In my Concepts in Physics course the other day at TC, we discusses race cars on a banked turn. We used free body diagrams (FBD) to illustrate where the centripetal force (the force that keeps the race car traveling in a circle).

Here is the basic FBD for a car on a banked turn. The dot in the middle of the rectangle (car) represents the car’s motion out of the page and toward the drawer or looker.

So, we’ve got the car on the incline. The force of gravity is always directly down — in the direction of a free hanging plumb line. The normal force (typically the force opposite the gravitational force) is always perpendicular to the surface, so not vertical in this case.

Now things start to get fun!!!!

We draw the components of the normal force in with dotted lines; pretty much, we’re making the normal force the hypotenuse of a triangle. You can clearly see the vertical leg of the triangle is equal and opposite to the gravitational force.

Left over is the little bit of horizontal normal force. Notice that it is pointing towards the center of the circle…it seeks the center. The definition for centripetal force is “center seeking”, so this must be the centripetal force on the car!!!!

The neat thing is, the faster the car travels around the bank, the higher it will go on the incline. The higher on the incline, the greater centripetal force. And the driver doesn’t have to do a thing! All these things happen naturally :o)

I thought this was a great demonstration to show students where this mysterious and confusing force comes from. It certainly isn’t magic!!!

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