Originally Posted by

**Bald Student**
I can answer two of these for you but do you mind if I ask what grade you're in?

Bigger disk breaks have more stopping power because of what's called the 'Law of the Lever'. The disk turns about its centre and the stopping force from the break pad is at the rim. The bigger breaks have a bigger radius and the 'stopping power' called torque is equal to the force from the breaking pads multiplied by their distance from the centre, i.e. the radius of the disk. The bigger the disk's radius, the bigger the torque.

The question about why a moving bike stays up is much harder (and it's why I asked your grade. I didn't learn it until I got to college). Anything which travels in a circle experiences a force away from the centre of the circle, this is called a centrifugal force. For example, the centrifugal force is what stops the planets from falling into the Sun, the gravity of the Sun pulls them in but since they're travelling in circles their centrifugal force pushes them back out and cancels out the Sun's gravity.

This is also why you have to lean your bicycle to the side when you take a corner quickly. When you take a corner you are traveling in a part of a circle and the **centrifugal** force pushes you away from the direction you are turning. By leaning into the corner you cancel out the force pushing you away from it.

When you ride along on the straight your bike naturally tends to fall one way or the other, the same as it would if it was still but since you're on it you can turn the handlebars a little to either side to turn in a very small part of a circle and use this **centrifugal **force to push you back up again. You can see this if you cycle in a straight line along wet grass. If you look back at your track you'll see that you were actually turning a little bit from side to side all the time.

The reason you can't balance on your bike when you're stopped is because when you turn the handlebars you're still not moving so you don't get any **centrifugal** force.

Let me know if you understand that and if you do I'll tell you some more.