Air Brake Chair
One of our most popular exhibits, the Air Brake Chair has brought smiles to generations of visitors. Be sure to try it next time you visit.
This exhibit demonstrates wind resistance and mechanical advantage. Visitors pull themselves up in a chair and when they release the rope, a fan type assembly spins and provides the braking effect that lowers the visitor back down gently.
- Sit in the seat.
- Hoist yourself up with rope.
- Let go!
The spinning the bike wheel pushes against the air, slowing your fall.
- What prevents the chair from falling faster?
- How many simple machines can you find in this exhibit?
- Why do the fan blades spin so fast?
We are currently looking for regional or industry connections related to this exhibit. If you know of one, please let us know.
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It is fairly easy to hoist yourself up because of the leverage gained due to the different diameters of the pulleys involved. The diameter of the pulley the rope turns is large compared to the diameter of the shaft that coils up the chair straps and lifts you. This difference amplifies the force applied (you pulling the rope), just like levers do. When you let go of the hoisting rope, your falling weight causes the bike wheel to spin. The yellow paddles attached to the wheel have to push against a lot of air as they spin and this air resistance is what slows your fall. This is the same resistance you feel when you stick your hand out the window of a moving car!
Notice another set of gears between the axle the seat is tied to and the fan blades or paddles on the bike wheel which spin the paddles faster than if they were connected directly to the axle. When different weight riders use the Air Brake Chair, they need to all return to the ground at safe speeds and increasing the speed of the paddles helps with this. Looking at the equation for drag, we can see why this is important
- $ F_D\, =\, \tfrac12\, \rho\, v^2\, C_D\, A $
- FD is the drag force,
- $ \rho $ is the density of the fluid,
- v is the speed of the object relative to the fluid,
- A is the cross sectional area, and
- CD is the drag coefficient.
Because the drag goes up with the square of the speed, a 250 lbs rider does not fall five times faster than a 50 lbs rider. The force of drag goes up exponentially allowing both riders to descend at safe speeds.