Beyond common sense.

[Boustrophedon]

I was musing on the Flat Earther thread and reading the Wiki about the Bedford level experiment and about the atmospheric distortions that would lead uninformed people to believe that the earth was flatter than it is. Thus even the evidence of one's own eyes can lead you to perfectly wrong conclusions.

As an example I completely failed to convince a teaching colleague that a plank of wood, equal in all other ways to another plank of wood but twice as thick would be eight times as stiff in flexure (about the perfectly bl..dy obvious axis, before any of you pedants jumps on me.) than the other plank (not my colleague.)
He argued that there was twice as much wood so obviously it was twice as strong, and that one ruler placed on top of another ruler was obviously twice as stiff. I later found the two rulers argument in a technology teaching textbook.

So are there any other examples where perfectly obvious common sense leads to conclusions that are totally wrong?

[Pucksoppet]

If cup of coffee A is twice as big as cup of coffee B, then it is obvious (to some) that coffee B is twice as small as coffee A.

It is also obvious that heavy things fall faster than light things, as demonstrated by a cannonball and an inflated balloon the same size as the cannonball.

It is obvious that turning a thermostat (such as found on ovens or living-room or office walls) to higher than the target temperature makes the temperature increase faster.

It is 'obvious' that you get significantly less wet by running in the rain.

[greyspoke]

There's the one about cycling attire...

[Bird on a Fire]

If cup of coffee A is twice as big as cup of coffee B, then it is obvious (to some) that coffee B is twice as small as coffee A.

Is this not just an issue with common parlance vs mathematical notation, rather than being wrong/misleading? In normal language it makes sense for the relationship between 250ml and 500ml to be described consistently.

It is also obvious that heavy things fall faster than light things, as demonstrated by a cannonball and an inflated balloon the same size as the cannonball.

Outside of a vacuum, this is true.

It is obvious that turning a thermostat (such as found on ovens or living-room or office walls) to higher than the target temperature makes the temperature increase faster.

I've heard this one before and accept its veracity, but I don't understand why it doesn't. I naively assumed that a higher target temperature would result in more power going to the heater and that this in turn would cause it to warm up faster - which half of my reasoning is wrong (or both?)

[shpalman]

It is obvious that turning a thermostat (such as found on ovens or living-room or office walls) to higher than the target temperature makes the temperature increase faster.

I've heard this one before and accept its veracity, but I don't understand why it doesn't. I naively assumed that a higher target temperature would result in more power going to the heater and that this in turn would cause it to warm up faster - which half of my reasoning is wrong (or both?)

Because most heater thermostats are either on or off, rather than regulating the power proportionally according to the difference between actual temperature and set point. (If you just had a proportional control, though, you would end up either always a little bit below the set point or oscillating around it depending on the various time constants of the system. This is what PID controllers attempt to deal with.)

The simplest sort of thermostat is a bi-metallic strip. But generally since the power going to the heater tends to be high, it's much easier to just switch it on or off than to try to regulate it.

[Gfamily]

With regard to thermostats, if you are cold and in a room that needs heating, you want to make sure that the room as a whole is properly at the desired temperature, rather than just the thermostat.

For instance, if the thermostat is in a slightly warmer place than the average for the room, the heating may find that it cycles a few times before the room as the whole feels comfortable.
So in the real world, turning the thermostat up higher may well result in the room reaching the desired temperature faster,

temp time.JPG (37.28 KiB) Viewed 2331 times

[shpalman]

It's complicated by the fact that you might feel more subjective warmth from a low but increasing temperature than when the temperature is warmer but constant. There is also the effect of feeling the warmth coming directly off the radiators.

[dyqik]

It is also obvious that heavy things fall faster than light things, as demonstrated by a cannonball and an inflated balloon the same size as the cannonball.

Outside of a vacuum, this is true.

No, it isn't. A 100kg man on a parachute falls more slowly than a 45g golf ball or a 2.3g penny. Or maybe even one of the those plastic parachutist toys of you're going to be picky and require the shape to be roughly the same.

[marbling]

I've recently had a heated discussion with my dad, to whom it was obvious that if every team plays every other team in the 6 Nations that the total number of matches would be 6x5. I did the maths the hard way and demonstrated that it would be 5+4+3+2+1. Neither of us worked out that it would be (6x5)/2, which is kind of obvious in retrospect.