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 Energy
Elastic
Flywheels
Momentum
Pros and cons |
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| A spinning drum has kinetic energy |
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Energy
For a car to move, it needs a source of energy. Cars with petrol engines get their energy from the fuel they burn. Electric cars get theirs from chemical energy stored in the batteries. Our clockwork cars run on a form of energy rather unusual for vehicles – mechanical energy.
The Jailbreakers' rubber-band car uses potential energy. This is 'stored' mechanical energy that provides power when it's released. For example, a stretched catapult, poised to fire, has potential energy (the potential to do damage, if you like). When you release the stretched catapult elastic, it pings and all that potential energy turns into 'movement energy', sending the projectile hurtling through the air. It's basically the same with the rubber-band car.
The Young Ones' flywheel car uses kinetic energy. Kinetic energy is another term for movement energy. A moving object has energy, and the faster it's moving, the more energy it has. Also, the heavier the object, the more energy it has. So the big idea with a flywheel is that the faster the wheel spins and the more it weighs, the more energy it will be able to output. Lots of energy in the spinning flywheel can be turned into lots of movement energy on the road.
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| The Jailbreakers are using elastic energy |
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| In the Jailbreakers plan the stretched rubber rotates round a spindle |
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| Hooke's Law holds as long as the elastic doesn't snap |
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Elastic
The word 'elastic' doesn't just refer to the stretchy rubbery stuff that holds up our pants. It's also a scientific word used to describe an object that changes its shape when a force is applied to it and then springs back to its original shape when that force is removed. We say that the object has elastic properties.
Robert Hooke was a British physicist, born on the Isle of Wight in 1635. When he was 25 he discovered a law of physics that describes and predicts how elastic objects behave – it's known as Hooke's Law. The law simply states that the stretch of, let's say, a spring, a piece of elastic or a bit of rubber is directly proportional to the pulling force acting upon it.
So, if you apply a pulling force to the spring, elastic or rubber it will stretch by a certain amount. If you double that force, you'll double the amount of stretch. Tripling the force will triple the stretch, and so on. In other words, the force and stretch are proportional to each other. This law applies as long as the object isn't stretched so much that it snaps or is damaged.
Some materials are really springy and stretchy, others are much tougher and harder to stretch. Materials that can be stretched a lot are described as having a 'low spring constant'. Materials with a high spring constant don't stretch very much.
The Jailbreakers' rubber-band car works because as the stretched rubber returns to its original length it rotates a spindle in the process. To do this effectively they need really stretchy rubber that creates lots of movement as it springs back. So they need to find some rubber that has a very low spring constant.
Rubber-band power is used quite a lot by model aircraft enthusiasts. But unlike the Jailbreakers' stretched rubber-band machine, they twist up lengths of rubber. Very expensive rubbers are used to get their propellers spinning. The problem for the aircraft flyers, as for our teams, is that rubber doesn't store much energy for its weight. For the flyers, it's all about how many 'turns' you can twist into the rubber. Also, there's a trade-off between a short fat motor that gives a brief burst of high power versus a long thin motor that gives a prolonged output of low power. Ideally, the Jailbreakers would like a bit of both.
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| The Young Ones use a washing machine drum as a flywheel |
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| The tricky part is releasing the power: a clutch is needed |
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Flywheels
The flywheel was developed by James Watt while working on his steam engine invention in the 18th century, and to this day you'll find one in every single petrol engine. The flywheel is a heavy rotating disc that's used to store kinetic energy and to provide a smoother output from the engine.
Flywheel trams use a flywheel to smooth the erratic demand for power that a passenger vehicle has – lots of stopping and pulling away again. Heavy vehicles need a large engine to get going but not much energy to keep going. The idea is to use a smaller engine and get a power surge from a flywheel when you need it. Flywheels are also used in record players and tape decks to maintain a steady playing speed.
The Young Ones exploit the tendency for flywheels to keep revolving once spun up to speed. It's easy to imagine that a fast spinning wheel will rotate for longer than a slow spinning wheel. But it's also true that, if released at the same speed, a heavy wheel will spin for longer than a light wheel, because it has more momentum.
The tricky part is getting the energy out of the flywheel, and in this case, transferring it into the stationary wheels on the bike to propel the vehicle forward. To do this the Young Ones will need to manufacture a clutch and a gearbox. A clutch uses friction to transmit the rotation of the flywheel to the gearbox and then on to the wheels.
Technical consultant Clive suggests using a clutch with a special kind of gearbox known as a fusee to get the wheels turning. This consists of a conical tube (like a traffic cone) with a spiral grove around it. It forms part of the rear axle and is in direct contact with the rear road wheels. A cord is wrapped around the cone and attached to a tube, which is attached to the clutch, which in turn is connected to the flywheel.
