Futten die Kaetzchens!

Bedeuten Sie "füttern die Kätzchen"?

fucking orangutan.

Terry is an ASSHOLE.

why you say this?? terry is such a sweet terry. look at that hair! admit it, you do not hate terry you really hate yourself, terry is blameless in all matters

Screw the haters, Terry rules!

Terry is the fucking dragon. You're all just ants in the afterbirth.

Yes, it's the angle of the ruler. Imagine the ruler angled down to the right, the the cart would roll left no matter which way the ruler was moved. The maker of this video is breaking our brains by framing the experiment in terms of direction rather than angle.

5 stars for breaking my brain for a few minutes

It isn't the angle. He is misdirecting the audience, by saying 'by moving the ruler to the left....' When, in fact, he is applying pressure on the ruler to the right. This causes the car to move to the left at a SPEED FASTER THAN HE MOVES THE RULER; the speed of the car is in direct proportion to the ratio of the radii of the large to the small wheels. So he is applying pressure to the ruler to the right while saying 'Moving the ruler to the left' (big fat lie), while THE WHOLE APPARATUS moves to the left at a speed faster than his pudgy little fingers can push the ruler to the right. The apparent speed of the ruler is then the speed of the car TO THE LEFT, minus the lesser speed generated RIGHTWARD by his actual pressure on the ruler, leaving a net leftward rulery speed of slightly less than the leftward car-ish speed (which is precisely what you see on camera).

Goddamn, I was thinking about this like a physics problem where the premises are solid, and there's no active deception!

One minor correction to your explanation though. the relative speeds of the top and bottom "tracks" (ruler and table) depend on the inner and outer radii of the *spools* only, which are the only part that changes the *linear velocity*. The inner portion of the spools touches the rubber wheel; the outer portion touches the ground. This gives the ground portion a higher linear velocity then the part in contact with the wheel has (which is the same linear velocity as the ruler). If you didn't have this shift of linear velocity, the cart couldn't even move.

Basically, if the inner and outer radii of the spools are r1 and r2, then the ratio of the cart velocity to ruler velocity is r2/(r2-r1), both always moving in the same direction. If there's no mechanical advantage, the ratio goes to infinity, i.e., the ruler velocity is 0 no matter how fast you move the cart.

Anyways, the force thing screwed with me though, because obviously the force applied to the top wheel needs to be in the opposite direction as the cart is moving to make the top and bottom wheels rotate in the proper direction. I think friction keeps the cart from moving backwards as well. All the backwards force does is set the top wheel in motion, moving the cart forwards. I was too naive to realize he was lying. Thank you!

You can't push an object and have it suddenly fly in the opposite direction of the force you applied, wheels or no wheels.

Idiots.

You can, actually. If the force is applied at the top of the wheel to the right, that has two effects: setting the top wheel in motion CW, and also applying an overall push (or "force") on the cart to the right.

This overall push to the right is what you're complaining about, the part of the push that you would think would send the cart moving backwards (to the right) in a frictionless system. However, that force is counterbalanced by the static friction between the bottom wheels and the table. The cart can't slide backwards due to friction. The wheels can only rotate. But for the wheels to rotate properly (CW) to send the cart backwards, the top wheel would have to rotate CCW, which can't happen due to the CW force from the ruler. In a sense, it's like pushing against a wall. The forces are balanced, you get no movement.

The setting of the wheel in motion actually causes the wheels to rotate, moving the cart to the left. The key point is that we still are in a case of static friction! Even as the wheels rotate, the static friction effect I just described still holds true! The wheels don't slide along the table. They rotate, staying in contact. So the static friction is preventing backwards movement of the cart, even as force is being applied to the right.

I hope that made sense. The key point is that the movement is due to rotation of the wheels in static contact with the surface, not sliding. Your point would be totally correct if we're just talking about pushing on free-floating objects in space.

Another way to visualize it is to imagine what would happen if you're sitting on the frame of the cart, rubbing a ruler across the top wheel. If you're sitting behind the wheel facing left, and you drag the ruler towards yourself, the cart starts to roll forward! And like OldScratch said, it rolls forward faster relative to the table than the ruler moves backwards relative to the cart. The cart carries the ruler along, and from the frame of reference of an observer on the table, both the cart and ruler move to the left (forwards), although the ruler moves more slowly. But obviously, from the person sitting on the cart, they pulled the ruler towards themself, applying force on the wheel towards themself. And static friction allowed the cart to actually move. Static friction is how any wheeled vehicle, like a car, moves on its own accord.

Basically, you could build this cart and drive around in it with no problem, applying force on the top of the rubber wheel exactly like OldScratch describes.

Ditto. I think it involves math and math=witchcraft.