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Episode 97: Plane on a Conveyor Belt, Cockroach Survival, Shaving Foam Shenanigans

  • Plane on a Conveyor Belt: A plane on a conveyor belt, matching its takeoff speed in reverse, can't take off busted
  • Cockroach Survival: cockroaches will be the only survivors of a nuclear blast busted
  • Shaving Foam Shenanigans: a frozen cyclinder of shaving cream will expand to fill a car when it thaws busted

The plane took off! To quote Adam: "I've read literally thousands of pages, of vitriolic commentary and arguments about airplanes on a conveyor belt, but nowhere did I find anyone who'd put an airplane on a conveyor belt." The MythBusters did and was one of the most straightforward mythbustings they've done: plane + conveyor belt = flight. To those who still have trouble wrapping your head around the problem: the wheels on a plane don't matter; they spin freely. The conveyor belt can't keep the plane stationary.

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.

 Cockroaches (50)Fruit Flies (100)Flour beetles (50)
Day215302153021530
100,000 rad100%100%100%60%100%100%10%50%90%
10,000 rad20%50%70%40%100%100%6%16%40%
1,000 rad10%30%50%30%100%100%2%10%26%
Control0%10%30%0%100%100%0%6%10%

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

Comments

I just descovered your writeups on Mythbusters, pretty awesome, a nice reference point..I'll now know which seasons to purchase :)

cockroaches have dormant cycles; that's when they can survive the nuclear blast - can't remember exactly but it's something like 1/3 of them are always dormant (on a large colony), and these, will be the ones that survive. Test a colony ;)

An airplane gets lift from air moving across its wings, no? In a wind tunnel, the plane is stationary and the air moves past the wings, creating lift. On a conveyor belt, however, there is no air movement (other than the propeller wash).

Both airplanes they used have extremely high lift/weight ratios, and relatively low take-off speeds. I know many will disagree, but I think that what the conveyor does is let the engine get up to speed, so that with only a small amount of forward motion relative to the conveyor belt, enough lift to take off was achieved.

Don't think of it as forward motion, however. Think in terms of lift, and revisit the myth with a heavier plane, and you will get "myth confirmed."

Ric, think of it in terms of the propeller pulling the plane. The wheels spin freely, so the conveyer may as well be static ground. The conveyer pulls the plane forward, size and such don't matter.

Agree with plane's lift due to airspeed. If the conveyor belt speed indeed matched the plane's takeoff speed, then the plane should not be able to move forward and taxi on the conveyor belt prior to taking off.
A more relevant experiment would be to match the maximum airspeed of the plane with the conveyor belt to ensure that no forward motion occurs.

MSI that wont make a difference as the wheels will just spin faster and the plane will still be able to move forward as the propeller will be moving it forward and this will make the make the wheels spin at the speed of the conveyor belt + the propeller speed (as in speed the aircraft would go on a stationary runway when propeller is in use) ergo still moving forward.

The cockroach test doesn't really fit the myth, because the true myth involves their reproductive cycle. Because cockroaches reproduce exponentially, the myth dictates that if even one cockroach survives the fallout, that survivor will reproduce and pass on that so called "radiation immunity". It's all about natural selection and evolution. Plus, the myth doesn't exactly states that it has to be a direct exposure to radiation (i.e. nuclear fallout expands at a decreasing exposure rate).

The conveyor belt myth is a good example of the difference in airspeed vs. ground speed for aircraft. In this case, the AS remains the same (takeoff speed), but the GS is TWICE because of the relative movement of the "ground" (the c-belt). For the dynamics of flight (stall speed, etc.), airspeed is what matters; travel time, on the other hand, is based on the GS.

On the Airplane/Conveyor belt demonstration: It is amazing to see how such a simple bias can cloud peoples' perception of the world around them. Everyone knows intellectually that planes move forward because of their propeller/engine, fly because of air moving over their wings, and that the air above a conveyor belt is not moving appreciably. But I must admit that being tethered to terrestrial locomotion is throwing off my conceptualization of how this works.

Even more baffling is that after having it explained to them some people cling to the well and definitively busted myth.

Regarding the Airplane/Conveyor belt myth I think this site explains it well: http://txfx.net/2005/12/08/airplane-on-a-conveyor-belt/

Summary - the plane is stationary (engine off) on a stationary conveyor belt. When the conveyor belt starts moving, the planes wheels rotate freely and the plane remains stationary (engine off) relative to it's fixed point on the earth.

When the engine on the plane starts and the propellers (or jet) spin the plane starts to move FORWARDS regardless of how fast the conveyor moves. Air flows over the wings and the plane takes off as per normal.

The conveyor belt could move 100x faster than the plane but regardless the plane will move forwards and take off.