Someone with a British accent constructed this device:
It's a small car with two spools for wheels. A toy wheel rests on top of the spools. Pushing the car to the right causes the spools to rotate clockwise and the big wheel to rotate "anticlockwise," as expected. He then proposes to lightly set the edge of a ruler on top of the big wheel and use it to push the top of the wheel to the right. What will the car do?
One stuffed animal says the big wheel will be turned clockwise, turning the spools anticlockwise, causing the car to move to the left. Another stuffed animal says the ruler will push the car to the right, but the wheels will push it to the left, and it just won't go. A third stuffed animal says it will go to the right, but can't say why. What do you say?
After drawing 3 free body diagrams, but not actually testing, I am thinking that the normal force against the wheels will stop the machine from turning. This is because of the actions acting against the wheels, vs the wheels acting against the disk. Good puzzle. EDIT: Is this a frictionless table?
Sorry I have another question. Does the wheel interlock with the other wheels or are they all just circular? If they interlock then I'm going to say that it doesn't move due to the configuration of the gears. If they do not interlock, then I am going to say that the wheels will move in the opposite direction of the big wheel.
"Honey, my stuffed animals are talking to me again. Time to up the meds."
Awww... don't post spoilers! It's fun to think about it and argue! I would have guessed that it didn't move, for every centimeter the top wheel turns the bottom wheels turn in the opposite direction one centimeter -- the different diameters shouldn't matter because it's the circumference of the wheels that are moving. Obviously I'm wrong. And the video totally blows my mind!
It will do nothing initially, until things start to slip. Once there is enough force on the ruler, it will move to the right (as the spools will start slipping against the ground)