Difficult / Famous Physics Interview Questions

A man sits in a boat in a Lake. There is a large rock in his boat. He throws it over the side of the boat. What happens to the level of the water in the lake? (goes up, down, stays the same?)

The level depends on the water being displaced and Archimedes says the weight of water displaced is directly proportional to the upthrust force on whatever's in the lake. With the rock in the boat, it's in equilibrium so its entire weight is being balanced by the water it displaces. When you drop it (because of its higher density than water) it moves down, meaning that the upthrust is less than before and hence the amount of the displaced water decreases. The boat starts off floating and ends up floating so you can take it out of the picture. Hence the level goes down...

If you could power a washing machine from the sunlight that hits the surface of an umbrella, how much mass does the sun lose a second?

I swear this is a famous interview question at one of the American universities, MIT or somewhere like that... anyway, the question is this: Why is it that when you are looking into a mirror and you raise your left hand, the image in the mirror raises its right hand? It seems simple, but it does make quite a lot of people stumble.

Correct - things that float displace their weight in water and things that sink displace their volume in water. Since density rock > density water, a floating rock displaces more water than one than has sunk.

No, the level stays the same. Come on guys

please explain

(and add any interview questions that you've heard of too!)

It depends on the density and volume of the rock, doesn't it?

So if you have a really dense, small rock, it pushes the boat down with more force so that a larger volume of water is displaced, so when you thow it out, it doesn't displace a large volume of water due to its small size, so the water level drops. But if the rock isn't as dense and has a larger volume, it applies less of a force downwards on the boat when it's in the boat. When you throw it out, if it's less dense than the small rock but occupies a larger volume, you have a larger volume of water being displaced and so the water level rises.

That might be total rubbish but that's what I would have said

well that's the same as what I've said above...
my question was towards the guy who said the level stays the same

Oh, right, sorry - probably misread your post.

N00b.

Here's a similar one I suppose.

There is a glass of water and a cube of ice. The cube of ice then melts. Assuming no evaporation, what happens to the level of the water? Actually, that's probably fairly easy. But whatever