General > General Technical Chat
Mess with your minds: A wind powered craft going faster than a tail wind speed.
electrodacus:
--- Quote from: IanB on September 18, 2021, 04:25:16 am ---2 W on the motor is more than enough to move the vehicle from left to right. So if you have 10 W from the generator, you have 2 W to turn the motor and 8 W spare to overcome friction and other losses. Since 2 W is much less than 10 W, there is no problem with conservation of energy.
Remember that it takes no power at all to keep the vehicle stationary. All you have to do to achieve that is to lock the motor axle so the wheels cannot turn. If it takes zero power to keep the vehicle stationary, it follows that any power greater than zero can move it to the right.
--- End quote ---
I think is best to mention what you are seeing in my slow motion video https://odysee.com/@dacustemp:8/wheel-cart-energy-storage-slow:8
If you want to take everything as it is there no modified reference frame then what it represents is vehicle driving against the wind direction with wind being the moving piece of paper and vehicle driving much slower than the wind speed.
If you want to look at it as vehicle driving in same direction as the wind speed then you need to consider the implications of modified reference frame.
You switch vehicle and road so vehicle kinetic energy is now the road (moving paper kinetic energy) and vice versa.
So when you consider everything correctly the result will be clear as you take much more energy from the moving paper (vehicle kinetic energy) than you put back in.
I just feel that there are to many things that you (all) do not understand and so it makes sense why I can not explain to you how the vehicle works.
Even if I'm able to explain one thing there are 10 more things that you wrongly understand (or better to say do not understand).
At this point I'm happy if I can make people understand the wheel only vehicle and why that can not move from left to right without energy storage.
Think at the exact moment that back wheel starts to move and what is powering that as the front wheel can not do it without the help of stored energy.
I wish I had a more robust toy vehicle so that there was much less energy storage so that vehicle can not move to demonstrate that to you also but not sure if that will have been of any help since you also need to understand the theoretical part of why the vehicle works the way it does.
NiHaoMike:
--- Quote from: electrodacus on September 18, 2021, 03:44:14 am ---People seems not to get the generator part as I hear many times in my business people saying things like while not charge my RV battery while I drive as there is power available at the alternator that is free to use since I still need to drive anyway.
They have a hard time understanding that taking 1kWh from the alternator and putting it in the battery will cost them an extra liter of gasoline so about $1/kWh or about 50x more expensive than getting that 1kWh from a solar panel.
So there is no free energy available at alternator since as soon as you take power out of alternator say 1kW then you add an extra 2kW load to the engine (alternators are typically just 50% efficient so the other 1kW is just heat in the alternator).
A liter of gasoline contains about 9kWh of energy if you burn it but since engine is typically just around 20% efficient you can see how 1 liter is needed for 1kWh took out from alternator.
--- End quote ---
Those figures are a bit dated, a modern Atkinson cycle engine can reach 40%. And what makes more sense is to charge the batteries using regenerative braking.
electrodacus:
--- Quote from: NiHaoMike on September 18, 2021, 05:14:56 am ---Those figures are a bit dated, a modern Atkinson cycle engine can reach 40%. And what makes more sense is to charge the batteries using regenerative braking.
--- End quote ---
We are not talking about best record efficient engine but about what people actually have and 1 liter /kWh is a rounded value but fairly close to what 90% of vehicles will get.
Obviously regenerative breaking is great as instead of wasting that energy as heat on the brake disks you can use it to charge the battery.
Tho you will need modifications done to your vehicle to be able to do that if you do not have an EV or hybrid vehicle.
In any case way to many people think charging the battery from alternator is free as in there is no fuel penalty.
Same happens here where people think they can take energy from the wheel of the blackbird and somehow that has no effect on the vehicle speed / kinetic energy.
ejeffrey:
--- Quote from: electrodacus on September 18, 2021, 03:44:14 am ---Of course there is a direction as the generator wheels in order to produce the power will need to oppose the moving paper direction so whatever you extract from there you can call that breaking power.
