Failed Intuition: The Boy Who Fell on His Ass

Lately, and I've mentioned it in several posts, I've been pondering about human intuition and how it so often gets things wrong.

My current belief is that we think logic has guided our intuition, whereas I think much of the time we just experience the world and file things away as this way or that without further thought. To me the Monty Hall Paradox is a good example of it.

Here is something that happened when I was maybe 7 or 8 years old. A few of us were sitting on the back of a pick up truck that was giving us a ride, and it slowed down to let us off. One of the kids decided to jump off before it stopped. He braced himself himself to fall in the direction opposite the way the truck was moving, and fell on his ass to the great amusement of everyone else. But I have to tell you this experience has stuck with me all these years, and the fact of the matter is I wondered if I could have made the same mistake. He was athletic and the truck was hardly moving, yet that he was going to fall in that direction was completely against his intuition.

Of course, his experience was that as you drive along in a car, if you throw something out the window, it goes whipping back with force, seeming to accelerate backwards. We were certainly not careful enough observers at that age to see that the object actually would tumble forward as it hit the ground. Thus intuition formed that objects thrown from a car, or dropped off the end of a truck presumably, had momentum opposite of the direction of the vehicle. Of course, the opposite is true, and he should have been prepared to start running in the direction the truck was moving as he jumped off. We all know that now, our intuition is reversed, and we no longer remember thinking something as stupid as imagining the momentum is opposite.

If this thread is interesting to anyone, I'll continue with the experiment of the person standing on a disk who starts revolving a second disk. Intuition will likely fail you on this one too! ... Perhaps someone knows this one.

A similar scenario was often portrayed on the show Knight Rider. On the show a sports car would be driving along, let's say at 60 MPH, behind a drink going a little slower, let's say 55 MPH. Then the truck would lower a ramp, and the sports car would slowly drive up the ramp and into the back of the truck.

Many have claimed this would violate the laws of physics, because once the car hit the ramp, it should have traveled at 60 MPH relative to the ramp, or 115 MPH total. However, the show was correct. A similar experiment was easily duplicated on the show Myth Busters.

Quote: odiousgambit

Of course, his experience was that as you drive along in a car, if you throw something out the window, it goes whipping back with force, seeming to accelerate backwards.

Of course, the boy is confusing velocity with acceleration. The velocity is in the opposite direction as the car movement, but the net acceleration is still going forward.

You get a strange sensation if you jump out of an airplane for the freefall. Within about 6-10 seconds you achieve a terminal velocity of somewhere near 120 mph (200 km/hr) but the acceleration goes to zero as the air friction balances out the force of gravity. So you feel weightless because there is no acceleration, but at the same time the wind is whistling past your ears, and distorting your face under the drag force. You don't intuitively associate such intense speed with no acceleration. It's a very exotic sensation, which is part of the reason people get addicted to skydiving.

I think that human intuition all geared at pre-industrial world. Before engines and motors the velocity and acceleration vectors were generally pointing in the same direction (like running or rolling down a hill or leaping).

People think they can jump up in the air just as the out of control elevator hits the bottom of the shaft, and be spared any terrible injuries.

To some extent that is true about our intuition about gambling. In pre-industrial society events happened somewhat randomly, and the idea of something bad happening to you over and over is not intuitive. Nobody got hit by lightning every month. But rolling dice, spinning wheels, dealing cards and other repetitive scenarios doesn't happen in nature. So the idea of getting a bad roll seven or eight times in a row is unnatural. It is intuitive to think the bad luck must stop, whereas the dice has no memory and simply does not care what you think.

Quote: pacomartin

To some extent that is true about our intuition about gambling. In pre-industrial society events happened somewhat randomly, and the idea of something bad happening to you over and over is not intuitive. ...

I most certainly agree that gambling has counter-intuitive aspects and this is on topic! As far as the above point, also we are vulnerable to the post hoc propter hoc fallacy, that's the one where you foolishly connect an event such as a falling star with, say, subsequent illness striking your village. Of course it was extremely valuable to connect eating a poison mushroom with illness, etc.; unfortunately the evolved mechanism is largely intuition on this, rather than logic.

Here's the promised bit on the man with a disk standing on a disk.

Saw this experiment once:

A man stands on a disk that can spin. He holds upright a second disk that is smaller and that can spin. He starts spinning the held disk and the disk he stands on starts spinning in the opposite direction.

