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The Little Trebuchet That Could

Science of a Trebuchet

Science of a Trebuchet
Building a Simple Trebuchet
Trebuchet Sketch
Construction Photos
Testing Procedures
Meaurements and Calculations
Beastly Changes and Discoveries
Birth of BEAST 2
beast 3
The Designers

Application of Physics Principles



Some physics concepts applicable to our trebuchet lab:

Projectile motion- applicable to movement of Christmas ball in air

Forces- a major factor in launching the projectile, the best ratio between applied force, air resistance, machine resistance must be determined to achieve best performance.

Kinematics- used to calculate projectile motion


The trebuchet is a first class lever. The force is applied to one end, the load is on the other end and the fulcrum sits between the two. Children playing on the see-saw is an example of the lever.  If a child on one end is allowed to fall from a high place all the way to the bottom through circular motion, the other child will be launched into midair (provided the first child is heavier than the second).  This is basically how a simple counterweight trebuchet works.  With the lever type trebuchet, the fulcrum can be adjusted to change the performance of the trebuchet.  If it is close to the weight and far from the projectile, it will require heavier weights, but it will also give great projectile distances.

The sling is another application of physics.  It is basically a piece of string tied to the projectile and attached to the arm, this gives the projectile extra hurling power coming off the arm.  The length of the sling can also be changed to maximize the performance.  The length of the sling forms the radius of the circle that the projectile travels through when the arm swings up and forward.  The formula v = 2πr/T says that velocity of an object traveling in a circle equals 2π multiplied by the radius divided by the time it takes to complete one revolution.  Therefore, if the period is the same, a larger radius will give a greater velocity, meaning the projectile will travel further.

Physics can also be implemented in calculation of the projectile’s speed and height during flight.  There are 5 equations that are used in the calculations:

D = (V2 + V1) t


D = V1t + at2


D = V2t – at2


V2 = V1 + at


V22 = V12 + 2ad

Given enough information, the maximum speed, height, and distance of the projectile can be calculated.

Newton’s famous 2nd law states F = ma.  

This basically says that force of an object = mass of object * its acceleration.  This formula is also applicable to the launch of the projectile.  For instance, knowing the velocity change of the projectile in air, we can calculate the acceleration (or deceleration) of the projectile as a result of air resistance.  If we use this along with the mass in Newton’s formula, we can see how much, in terms of force, the air resistance is.