Catapults were used in Medieval times to hurl rocks at enemy fortresses.
The main parts of a catapult. Fire!

Fast forward to about 2:30 of this clip from Gladiator to see a catapult throwing its flaming cargo at the enemy.

A trebuchet is a specialized catapult with a sling attached to a length of rope at the end of the catapult to "whip" the payload even further.
No one has yet been able to build a working trebuchet, but maybe that will be you!

Today we will focus on building our own catapults to launch marshmallows across the room.

The first thing to do is to build your robot catapult / trebuchet. You will need to include the following parts on your catapult:
  • a pouch to hold the payload
  • a strong, stable body that will will hold the pouch and not fall over
  • an arm with a key that slips off when it's time to release the payload
  • motors to winch the arm back to shooting position
  • sensors to start and stop the throwing motion (like the guy yelling "Fire!" in the Gladiator clip)

Next, make a NXT .rbt program to control the catapult. Your program will need to include:
  • controls for the strength and duration of the motor, answering these questions:
    • how do you control the amount that the motor brings back the arm?
    • how powerful does the motor need to be?

  • sensor controls for firing the catapult:
    • how will you control pulling back the pouch into a position to load it?
    • how will you control the firing mechanism?
      • popular options for both of these are either a touch sensor (press the button) or the sound sensor (ready -- fire!)

Here are your marshmallow challenges:

First Challenge: Going the Distance

Build a catapult that can throw the marshmallow as far as you possibly can. We will keep track of personal bests!

Second Challenge: Precision Shot

Three cups of varying sizes will be located in a straight line. Program your catapult to throw a marshmallow from the start line into each cup, using the same program!

Final Challenge: Ping Pong Ball Thrower

For this challenge, we will borrow an interesting challenge from Dr. E's Mindstorms Challenges: a Ball Thrower. From the webstite, your goal is to "design a machine to throw a small plastic ball as far as possible. Or as high as possible. Or as accurately as possible. The choice is up to you. In your submission, tell us which goal you designed your launcher for–distance, height, or accuracy—and show us how it works (but be sure to aim it away from people)!" For our purposes, we will be using whiffle-style ping pong balls.

Student Examples From Previous Leap Camps

Other Examples of Catapults for Inspiration

On his amazing website full of Mindstorms tutorials, Dr. Graeme offers the following rules for a marshmallow contest:

Robotic Mediaeval Marshmallow Throwing Contests – Rules
Believe it or not, there are standard rules for this type of contest! Some of the rules that are often used are listed below.
The purpose: To throw your marshmallow as far as possible.
The arena: A start line on any bit of floor that does not mind marshmallows being scattered over it.
Making the robots: You can use bits from your MindStorms kit to build a robot powered only by Lego motor(s) - plus about a metre of string or fishing line if you want to make a trebuchet instead of a catapult.
The start of a bout: The robots will be placed behind the start line.
During the contest: Once a marshmallow is launched, your machine must stop moving by itself. If your machine falls over during or after throwing your marshmallow, you will lose that contest.