Follow your Gut?

Next in the series, I'm looking for questions or problems which specifically make your gut tell you one thing, but that when you think about it more you realise you're wrong.
Not a teaser, it needs to be a question that makes you THINK your first gut reaction is right, but actually would be wrong.
Physics based would be preferred, but it must be an everyday kind of thing.

OK - two men put a large rock into a boat and row out into a lake. They throw the rock over the side. What happens to the water level?

It stays the same... right idea though, it's got the everyday thing, a similar one is if you've got a glass containing one ice cube and the rest is water, to the very brim, one more drop and it overflows, what happens to glass of water when the icecube melts?

Originally posted by agryson
It stays the same... right idea though, it's got the everyday thing, a similar one is if you've got a glass containing one ice cube and the rest is water, to the very brim, one more drop and it overflows, what happens to glass of water when the icecube melts?

The level of water lowers a little because water is more dense than ice, though the number of water particles in the glass doesn't change (if we assume there is no evaporation).

Originally posted by agryson
Next in the series, I'm looking for questions or problems which specifically make your gut tell you one thing, but that when you think about it more you realise you're wrong.
Not a teaser, it needs to be a question that makes you THINK your first gut reaction is right, but actually would be wrong.
Physics based would be preferred, but it must be an everyday kind of thing.

If a woman is unmarried, has short hair, reads Gloria Steinem books, and likes numbers, is she more likely to be a banker or a feminist banker?

hmm... that banker question has me stumped.
As for the ice one, actually the level doesn't change at all, the ice is less dense, by about 10% so 10% of the ice cube is actually above the level of the water and once it melts has the same density so you see no change in the level of the water.
That's an example of what I'm looking for...
Any others?

Originally posted by CauselessOne
OK - two men put a large rock into a boat and row out into a lake. They throw the rock over the side. What happens to the water level?

I'm fairly sure the water level actually gets lower. This is because when the rock is in the boat it is displacing it's weight and when it is thrown overboard it is only displacing it's volume. Since the rock is denser than water it displaces more when it's on the boat than when it's thrown overboard, meaning the water level decreases when it is thrown over.

Originally posted by PaddyB
I'm fairly sure the water level actually gets lower. This is because when the rock is in the boat it is displacing it's weight and when it is thrown overboard it is only displacing it's volume. Since the rock is denser than water it displaces more when it's on the boat than when it's thrown overboard, meaning the water level decreases when it is thrown over.

Exactly right - unless the lake is deep enough for it to float, of course ...

hmm... there you go, exactly what I'm looking for then!
(May you burn in hell Archimedes!)
Any others?

I don't know how "real life" this is, but I've seen this in Physics classes:

Suppose you have a tricycle and a piece of string is tied to one of the pedals. If the pedal is at the top of it's rotation and the string is pulled forward gently (by someone standing in front of the tricycle), the tricycle rolls forward. But what will happen if the pedal is at the bottom of its rotation and the string is pulled forward?

You are driving along the road in a car with the windows rolled up and a helium balloon floating around, say, in the middle of the car, when you hit the brakes. Which way does the balloon move in the car?

Suppose an electric kettle is 70% efficient when used to heat 1 L of water. Does the efficiency of the kettle increase, decrease, or stay the same if it used to heat 1.5 L of water (assume the kettle does not over-flow)?

How's about this?

You are in a room with a talking riddle-chimp. The riddle-chimp says "you have two options. In option one, I will ask you to say a statement. If this statement is true, I will give you £100. If it is false, I will give you a golden watch, worth £90. In option two, I will ask you to say a statement. If it is true, I will give you £150, if it is false, I will also give you £150." Which option should you choose?

Originally posted by smomofo
I don't know how "real life" this is, but I've seen this in Physics classes:

Suppose you have a tricycle and a piece of string is tied to one of the pedals. If the pedal is at the top of it's rotation and the string is pulled forward gently (by someone standing in front of the tricycle), the tricycle rolls forward. But what will happen if the pedal is at the bottom of its rotation and the string is pulled forward?

I say, if the string is pulled forward when the pedal is at the bottom of its rotation two things can happen:

If it is a tricycle with back pedal brakes, then the tricycle will not move or will jerk a bit and then stop.

Or the chain does not lock when back pedaled, and tricycle does not move.

So it doesn't move....seems too easy...

In the tricycle question, the bike still moves forwards.