Annotated Mythbusters

Plane on a Conveyor Belt

Myth: A plane on a conveyor belt, matching its takeoff speed in reverse, can't take off

This is a Internet meme popularized by sites like kottke.org. The original problem states that the conveyor belt matches the forward speed of the plane in reverse, but for this particular mythbusting they tested the conveyor belt moving at the plane's takeoff speed. To control for this adjustment, they measured the takeoff distance of the plane and only allowed it that much conveyor belt to take off.

Small-scale test

They found a small R/C park flyer airplane with a take-off speed of 11.3mph. Adam held up the tail of the airplane as Jamie accelerated the plane up to speed. The wheels were a bit wobbly, but the plane easily moved forward off the front of the conveyor belt. The plane didn't actually take off as the belt was too short.

They switched to a roll of butcher paper that Adam pulled with his segway. They figured out the take-off distance of the plane and measured out that much butcher paper as their mock conveyor belt. The plane easily took off before the end of the butcher paper was reached.

Science explanation: Most people think of a car on a conveyor belt, which will remain stationary when a conveyor belt moves at the same speed in the opposite direction. The car remains stationary because the wheels on the car are what moves the car forward. Airplanes move because of their propellor -- the wheels are irrelevant and spin freely. Thus, a plane takes off.

Full-scale test

They decided to go big with 2,000 ft of tarp and a 400lb ultralight plane. They set up at Castle Airport and had Art Johnson as their ultralight pilot. They measured the takeoff distance of the plane to be 85 ft and the take speed at 25mph. After a day's delay to wait for the winds to die down, they were ready to do their test.

Jamie's pickup truck pulled the tarp at 25 mph while Art accelerated the plane. Art didn't think his plane would take off, but lo and behold, it immediately accelerated. In Art's words, "it took off normally."

busted

Cockroach Survival

Myth: Cockroaches will be the only living thing to survive the radiation from a nuclear blast

They focused on the radiation left over from a nuclear blast as the blast itself would most certainly kill a cockroach. This myth derives from the fact that the soft tissue in bone marrow and intestinal track makes humans more suspectible than insects.

They tested two aspects of this myth: that cockroaches can survive the radiation from a nuclear blast and that they would be the only survivors. They decided to also test fruit flies (classic science subject) and flour beetles (very tough). They chose the german cockroach to represent cockroach-kind: they can cause allergic reactions in human and are known to consume human night-time dribble.

Setup

• Cobalt 60 radiation source at Pacific Northwest Laboratory
• Plastic boxes to hold insects (aka "Holiday End")
• 50 flour beetles per test
• 100 fruit flies per test
• 50 german cockroaches per test

Michelle Johnson told them that the PNL's Cobalt 60 was capable of delivering 55,000 rads/hr, which is lethal to humans in 10 minutes (1000 rads is the lethal dose).

Test and count

They exposed the three different sets of bugs at 1000 rads, 10,000 rads, and 100,000 rads. The bugs in the 1000 rads and 10,000 rads tests appeared fine but 90% of the cockroaches in the 100,000 rads group immediately died. To evaluate the longer term affects of the test on the insects, they took them home along with a control group that hadn't been exposed. They monitored the insects over the next 30 days and counted how many died.

Based on these results, the cockroaches clearly weren't the best survivors of a nuclear blast. They are quite hardy, able to survive radiation doses at 10,000 rads, which is 10x the lethal dose for humans. However, the flour beetles did much better and the fruit flies might have done better if their normal lifespan wasn't 30 days.

busted

Shaving Foam Shenanigans

Myth: If you take a can of shaving cream and freeze it, take off the outside, and put it inside someone's car, it will fill the entire car

The build team went to work putting this fan-submitted prank to the test. After Tory and Grant finished attacking each other with shaving cream, they got to the business of freezing shaving cream. Tory cut away the container from a can that had been in the freezer to get to the cylinder of foam inside. He stuck the foam on the center console of a Volvo. After five hours the cream had only slightly expanded. Most of the mess was from it melting ocver the center console.

Disappointed with this prank (Tory: "it's of microscopic proportions"), they upped the ante with 50 cans, which they deposited over the seats of the car. The cream expanded a bit to make a mess of the seats, but the expansion was nothing to write home about. You'd probably do just as well squirting shaving cream into the car manually.

Just to show what expanding foam really looks like, they switched over to A-B foam, aka polyurethane foam. They used 25 gallons (3.5 cubic ft) of the liquid, which they wanted to see would fill the 90 cubic ft car. The build team acted quickly to place drums of the foam inside the car. After 30 seconds of mixing, the A-B foam starts expanding very rapidly.

The foam filled most of the car, including up to the roof, and some of it even spilled onto the concrete around the car. When dry, the A-B foam becomes rigid, which allowed Kari to hit one of the car windows with a hammer without the hammer going through the window. The car was ruined far beyond the level of a prank.

Tory: "I'm totally satisfied. I know if I walked up to my car and it look like that, I'd be pretty upset. It would ruin my day, not to mention my car."

busted