# Friction and Wheels

1. Jan 20, 2012

### aznweirdo45

Hi guys,

I've been studying rolling recently but I'm having trouble understanding some of the concepts regarding friction and wheels. My first question is, how does friction cause a wheel to roll in general? And if there's no friction, why would the wheel slide instead?

In my Physics textbook, it says that "if a net force acts on the rolling wheel to speed it up or to slow it, then the acceleration tends to make the wheel slide at the point of contact, and the frictional force must act at the point to oppose that tendency." Why would accelerating it make the wheel slide to the left? And again, how does friction oppose this and make the wheel go forward instead? I'm having a really hard time visualizing all of this so any diagrams would be greatly appreciated. Thanks!!

2. Jan 20, 2012

### Simon Bridge

everything you want to know

In general? It doesn't. You gotta push it, but what you mean is:
If you've ever skidded a car you know that a wheel can slide with friction too.

The center of the wheel wants to go in one direction, but the bit of the wheel in contact with the ground wants to stay where it is - because of friction. That will make the wheel try to turn about the contact point. If it was not round, but a column, it would fall over.

Same reason a flat block would slide if you push it: you've overcome the static friction.