weight is kept at a higher height and the arms spin a lot more generating more angular momentum.

Whipper Trebuchet

Materials Used to build the Whipper Trebuchet:
8 x PVC pipes of total length [total length 5.90mts]
PVC connectors- [11 x T-shape connectors and 1 x L-shape connector]
3 nos nut bolts are used to hold the PVC pipes together

6 nos NUT AND BOLTS used as weights weighing a total of 5 kgs

What is the Whipper Trebuchet?

The whipper trebuchet is a modern take on trebuchets, geared toward throwing lighter objects long distance. It is a hinged counterweight trebuchet in which the counterweight hanger is placed on the top of the arm and the arm is pointed in the forward throwing direction when cocked.

Another important thing is the sling. It is a piece of thread that connects to a holding pocket. This holding pocket is attached from both ends. One of the strings is fixed to the pole and the other is looped and kept on the pole. When the mechanism is spinning the looped string is pulled off and the pocket opens up releasing the ball.

Advantages of Whipper Trebuchet

  • Gravity is the main power source
  • Increased potential energy
  • Long acceleration path
  • Excels with lightweight objects
  • More “drop” more energy

The mechanics of a trebuchet involve various factors that influence its performance. Some important considerations include:

Counterweight: The mass of the counterweight affects the projectile’s velocity and range. A heavier counterweight typically leads to a higher launch speed.

Arm length: The length of the throwing arm determines the leverage and angular acceleration of the trebuchet. A longer arm can generate more power but may reduce accuracy.

Projectile mass: The mass of the projectile affects its range and trajectory. A heavier projectile generally travels a shorter distance but may have more impact.

Release angle: The angle at which the projectile is released influences its trajectory. Different angles can optimize distance or height, depending on the objective.

Friction and air resistance: These factors can influence the actual performance of a trebuchet and may need to be considered in more advanced simulations.

Finally we managed to build the award winning trebuchet that managed to launch the tennis ball to a distance of 27 mts






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