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Car Flinger science
 Energy transfer and acceleration
The trebuchet
The catapult
Pros and cons |
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Energy transfer and acceleration
Throwing an object is all about energy transfer and acceleration. Energy has to be transferred from the launch system to the projectile (object being thrown), accelerating it to as high a velocity as possible. When maximum velocity has been reached, launcher and projectile must part company in such a way that the projectile continues on its way unhindered. This may sound high-tech, but it's what happens when you throw a cricket ball, for example.
The principle is the same for throwing a Mini. The problem facing the teams is that, at around 350kg, a Mini is quite a bit heavier than a cricket ball. So they need to generate a lot more energy.
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| A trebuchet consists of a beam, a counterweight and a sling |
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| When the sling and the beam come into alignment, the Mini should release ... |
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| ... otherwise it could crash back down on the trebuchet |
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The trebuchet
The Barley Pickers opt to build a trebuchet – a medieval siege engine used to hurl projectiles (rocks, dead horses or people's heads, for example) over or at castle walls. The name comes from the French verb trébucher, meaning to lurch or fall over, and this is a fairly good indication of how it operates. A trebuchet consists of a lever and a counterweight. The potential or stored energy in the counterweight is converted into kinetic or movement energy as the weight falls towards the ground. The lever multiplies this kinetic energy. This is known as mechanical advantage.
A lever is simply a bar or rod that tilts on a pivot or fulcrum. If you apply a force by pushing or pulling on one part of the lever, the lever swings around the fulcrum to produce useful action at another point. The force applied (by means of the counterweight) is called the effort; the weight raised or resistance overcome is called the load.
In the Barley Pickers' trebuchet, a lever or arm made from a telegraph pole is supported by a huge frame and joined at one end to a 4-tonne counterweight and at the other to a sling looped around the Mini. The counterweight is hoisted high into the air and released. As it falls, it swings the arm on its pivot, first dragging the sling along a launching trough, then lifting it skyward. One end of the sling is firmly tied to the end of the arm; the other is looped over a release hook. When the sling and the beam come into alignment, the sling slips off the hook and the projectile is released. The point of release (and thus the trajectory) can be adjusted by bending the hook forwards or backwards.
If the fulcrum is located in the middle of the arm, as one end goes down, the other will move up by the same distance and both ends will be travelling at the same speed – rather like a see-saw. Mechanical advantage in a trebuchet derives from the fact that the fulcrum is offset to a position approximately a third of the way along the arm from the counterweight (two thirds from the other end). This means that the projectile is travelling twice the distance as the counterweight in the same amount of time – in other words it's moving twice as fast. The sling further increases the distance of the projectile from the fulcrum, boosting acceleration even more.
The Barley Pickers' trebuchet is easily the biggest machine ever built on Scrapheap. Its base frame is about 6 metres square and the arm reached up to a height of more than 10 metres.
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| The elastic will provide maximum energy if it is stretched to six times its length. So for a 9-metre ramp ... |
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| ... you want 1.5 metres of elastic |
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The catapult
The Cat-alysts opt to build a giant catapult, which uses elasticity rather than weight as a source of energy.
Their catapult is built around a trailer bed extended to make a ramp nearly 12 metres long. One end is raised to an angle of 25 degrees and two elastic bungee ropes are attached to this raised end. The other end of the elastic is attached to a trolley or dolly which carries the Mini along a track that runs the length of the ramp. The dolly is pulled back to the bottom of the ramp and released. At the end of the ramp, it hits a buffer and stops dead; the Mini's momentum means it carries on and shoots off the end of the ramp. (See Blind Navigation science for more about momentum and Clockwork Cars science for more about elasticity and potential energy.)
The point of the track-and-dolly system is to keep friction to a minimum. Friction acts against the accelerating mass and would slow the Mini down. To further reduce friction, the Cat-alysts lubricate their track with washing-up liquid.
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| The trebuchet is a tried-and-tested design. It has been around since medieval times. |
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Pros and cons
The Cat-alysts
The bungee cords of the Cat-alysts' machine can store lots of energy
There are few parts to fail.
On the other hand ...
The cords must be released immediately, otherwise they will lose their elasticity. They must also be exactly the same size and strength, or the dolly and projectile will veer off to one side.
The angle at which the ramp can be set is limited, restricting the amount of lift to the projectile.
Friction between the dolly and the track could be a problem – as could keeping the dolly on the track.
The Barley Pickers
The trebuchet is a tried-and-tested design making excellent use of basic engineering principles.
On the other hand ...
It's massive – a real mega-build.
They must get each of the variables – weight of counterweight in relation to projectile; length of arm; position of fulcrum; angle of release hook; length of sling – absolutely right. Otherwise the Mini could end up going backwards or come crashing down on top of the machine.
The whole structure will be under enormous strain when in operation.
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