So if you have 10W of breaking power you will need a motor on the other wheels (back wheels) that also receives 10W just for vehicle to remain stationary(ideal vehicle in real world you will need more power on the back wheels to also cover the friction losses).
--- End quote ---
We are actually maybe getting somewhere here. The paper does positive work on the cart. Work (energy) is force * displacement while power = force * velocity. If the force and velocity are in the same direction then the power input is positive and can increase the kinetic energy of the car. If they are in opposite directions it the work is negative and will decrease kinetic energy. While the paper is moving backwards and the cart is moving forwards, the bottom surface of the wheel is moving backwards so the work done is actually positive. This power input is what allows the cart to accelerate, or in the steady state with no acceleration provides energy to overcome frictional losses. Due to the gear ratios the acceleration of the cart is in the opposite direction of the force applied to the wheel, but that is no big deal because energy doesn't have a direction.
The table provides a forward force but since it is stationary and so is the bottom of the wheel it does no net work. The table is still important because we still need a forward force to balance F=ma. But the table can provide that balancing force without doing any work, positive or negative.
electrodacus:
--- Quote from: ejeffrey on September 18, 2021, 06:10:05 am ---
--- Quote from: electrodacus on September 18, 2021, 03:44:14 am ---Of course there is a direction as the generator wheels in order to produce the power will need to oppose the moving paper direction so whatever you extract from there you can call that breaking power.
So if you have 10W of breaking power you will need a motor on the other wheels (back wheels) that also receives 10W just for vehicle to remain stationary(ideal vehicle in real world you will need more power on the back wheels to also cover the friction losses).
--- End quote ---
We are actually maybe getting somewhere here. The paper does positive work on the cart. Work (energy) is force * displacement while power = force * velocity. If the force and velocity are in the same direction then the power input is positive and can increase the kinetic energy of the car. If they are in opposite directions it the work is negative and will decrease kinetic energy. While the paper is moving backwards and the cart is moving forwards, the bottom surface of the wheel is moving backwards so the work done is actually positive. This power input is what allows the cart to accelerate, or in the steady state with no acceleration provides energy to overcome frictional losses. Due to the gear ratios the acceleration of the cart is in the opposite direction of the force applied to the wheel, but that is no big deal because energy doesn't have a direction.
The table provides a forward force but since it is stationary and so is the bottom of the wheel it does no net work. The table is still important because we still need a forward force to balance F=ma. But the table can provide that balancing force without doing any work, positive or negative.
--- End quote ---
You also need to consider what the example represents.
Moving paper is the energy source for the vehicle and is the analog of win in a wind powered vehicle. So in my video you see the equivalent of a vehicle moving in the opposite direction to wind direction at around half the wind speed.
If you want to see things in modified reference frame and want to consider the moving paper as the road so the vehicle and road reference frame are switched then you need to consider that kinetic energy of the vehicle and road (moving paper) have been switched so when trying to interpret what happens with vehicle speed you will need to look at the paper not vehicle.
So an ideal vehicle traveling at a fixed speed of say 10m/s will have a certain kinetic energy based on speed and weight and you will consider ground as stationary and thus no kinetic energy.
If for some reason you want to look at this system from a modified frame of reference and switch the vehicle characteristics with ground and then continue the experiment from there then you need to look at now moving ground kinetic energy to know what happens with vehicle speed.
I do not get why people will want to change reference frame and deal with all the complexities of that (seems people wrongly think changing reference frame has no implications and then get to wrong conclusions).
So in my video when analyzing that vehicle you either look at it as wind powered vehicle traveling against wind direction at half the wind speed and all is what it seems no modified reference frame.
Or you consider the modified reference frame to analyze a vehicle traveling above wind speed in the same direction as the wind but then you need to look at what happens with moving paper (road) energy as that is switched with the vehicle kinetic energy.
So for example in that period where only the front wheel rotates the vehicle seems stationary while energy is put in to rubber band but if you interpret correctly the modified reference frame you will need to look at how much energy you took from the paper to charge that rubber band and since that is not zero it means vehicle speed was reduced during that period (not obvious if you just look at the vehicle).
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