MULTI TASK TREBUCHETS IT’S ALL ABOUT ENERGY!

OBJECTIVES Distance - Throw a single projectile weighing a minimum of 40 grams as far as possible Accuracy - Throw a single projectile weighing a minimum of 40 grams as close as possible to two targets: Target 1 - 6.25 meters away with a radius of 125 centimeters Target 2 - 10.00 meters away with a radius of 200 centimeters Strength (high school only) - Throw as many projectiles as possible through a vertical target given two chances

KEY RULES Projectile= 40 g. minimum Counterweight= 1.50 kg maximum Rotating arm pivot above platform/ground= 60 cm Counterweight height above platform= 80 cm. maximum Throwing arm must be heavier than counterweight arm.

Trebuchet Overview What is going on when a trebuchet is used? A dropping mass (counterweight) causes a throwing arm (1st class lever) to rotate. The throwing arm has sling attached that holds another mass (projectile) that is much lighter than the counterweight. As the counterweight drops the throwing arm begins to drag the projectile along the surface and rotate around the point where it is attached. When the throwing arm rotates high enough the projectile leaves the ground and rotates around until it slides off of the attachment point (finger). It then flies at the angle it releases until it impacts the ground.

Factors influencing performance Multi-Task Trebuchet Challenge Exploring and Succeeding in Designing a Trebuchet Factors influencing performance Counterweight Projectile Rotating Arm Sling Design Release mechanism Friction Building techniques Potential Energy Work & Efficiency Newton's Laws of Motion Materials

Efficiently using as much of the available energy as possible is key Potential Energy How much energy is available from the counterweight? The potential energy is the product of the maximum height of the counterweight and its mass times gravity. PE = m g h Efficiently using as much of the available energy as possible is key

factor impacting performance Mass and Performance Since I can’t change the projectile, Why do I need to experiment with it? The size, shape & mass of the projectile influence performance. Lighter projectiles should fly farther given similar conditions. Projectiles move fast! Mass may not be the only factor impacting performance

Mechanical Advantage How does changing the length of the rotating arm influence performance? The relationship between counterweight and projectile begins with choices about this 1st Class Lever. This lever arm allows teams to control the mechanical advantage by the positions of the fulcrum between the “effort” or counterweight & “resistance” or sling attachment point. The ratio of the lengths of both segments determines the mechanical advantage Fsling = Fcw (---------------------------------) Length CW arm Length Proj. arm This relationship between the arm lengths is the mechanical advantage, a ratio less then 1 for most trebuchets

Friction Is friction a big factor for this type of project? Friction can waste the energy they need for throwing farther or more precisely. Again this simple device has a few areas where friction can cause problems The rotating arm is the most likely energy thief. Smooth and consistent! Check out different design ideas: More moving parts means more friction. Balance the advantages and disadvantages. Materials have differing abilities to slide against each other smoothly. Oil is an idea: but beware the end result isn’t sticky. It usually solve heat problems

Sling Length Combined with the release mechanism, sling type (attached or un-attached to the projectile), and sling length are important factors to explore. Within reason: increasing sling length delays release and decreasing sling length quickens release arm rotation sling rotation projectile Repeatability: Another place where friction might rob you of reliable performance

Release Mechanism Release mechanism? What do you mean? The sling is usually looped over an notch or pin at the end of the throwing arm. As the sling and mass swing around, the string reaches a point were the mass pulls the sling loose from the attachment point releasing the projectile. It’s important to find the relationship between the shape/angle of the attachment point and when the projectile is released. This goes hand-in-hand with sling design. arm rotation sling rotation projectile

COMPUTER SITES Google-trebuchet challenge (trebuchet game) Washington MESA- Model trebuchet plans for class study Mesa.ucop.edu/mesa_day dbraunst@csulb.edu