DDWFTTW - Physics on a shoestring

[HiDave606]

Okay, so I think I've got this figured out. The prop is "flying" through the air, which is at zero velocity. If the air were moving at the velocity of the treadmill, the prop would still be flying through the air, the "air speed" would remain the same, the ground speed would be reduced by exactly the speed of the treadmill, so no net forward speed. Right?

[HiDave606]

This particular demo is even simpler to explain than others. The treadmill is turning the wheels, which turn the prop, which pushes the vehicle forward. So the power to the prop is FROM the treadmill. If the vehicle were not constrained, it would not go forward, except from inertia in the blade.

[spork33]

@HiDave606

So your theory is that the shoestrings holding it back are what makes it go forward. No.

[kasics]

There is weary simple explanation of this phenomena. In the room wind speed is 0 and vehicle does not move forward. The treadmill is turning the wheels and propeller, but there is no approaching wind so the thrust from propeller will push the vehicle forward since there is no wind coming from front, as it would be if the vehicle would be on the ground. As soon as you put that vehicle on the ground and push it (in 0 wind speed) it will stop because approaching wind will be=to the thrust.

[spork33]

@kasics

It's a wind powered vehicle. Of course it will not work on the ground with no wind.

[ColinWatters]

Why are people surprised at what happens? Consider this... The propellor on a aircraft pulls it forward right? So why would changing the type of transmission from gears to a funny sort of "belt drive" make a difference? Isn't It obvious the prop on the cart is going to try and pull the running machine forward in the same way?

[subductionzone]

Excellent video. A very clever solution to show it generates its own force when at "wind speed" . I was amazed at how fast it got back to exerting a force forwards when picked up and dropped.

[spork33]

The reason it initially takes a while to pull the strings forward is that the treadmill itself accelerates slowly. With the treadmill at speed, the cart will very rapidly come to steady state (DDWFTTW)

[subductionzone]

Yes, but in your previous videos JB always held it for a while before letting go of it. It is now obvious that he was more concerned with getting the cart to go straight rather than waiting for it to get up to speed. That this is downright zippy!

[CousinoMacul]

Nice demonstration.

[Odziz]

That's freakin awesome.

So when are you going to build a full size version ?