This one sort of stumped me when I saw it since if you asked me if it was possible I would have said no.
On a recent episode of Tosh.0 there was a short clip of him sitting stationary in a 4-wheeled cart inside a warehouse like environment. The cart had a makeshift mast and sail. He was holding a gas powered leave blower.
He fired up the leave blower, aimed at the sail, it filled with air and sure enough started to roll him across the floor.
Am I missing something here or did they pull a fast one on me?
Without the sail the leafblower would work just fine as a motor – push out air one way, cart goes the other way. A sail is just redirecting that air flow. Ultimately the air is coming out of the cart-sail-leafblower system in some (net) direction, and the cart moves in the other direction.
i doubt it would be a fast one, more likely pretty slow.
if additional air flow came from off camera then it would move.
No reason to believe it wouldn’t work. On “Mythbusters,” Tory Belecci, Grant Imahara, and Kari Byron reversed the rotor on an airboat (so that the airflow from the rotor blew in the direction of the bow), attached a sail to it, and were able to make the boat go forward by blowing against the sail with the rotor.
It would surely be more efficient, though, to just remove the sail and point the blower straight back.
Maybe one good way of thinking about it would be to consider what would happen if the leaf blower had a u-shaped nozzle, so that it’s aiming forward but the air is blowing backwards. The air goes forward, hits the end of the tube and gets re-directed backwards where it generates some thrust. This is kind of the same thing, but less efficient.
I don’t see how this would work. Let’s review the equal and opposite rule. If the operator just sat in the cart and used to blower to blow air out the back, the car would go forward. The air is pushed out the back, and the air pushes the blower in an equal amount but opposite direction forward. Since the blower is attached to the cart (by the operator), the cart moves forward.
Now he takes the blower and points it at the sail. Based upon our previous experience, if the air moves forward toward the sail, then the blower must be pushed backward. The forward air hits the sail and tries to push the cart forward. But the blower (which is being pushed backward) is attached to the same cart. The two should mostly equalize out so that the operator goes nowhere.
If anything, the sail is not catching all the air, so the cart should move backward. They could reverse the tape and make it look like it going forward.
All we need now is a treadmill.
Damnit…beaten by 4 min.
They did- sorta. But it was hwaaaay better just to use the fan to power the boat off the stern. Directing the air into a sail was wonky, hard to get right, and slow.
Except air is pulled into a blower from the side and uses energy from either gas or electricity to do it. It’s not a closed system, so it’s not an “equal and opposite”-applicable situation. The intake part of the equation would be resisted by the cart’s (lack of) sideways motion.
ETA: Also, Daniel was pointing the blower up at a 45 degree angle since he was sitting beneath and behind the sail. So presumably some of that back-energy was directed into the ground.
Absolutely all systems, everywhere, everywhen, are subject to Newton’s third law. The reason it works is that the air, after hitting the sail, bounces back.
Agreed, but that wasn’t the point. They established experimentally that, at least for a certain range of parameters, it didn’t just sit there cancelling out, or drift backwards. They also noted that there was no discernible wind to consider, and, at any rate, they (slowly) propelled the boat in different directions this way.
One of their more interesting results, IMO. Besides, the airboat was pink.
I think I understand it now.
The force moving the cart forward was not air from the blower pushing on the sail, but rather air from the blower being redirected by the sail to the rear again acting as thrust?
Yes. What matters is how air is moved in relation to the whole boat-sail-leafblower system.
Right, but they weren’t trying to be efficient. IIRC, they were trying to bust a Roadrunner cartoon where Wile E. Coyote gets a sailboard and uses an electric fan to propel himself after the Roadrunner. It worked for Wile E. until he ran out of extension cord.
Newton would be spinning in his grave…
…and then he would spin again, in the opposite direction
How come nobody’s talking about Wile E. Coyote’s boat motor in the bathtub? That one is way better.