Public summary: 

Fire bean-bag bunnies across the room using our trebuchets! Can you work out how to make them go really far?

Using a medieval inspired siege-weapon to launch bean bag bunnies
Useful information
Kit List: 

2 x collapsible trebuchet
4 x bean bag rabbits (mostly missing, have been replaced by rice in balloons)
(all in a big red box)
bolts (with wingnuts) to assemble trebuchet in plastic tub
release pins (also in plastic tub)
Note for future: An artificial soft surface (e.g. piece of roll mat) would lower the wear rate of the weights, esp. on tarmac or dry mud. If they start leaking nuts during a session, repair with gaffer tape!

Packing Away: 

Dismantle, and store the wing nuts in the plastic tub provided. Place in box. Weights at the bottom is generally a good plan.

Frequency of use: 

New for 2010.

The Challenge:

Split the group into two teams. This could be two groups of children at schools or a child and their parents at events.
Each team gets a trebuchet.
Compete to try and launch the bean bag the furthest or to get the bean bag into a box if space is limited.

Optional challenge to beat the day's record.


The trebuchets can be modified in two main ways, the pivot position and and the angle of release.

The optimal pivot position seems to be the third or fourth hole from the weight (don't tell them this). With the weight close to the pivot, it cannot accelerate the arm fast enough. This can be explained via the analogy of opening a door via pushing it near the hinge, which is much harder than doing it normally. However, with the mass too far from the hinge, the acceleration of the arm is greater, but speed of the bean bag end is small as it is now too close to the pivot. This time, the structure of the trebuchet also needs to be taken into account, as the weight hits the ground before completing the full swing. As it is not good at either end, the best position needs to optimise the balance between these two ideas.

The angle of release can be altered by the position of the spaghetti spoon launcher. The best launching angle should be 45 degrees, or a bit less accounting for air resistance. However, it will not necessarily fire at the same angle when the pivot position changes so this may also have to be adjusted.

Some of the bean bags have a string attached, which act as a sling to launch the projectiles further. At the moment that the bean bag leaves the trebuchet, it is swinging relative to the arm, so its relative speed adds up with the normal launch speed. Show them this after they have tried it normally.

Things to talk about:

* Levers: the more the distance to the pivot the less the force, but the greater the distance moved. It might be worth starting off with a brief explanation of how levers work using an arm.
* Why it's best to have the projectile far from pivot.
* Angle of release: 45 deg should always be best for range, neglecting wind. How can we control the angle? - the rabbit is ejected from the trebuchet when centrifugal force overtakes any force the holder exerts on it. (Centrifugal force gets bigger as the arm accelerates through the swing.)
* Using a sling to get extra velocity.

(more suggestions welcome)

Risk Assessment
Date risk assesment last checked: 
Thu, 16/01/2020
Risk assesment checked by: 
Date risk assesment double checked: 
Mon, 20/01/2020
Risk assesment double-checked by: 
Beatrix Huissoon
Risk Assessment: 
Hazard Risk Likelihood Severity Overall Mitigation Likelihood Severity Overall
Projectile-carrying arm Injury from projectile-carrying arm flying up and hitting audience member or demonstrator in the face. This is a bigger problem when the weight is far from the pivot, as the arm flies up with large force. Also has the potential to be especially nasty if pointy bits of spaghetti spoon hit someone in the eye. 3 4 12 Instruct kids and demonstrators to fire the trebuchet by standing to one side. Do not let them stand over it and put their face in the way.
In the event of an accident, call a first aider.
2 2 4
Weights Injury due to weight dropping suddenly and hitting audience member or demonstrator. 3 2 6 Weight - has been made from nuts - no sharp edges and will deform on impact. Make sure small humans stand out of the way of the weight as it drops. Also don't let them be thrown/launched.
Call first aider if required.
2 2 4
Projectile Could hit audience member or demonstrator and injure them. 4 1 4 Projectile - light bean bag, trebuchet designed to have limited range and velocity, and will not hurt on impact, even at high speeds. Make sure people do not stand in front of the trebuchets when firing. Avoid busy areas/thoroughfares. If the area happens to get busy mid-demonstration, either ask people to evacuate the 'smash zone' or wait until they have moved.
Call first aider if required.
3 1 3
Arm and support gap Trapping fingers between arm and support when the trebuchet is fired. 2 2 4 The supports are solid to reduce possibility of fingers being in way of arm. Given the launch position, it is highly unlikely that the person discharging the weapon will be at risk of this. Therefore, one must be aware of anyone else who may get in the way.
Call first aider if required.
2 2 4
Spare projectiles Spare projectiles may act as a trip hazard on floor. 2 3 6 Ensure all spare projectiles are near to the experiment, and don't end up in walkways. Preferably keep them in the box when not in use.
Call first aider if required.
1 3 3
This experiment is sometimes run outside during CBS!, see separate risk assessment.
Publicity photo: 
Experiment photos: