## Banked Turns

• In order for a car to move around a circular track, something must provide a centipetal force.
• One source of the centripetal force is friction between the tires and the road. But at high speeds, on small tracks, this might not suffice; the car might start to skid sideways.
• If the road is banked, so that the outer edge is above the inner edge, then a portion of the normal force from the road on the tires points towards the center of the track; this fraction of the normal force can provide enough centripetal force to keep the car moving in a circle.
• In fact, if the track is banked just right, then a car moving at the critical speed
```         v  =  sqrt [ g * R * tan(theta) ]
```
will have all the necessary centripetal force supplied by the normal force. This means that the tires do not need to supply any friction at all to keep the car from slipping sideways.

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