3 Wheel thing (Veritasium)
In my last post, Good to be wrong (sometimes), I wrote about Veritasium's video about the wind powered car that can travel faster than the wind. In the video an analogy is made with the 3 wheeled thing in this video clip. I couldn't work out how this worked so I made one in Algodoo and all was revealed.
The important thing to notice is that the way it starts moving is not the same as the way it moves faster than the plank. At first the big wheel rotates forwards (clockwise), this rotates the small wheels backwards so the cart moves back slightly but the forward friction on the big wheel is greater than the backwards friction so it starts moving forwards this causes the small wheels to rotate forwards, the friction between the small wheels and the big wheel is greater than the friction between the big wheel and plank so the big wheel starts to rotate backwards. For a short time there is no relative movement between the big wheel and plank, the cart and plank are travelling at the same velocity. The forces on the cart ( friction forwards at the top and friction backwards at the bottom) are unbalanced so the cart moves faster, the small wheels therefore rotate faster so the big wheel moves faster, the big wheel is now slipping backwards against the plank, th friction force on the top of the ball is still forwards due to the backward rotation of the wheel so the cart accelerates forwards, it is now moving faster than the plank.
The counterintuitive thing is that the friction between the plank and the wheel is always forwards no matter whether the wheel is rotating forwards or backwards, this is because the plank is always faster than the surface of the wheel. Imagine you are sitting on a bike next to a flat sided truck. As the truck starts to move you place your hand on the side of the truck, friction , to go faster yo will have to stroke your hand backwards on the side of the truck between your hand and the truck propels you forward until you are travelling at the same speed as the truck. The difference here is that the energy is coming from you not the truck. Imagine now that you have a series of gears connected to your wheels that drives another wheel that is resting against the tuck. If this wheel turns fast enough you will be propelled forwards, the energy has now come from the truck.
That should mean that the truck slows down more if the energy is derived from the truck rather than the rider, I haven't quite got my head round this yet.