# Difficult / Famous Physics Interview Questions

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#1
I'm wondering if anyone knows any Physics interview questions from Oxford or Cambridge or just common braintwisters.

Cheers
1
9 years ago
#2
I can't remember what I was asked in my Oxford physics interviews but at my Leeds interview, I was asked to use gravitational forces equations and circular motion equations to show that the total mass of all the stars in a galaxy isn't high enough to account for the movement of some things that underwent circular motion around the galaxy, thereby showing that most of the mass of a galaxy is not the stars etc. but something that can not currently be detected, ie. Dark Matter.

Sorry if this isn't the kind of thing you were looking for.
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#3
Nononono this is exactly what I'm looking for, even if you don't remember any meaningful information I'm interested in what the interview topics were, ie in your case Gravity & Circular Motion.
Cheers
0
9 years ago
#4
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?)
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#5
(Original post by Mbob)
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...
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9 years ago
#6
(Original post by Brutal Chav)
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...
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.
0
9 years ago
#7
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?
1
9 years ago
#8
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.
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#9
(Original post by bluesky42)
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?
The sun loses mass because of energy loss, so I need to find the energy it loses each second using the inverse square law combined with an estimate of the solar constant using the assumption that an umbrella can power a washing machine...

Assuming the umbrella's a perfect absorber of what ever the hell comes out of the sun. Let the umbrella's area be about 0.5m^2 and the washing machine operate at 1kW; If 1kW falls on 0.5m^2 on the earth (assuming the umbrella always faces the sun directly) then 2kW fall on a square meter.

Assuming the sun radiates energy equally in all directions, an imaginary sphere, of radius 1.5x10^11m would enclose an area of ~2.8x10^23m^2. If one square meter on that area (the umbrella) receives 2000J each second, then the sun radiates out ~5.7x10^26J each second. Putting it into E=mc^2 gives a rate of loss of mass of about 6.3x10^9kg/s.
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#10
(Original post by lukas1051)
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.
When you say the image raises its right hand, it confuses me because I'm still seeing my left hand, it's just on the right side. I'd like to give some smart-ass answer to this question but I've already heard it before & I think there's a really good video on it http://www.youtube.com/watch?v=6tuxLY94LXw
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8 years ago
#11
(Original post by Mbob)
Correct - things that float displace their weight in water and things that sink displace their volume in water. Since density rock &gt; density water, a floating rock displaces more water than one than has sunk.
No, the level stays the same. Come on guys
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#12
No, the level stays the same. Come on guys

(and add any interview questions that you've heard of too!)
0
8 years ago
#13
(Original post by Brutal Chav)

(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
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#14
(Original post by TenaciousMonkey)
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
0
8 years ago
#15
(Original post by Brutal Chav)
well that's the same as what I've said above...
my question was towards the guy who said the level stays the same
0
8 years ago
#16
(Original post by TenaciousMonkey)
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
4
8 years ago
#17
Got that one at my Cambridge interview a couple of years ago, properly fluffed it in a panic but when you think about it it seems so easy.
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8 years ago
#18
(Original post by dknt)
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
how would you do this? wouldn't it depend on how much of the ice is floating above water level and how much is submerged? obviously ice is less dense than water so if it was in a closed container with no air the water level will drop. please reply my physics interview is on Wednesday!
0
3 years ago
#19
One of the questions ive heard is, you're at the beach,a nd you have a spade and a bucket. Can you work out how many grain of sands there are in your bucket?
0
1 year ago
#20
A) Yes, just empty out the bucket and it's 0.
B) No however you could find an estimate by working out the volume of the bucket and then using an estimate of the volume of a sand grain and then diving the two.

I can't think of any other ways, am I missing something?
Last edited by WackyWackyWoosh; 1 year ago
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