Re:Elon Musk and the Hyperloop
by robomartin on Thursday November 22, 2012 @12:33AM
(#4817616)
> You can pull a 10 tonne yacht with one hand on a rope. Try pulling a 10 tonne truck...
This probably deserves a little more attention.
Having done a lot of sailing as well as ocean-fishing on different size vessels I know exactly what you are talking about. What you might be confusing is that at zero speed there's a huge difference between something floating on water and a truck resting on the ground.
In the case of a large boat floating on water, so long as you are not after making it move quickly you are almost just fighting inertia. So, yes, a fifteen year old kid pulling on a rope could make a large boat move.
When it comes to a truck sitting on the ground you have to contend with rolling friction as well as the static friction (because nothing is moving) and the viscous drag of all the mechanical elements, oil and grease in the drivetrain. It could take hundreds of pounds of force to get it to budge. Once your break through static friction then you have a much reduced dynamic friction and viscous drag (oil and grease in the drivetrain) --again, so long as speed is very, very low-- as well as the rolling friction of the tires.
If, on the other hand, that truck was resting on a frozen lake bed and if the tires are not frozen-glued into the ice, and, if the ice is not deformed under the tires, the same fifteen year old kid could move it just as well as he moved the boat. In fact, it might even be easier because there isn't a significant v^3 component to the power required to make it move.
Once you start to add speed to the problem it is easy to see that moving the same object through water will require at least 830 times more water.
At speed the power required to counter drag comes into play.
Power = 1/2 * fluid density * velocity ^3 * coefficient of drag * area
Here's an easy experiment: Take a tennis racket and duct-tape the strung area in order to make it a solid paddle. No air or water should get through. Now swing it in air as fast as you can. Now go into a swimming pool and try to swing it just as hard under water. Unless you are super-human you will not be able to. In fact, you won't be able to even get close to the speed in air. Again, simply stated, you need over 800 times more power to achieve the same speed in water with the same object.
If it takes one horsepower to move something in air at a given speed you'll need over 800 horsepower to achieve the same speed in water. It's the difference between a model airplane engine and a Formula 1 race car engine.
Get down to the "Power" heading. That's the formula you want to look at.
To get back to your yacht vs. truck example. What you would really have to experience is pulling each of them at 20 or 30 miles per hour with a rope (chain?). Of course, it would take super-human strength and endurance to be able to do that.
This probably deserves a little more attention.
Having done a lot of sailing as well as ocean-fishing on different size vessels I know exactly what you are talking about. What you might be confusing is that at zero speed there's a huge difference between something floating on water and a truck resting on the ground.
In the case of a large boat floating on water, so long as you are not after making it move quickly you are almost just fighting inertia. So, yes, a fifteen year old kid pulling on a rope could make a large boat move.
When it comes to a truck sitting on the ground you have to contend with rolling friction as well as the static friction (because nothing is moving) and the viscous drag of all the mechanical elements, oil and grease in the drivetrain. It could take hundreds of pounds of force to get it to budge. Once your break through static friction then you have a much reduced dynamic friction and viscous drag (oil and grease in the drivetrain) --again, so long as speed is very, very low-- as well as the rolling friction of the tires.
If, on the other hand, that truck was resting on a frozen lake bed and if the tires are not frozen-glued into the ice, and, if the ice is not deformed under the tires, the same fifteen year old kid could move it just as well as he moved the boat. In fact, it might even be easier because there isn't a significant v^3 component to the power required to make it move.
Once you start to add speed to the problem it is easy to see that moving the same object through water will require at least 830 times more water.
At speed the power required to counter drag comes into play.
Here's an easy experiment: Take a tennis racket and duct-tape the strung area in order to make it a solid paddle. No air or water should get through. Now swing it in air as fast as you can. Now go into a swimming pool and try to swing it just as hard under water. Unless you are super-human you will not be able to. In fact, you won't be able to even get close to the speed in air. Again, simply stated, you need over 800 times more power to achieve the same speed in water with the same object.If it takes one horsepower to move something in air at a given speed you'll need over 800 horsepower to achieve the same speed in water. It's the difference between a model airplane engine and a Formula 1 race car engine.
There's a good article in Wikepedia on drag:
http://en.wikipedia.org/wiki/Drag_(physics)
Get down to the "Power" heading. That's the formula you want to look at.
To get back to your yacht vs. truck example. What you would really have to experience is pulling each of them at 20 or 30 miles per hour with a rope (chain?). Of course, it would take super-human strength and endurance to be able to do that.