So far ho hum.

He then positions the held spinning disk's axis at 90 degrees to the disk he is standing on. The disk he is standing on stops.

Well, so far that part might be surprising, but just a bit.

He then turns the held spinning disk straight down. The disk he is standing on starts spinning in the opposite direction!, and the man himself spins in the direction opposite what he first propelled himself in! Now that, to me, was totally beyond the expectations of myself and for that matter the general audience. Practically Twilight Zone stuff. Conservation of Angular Momentum I think it is called. Angular Momentum shall be preserved, Thus spake Zarathustra!

edit: I looked online and saw some questions about what happens when you stand on a spinning disk and walk to the edge, etc., but didnt find this exact thing. Didnt look all that hard tho.

Quote: Wizard

...Then the truck would lower a ramp, and the sports car would slowly drive up the ramp and into the back of the truck.

Many have claimed this would violate the laws of physics, because once the car hit the ramp, it should have traveled at 60 MPH relative to the ramp, or 115 MPH total. However, the show was correct. A similar experiment was easily duplicated on the show Myth Busters.

I never understood why people would believe that.

Anyone who has had to stop a car quickly, or has watched a drag racer spin his wheel when he hit the gas, would know that inertia is a far stronger force than a skidding tire. INERTIA is the law of physics they should be referring to.

Quote: pacomartin

People think they can jump up in the air just as the out of control elevator hits the bottom of the shaft, and be spared any terrible injuries.

Bill Cosby did a bit about this. He said, "Might not work. Might be 8 crumpled bodies on the floor, and one head thru the roof, but they will say 'Man he sure did try.'"

The reality is, if you DO manage to time it right, you'll simply reduce the speed that you hit the bottom of the elevator shaft by a mere couple mph. (How high/fast can a person jump up?) The result will still be 9 crumpled bodies. You'll just be on the top of the pile.

As I recall, MythBusters did this one too. One thing about the episode that I remember is that they were shocked at how easy it was to disable the fail-safe!

Quote: pacomartin

You get a strange sensation if you jump out of an airplane for the freefall.

My sensation was one of not being able to jump!

Many years ago in college, some buddies and I went skydiving. It was our first trip, and they gave us the lessons on what to do, and took us up in the plane. We weren't jumping tandem, so out chutes were hooked on (?) to something so that they would open automatically. As I stepped out on the landing strut (?) of the plane, I looked behind me, and saw the tail wing. This was a huge mistake. At this point, my sense of "what is right" told me that when I let go, the plane would keep moving forward, and the tail wing would cut me in half.

It took much cajoling and threatening on the part of the jump instructor to finally get me to let go. I was naive at the time and honestly believed that the only way down was to jump. But it's a good example of my expectation being based on some silly notions, rather than honest science and mathematics ruling my brain.

Quote: DJTeddyBear

I never understood why people would believe that.

Anyone who has had to stop a car quickly, or has watched a drag racer spin his wheel when he hit the gas, would know that inertia is a far stronger force than a skidding tire. INERTIA is the law of physics they should be referring to.

There's more going on. To begin with there's the law of conservation of momentum, which states momentum cannot be destroyed, only transfered. So if a moving object hits another object, it transfers some or all its momentum to the second object, whether the second object is moving or not.

Let's say you're doing 55 mph on the freeway and are struck by a car with the same mass as yours, doing 65 mph. Your car will increase its speed, but not not to 65 mph, much less to 55+65 mph. If, however, you're hit by a tractor trailer massing ten times the mass of your car moving at 65 mph, you'r speed will increase a great deal more. It's a matter of kinetic energy as well as momentum.

Next there are relative speeds. A car doing 65 passing a car doing 55 has a speed of only 10 mph relative to the slower car. In the Knight Rider "myth" the sports car effectively matches speed with the trailer, winding up with a relative speed of zero, no matter how fast they're moving relative to a motionless observer. In fact, in order to make it up the ramp and inside the trailer it would ahve to accelerate a little in relation to the trailer.

Now, the combination of tractor trailer and car would have the sum of the momentum of each separately. Meaning if the trailer needs to stop after swallowing the car, ti will take longer to do so, becasue it has more momentum than it did before. Momentum is added, not transferred.