Jim's maths and physics problems

[JimC]

Problem 4

An illuminated object is 23 cm in front of a convex lens with a focal length of 17 cm. Calculate the position and magnification of the image produced, and comment intelligently on the nature of the image.

[Trolldor]

Problem 4

An illuminated object is 23 cm in front of a convex lens with a focal length of 17 cm. Calculate the position and magnification of the image produced, and comment intelligently on the nature of the image.

I calculate the image to be roughly in front of the lens, and it appears to me to be a bleak representation of the future of a Capitalist driven society.

[JimC]

Problem 4

An illuminated object is 23 cm in front of a convex lens with a focal length of 17 cm. Calculate the position and magnification of the image produced, and comment intelligently on the nature of the image.

I calculate the image to be roughly in front of the lens, and it appears to me to be a bleak representation of the future of a Capitalist driven society.

Fail...

(especially since it is on the far side of the lens)

And where was last week's physics homework, young man?

[Trolldor]

Problem 4

An illuminated object is 23 cm in front of a convex lens with a focal length of 17 cm. Calculate the position and magnification of the image produced, and comment intelligently on the nature of the image.

I calculate the image to be roughly in front of the lens, and it appears to me to be a bleak representation of the future of a Capitalist driven society.

Fail...

(especially since it is on the far side of the lens)

And where was last week's physics homework, young man?

It is both in that box on your desk and not in that box on the desk.

[JimC]

Problem 4

An illuminated object is 23 cm in front of a convex lens with a focal length of 17 cm. Calculate the position and magnification of the image produced, and comment intelligently on the nature of the image.

I calculate the image to be roughly in front of the lens, and it appears to me to be a bleak representation of the future of a Capitalist driven society.

Fail...

(especially since it is on the far side of the lens)

And where was last week's physics homework, young man?

It is both in that box on your desk and not in that box on the desk.

Would have been a good answer...

Except the homework was all classical physics...

That Schroedinger cat has a lot to answer for...

[Twiglet]

How did you know you would expose farsight's twaddle so easily. We've been trying for nigh on 50 pages?

To be fair, a lot of people have been calling BS for a while. The only thing I did differently was to ask questions which many people visiting here are capable of answering, or understanding the answers given. It's hard to expose flaws in someones understanding when they talk about Ph.D. level stuff. It becomes a battle of BS, and when you have a good salesperson vs a scientist, the salesperson will win out with years of practice.

Asking simple questions on familiar territory is a good way to expose when someone is making stuff up. That's why they don't have physics or maths oral exams. You sit a test and answer questions to demonstrate competence and understanding. You have to actually supply proofs and choose what equations will solve the problems.

I can well understand people getting stuck on really complex second order differential equations or (insert gibberish here), but how can someone who doesn't even understand Newton or simple algebra be credibly engaging with something like String theory?

I like that we've gone to these Q&A, IMO it is one of the best ways to get an appreciation for science - by seeing what the answers look like and how they are arrived at. It's not a spectator sport.

[colubridae]

Problem 4

An illuminated object is 23 cm in front of a convex lens with a focal length of 17 cm. Calculate the position and magnification of the image produced, and comment intelligently on the nature of the image.

I calculate the image to be roughly in front of the lens, and it appears to me to be a bleak representation of the future of a Capitalist driven society.

Fuck me not even gone back to the intenet.

hope it's 1/u + 1/v = 1/f

in which case

1/23 + 1/v = 1/ 17



1/v = 1/17 - 1/23


gives 1/v = (23-17)/ 23*17

v = 391/6 = 65 1/6 cms


is it erect and real?

stretching memory now.

[Twiglet]

Problem 4

An illuminated object is 23 cm in front of a convex lens with a focal length of 17 cm. Calculate the position and magnification of the image produced, and comment intelligently on the nature of the image.

1/u +1/v = 1/f

u=0.23, f=0.17

1/v= (1/0.17) - (1/0.23)

v=0.651 metres

The image is inverted.

[Don't Panic]

Problem 2

A 2 kg mass of copper at 250 degrees C is added to 20 litres of water at 20 degrees C.

Assuming no loss of heat energy to the surroundings, calculate the temperature at thermal equilibrium, given that the specific heat capacity of water and copper are 4200 j/kg/degree and 390 j/kg/degree respectively.

Hmmm,

2*390(T-250)=20*4200(T-20)
780T-195,000=84,000T-1,680,000
1,485,000=83,220T
T=17.84

That makes no sense, think I fucked up somewhere.

The error was that the copper is losing heat energy, and so it is a negative term, leading to:
2*390(250 - T) = 20*4200(T - 20)
When solved by the same method you used, the temperature at thermal equilibrium comes to 22.12 degrees C...

I realised that half an hour ago while sitting in the bath.
Hadn't my brain switch on this morning.

[Clinton Huxley]

I had a rather nice Casio calculator at school. I could press the enter button 61 times in 10 seconds.

That's the only contribution I can make here.

As you were.

[Twiglet]

This one might be quite fun!

Calculate escape velocity from the Earths surface

You may use the following assumptions:

radius of earth = 6400km
mass of earth=5.97 × 10^24 kilograms
gravitational constant = 6.673 × 10-11 m3 kg-1 s-2

In practical terms, explain why a higher velocity would be needed to achieve orbit.

[Feck]

tried the chemistry one


2 C8H18 +25 O2-----16CO2+18H20


and I did the maths for the masses but when I added up both sides of the equation to check at the end they did not match ....So I gave up like the lazy sod I am !

If nobody else does this one I might try it again later (without the decimal places probably )

[JimC]

tried the chemistry one


2 C8H18 +25 O2-----16CO2+18H20


and I did the maths for the masses but when I added up both sides of the equation to check at the end they did not match ....So I gave up like the lazy sod I am !

If nobody else does this one I might try it again later (without the decimal places probably )

Correctly balanced equation.

Using stoichiometry, for every 1 kg of octane, 3.50 kg of O2 is consumed, and 1.42 kg of CO2 and 3.08 kg of H2O are produced...

Is it sad that I have made an Excel sheet (with look-up tables, no less) that performs stoichiometric calculations?

[JimC]

Problem 4

An illuminated object is 23 cm in front of a convex lens with a focal length of 17 cm. Calculate the position and magnification of the image produced, and comment intelligently on the nature of the image.

1/u +1/v = 1/f

u=0.23, f=0.17

1/v= (1/0.17) - (1/0.23)

v=0.651 metres

The image is inverted.

Both you and colubridae were correct, the image would be 65.17 cm on the other side of the lens. The image is real and inverted, and has a magnification of x2.83

I always approach this with my students via lots of prac work (curved mirrors likewise), making a large range of measurements of object and image distances, and teasing out the relationships using the data. Plus lots of ray diagrams, of course...

[Xamonas Chegwé]

Problem 4

An illuminated object is 23 cm in front of a convex lens with a focal length of 17 cm. Calculate the position and magnification of the image produced, and comment intelligently on the nature of the image.

1/u +1/v = 1/f

u=0.23, f=0.17

1/v= (1/0.17) - (1/0.23)

v=0.651 metres

The image is inverted.

Both you and colubridae were correct, the image would be 65.17 cm on the other side of the lens. The image is real and inverted, and has a magnification of x2.83

I always approach this with my students via lots of prac work (curved mirrors likewise), making a large range of measurements of object and image distances, and teasing out the relationships using the data. Plus lots of ray diagrams, of course...

Actually, they're called Mandelson diagrams here.