When the clutch is engaged the tube connected to the clutch will begin to turn, this causes the cord to start winding around the tube and off the cone. The flywheel energy is used to pull out the cord, like a whip-and-top toy, and turn the road wheel. When all the cord is unwound from the cone, the cord simply detaches from it. This allows the rear wheels to coast along. To recharge their car, the Young Ones must wind the cord back around the groove on the cone and start the process again.
The conical shape is important because it creates a series of continuously changing gear ratios. The cord is unwound from the wide end of the cone, which makes getting the wheels turning in the first place much easier. As the vehicle picks up speed the cord is pulled off from a progressively narrower part of the cone and the gear ratio is reduced. This helps the car pick up speed and makes the most of the flywheel energy. It works just like the gears on a pushbike except the gear ratio changes automatically as the diameter of the cone decreases.
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Momentum
'Momentum' is the amount of motion that a moving object has. It can be defined as 'mass in motion'. All objects have mass – mass is the amount of matter that makes up an object. So if an object is moving, then it must have momentum, as it has mass in motion.
The amount of momentum that an object has depends upon two variables – the mass of the object that is moving and how fast that object is moving. As an equation, the momentum of an object is equal to the mass of the object multiplied by the speed the object is travelling.
So the greater an object's mass and/or the greater its speed, the greater will be its momentum. Both variables are of equal importance in determining the momentum of an object. Consider a truck and a roller skate moving down the street at the same speed. The considerably greater mass of the truck gives it considerably greater momentum.
You would have thought then that the clockwork cars need to be as heavy as possible so they have lots of momentum to keep them rolling as far as possible. Well, the problem is that heavier vehicles take more energy to get them going in the first place because they can't accelerate very quickly. Our teams won't be able to provide the energy to get a heavyweight clockwork car off the starting blocks.
As heavy vehicles need more energy to get them moving, our teams will want to build lightweight machines. Tubular materials are a good construction choice as they are hollow and so obviously weigh less. Aluminium is a good material as it's light yet strong, though it's not always as easy to work with as steel.
Our challengers will want to avoid eating a whopping fry-up on test day because the weight of the rider will also bump up the overall weight of the vehicle.
A bit of a problem for the Young Ones to solve concerns the size of their flywheel. A big flywheel can store more energy than a small one, but it will also weigh more of course.
Another consideration for our teams is 'rolling resistance'. Rolling resistance is a mixture of things, including how stiff the wheel bearings are (which affects how freely the wheel will spin) and the amount of friction between the wheels and the road.
A vehicle with a lot of rolling resistance requires more energy to move. So to reduce the friction, the teams will want to pump up their tyres really hard and will probably want to use really skinny wheels, perhaps those off a racing cycle. They might also opt for a design that runs on three wheels instead of four. Though the downside to that is a less stable vehicle.
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Pros and cons
Jailbreakers
The Jailbreakers quickly come up with a relatively simple stretched rubber-band design and stick to it throughout the build day. When they test the machine in the build area it looks as if it works very well. Their machine is light, which is very important. It's designed to deliver power in short bursts and then be quickly recharged.
On the other hand ...
The rubber that the Jailbreakers use isn't ideal. It's very old and loses its spring quickly. This means that each time they recharge, less and less energy is being stored. Perhaps if they had scavenged a bit harder they may have found something more resilient and better suited to the task.
When the Jailbreakers test their machine during tinkering time the chain snaps. This is because the chain and sprockets they use aren't really strong enough to cope with the energy stored in the rubber. It's too late for them to change them so the only alternative is to take some of the rubber off. This means they store less energy and consequently don't travel as far on each wind-up of their machine.
Young Ones
Flywheels are a very efficient means of storing energy that should be capable of generating lots of speed and distance. The Young Ones' design is based on some very sound theory. Even though they don't have time to execute the design exactly how they planned it, the finished article is good enough to outrun the Jailbreakers. Although the flywheel takes a while to fully charge up, once it's spinning it delivers power for longer than the Jailbreakers' device.
In effect, the flywheel is a large gyroscope. Consequently it has a tendency to generate what's called 'gyroscopic inertia' in their vehicle – that is to resist changes in direction. Despite concerns about the gyroscopic effect making steering difficult, the Young Ones passed this stage of the test with flying colours.
On the other hand ...
Technical consultant Clive's plan to build a flywheel from a washing machine is certainly novel. However, it was quite a tricky thing to get right and took lots of time. A simpler solution could have been to use a car or truck wheel instead. They have the advantage of being well balanced and are already mounted in good bearings. This would have left them more time to concentrate on the really tricky part of building a flywheel-powered car – the fusee or clutch.
The Young Ones had to settle for a much simpler version than Clive had hoped for. This meant the energy was 'dumped' onto the bicycle wheels very quickly rather than gradually releasing the energy as they travelled along. This meant they didn't travel as far on each recharge as a proper fusee would have let them.
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