Flat tyres will get you nowhere
There was an unusual event in the staff room today, everyone was talking about a physics problem. This was sparked of by an EE topic suggested by one of my students, Boyan. He is an enthusiastic biker as you can see from his photo (pinched from facebook). His observation was that when he rides his bike downhill it goes slower when the tyres are flat than when they are fully inflated. Anyone who has ridden a bike with soft tyres will know that it is more difficult so the investigation immediately draws interest as happened in the staff room. Ones initial reaction is that it must be to do with friction but friction makes the wheels roll it doesn't really slow them down, rusty bearings would do that. If the bike rolls more slowly then energy must be going somewhere other than the KE of the bike, but where?
I made a simulation of the wheel and tyre in Algodoo, shown below. It doesn't look like a bike but the mechanics is I think similar. The inner mass is the wheel and the outer ring plus springs is the tyre. The lower wheel has stiff springs representing pumped up tyres and the upper wheel has soft springs (flat tyre). As you can see the upper wheel travels slower as some of the PE lost is used to stretch the springs which are set to loose energy as heat.
Tags: EE