Parachutes

Last year at the “Blues on the Bay” event at Kaneohe featuring the Navy’s Blue Angels, the U.S Army’s Golden Knights parachute demonstration team was one of the opening acts. They jumped from a CH-47 helicopter and drifted slowly toward the ground after deploying their parachutes. Their parachutes allowed them to have acceleration much slower than -9.80 m/s^2. If they had jumped without parachutes, they would have had acceleration closer to -9.80m/s^2 because the body of a person is much less than the surface area of a parachute. The bigger the surface area, the more drag is created. Either a small pilot chute or a static line attached to the aircraft is used to aid in the deployment on the main chute. An improperly packed parachute may not deploy, and therefore will not create the drag necessary to safely descend back to earth. Even without a parachute, however, a person will only accelerate until he or she reaches terminal velocity (about 50-60m/s for a human body), and there has been at least one case in which a skydiver survived a freefall to Earth
(See Mikey Holmes : http://www.esquire.com/dont-miss/wifl/parachutefailure0807).

Disk Brakes

Today I decided to replace the V-brakes on my bike with disk brakes from an old bike. Both brakes work by applying kinetic friction (surfaces are not completely smooth when looked at through a microscope, so when two surfaces rub together, they don’t just glide past each other, they experience friction), however the way they are applied differs. V brakes apply kinetic friction to the bike’s rim, while disk brakes apply kinetic friction to a disk mounted on the rim that passes through a slit in which two brake pads are squeezed to apply kinetic friction to both sides of the disk.

Here is a picture of the disk brake mechanism:


Because of this, disk brakes have an advantage over V-brakes because disk brakes don’t wear down a bikes rim (you can see some black marks left from the V brakes). Brakes help to decrease my bikes velocity through negative acceleration, assuming my bike is traveling in the forward direction. I can vary the amount of negative acceleration by varying the force applied to the brake cables through use of the brake handles. During installation, some oil dripped onto the brake pads, which reduced friction between the pads and the disk. Unfortunately, that was not what I wanted :(

Here's a picture of the V-Brakes on my bike...


And here's my bike with the disk brakes installed:


The finished product: