Search found 474 matches

by joe
Wed Aug 31, 2016 8:19 am
Forum: Physics Questions
Topic: Mathematical obscuration?
Replies: 30
Views: 27755

Re: Mathematical obscuration?

If you are preparing a formal argument, please make sure that you use the formal definition of limits. See for example https://en.wikipedia.org/wiki/Limit_(mathematics) and there are quite a few other introductions to limits on the web. Please do not use the terms 'infinitesimal' or 'infinite', beca...
by joe
Fri Aug 26, 2016 7:58 am
Forum: Physics Questions
Topic: Mathematical obscuration?
Replies: 30
Views: 27755

Re: Mathematical obscuration?

h and delta x are both very small quantities, as required in the definition of limits.
by joe
Thu Aug 25, 2016 3:32 pm
Forum: Physics Questions
Topic: Mathematical obscuration?
Replies: 30
Views: 27755

Re: Mathematical obscuration?

"delta x goes to zero but delta x is not treated as zero"

In the definition of limit, delta x is a small quantity.
by joe
Thu Aug 25, 2016 1:43 pm
Forum: Physics Questions
Topic: Mathematical obscuration?
Replies: 30
Views: 27755

Re: Mathematical obscuration?

ASH is correct to say " It is correct to say that lim h->0 (2x+h) = 2x. " Boris then writes " Ok, then, please, prove mathematically and/or geometrically that it is correct to say so. " It follows directly from the definition of a limit. See for example https://en.wikipedia.org/w...
by joe
Wed Jul 13, 2016 4:37 pm
Forum: Primary School Physics
Topic: Welcome to the forum
Replies: 0
Views: 24306

Welcome to the forum

Kids love to ask why. And some of those who never stop asking why turn into scientists. So scientists are usually happy to try to answer some of those questions from curious children. This forum was created at the request of some teachers. I'll moderate it and try to find suitable authoritative answ...
by joe
Fri Apr 01, 2016 7:27 am
Forum: High School Physics
Topic: HSC 2015 Q19
Replies: 1
Views: 19622

Re: HSC 2015 Q19

(A) is not true. One should never say 'negligible' without a phrase 'in comparison with ..." (B) The statement itself is true. If the answer had included "They are also both in orbit around the Earth", then this would be easily the best answer. (C) is not true. (D) With respect to any...
by joe
Thu Mar 03, 2016 2:50 pm
Forum: Physics Questions
Topic: Force and Energy?
Replies: 1
Views: 4770

Re: Force and Energy?

Position, velocity and acceleration of particles are all classical concepts that do not simply correspond to the properties of very small things. An electron does not travel around a nucleus (if it did, it would be accelerating* towards the nucleus (centripetal acceleration) and therefore would radi...
by joe
Fri Jan 08, 2016 2:04 pm
Forum: Physics Questions
Topic: Multi-variable systems?
Replies: 43
Views: 37540

Re: Multi-variable systems?

I can only see the brute force method: Write down the distance D between the given point and an arbitrary point (X,Y,Z) on the curve (using the d= equation you have). So you have D = D(X,Y,Z) At the shortest distance, ∂D/∂X, ∂D/∂Y and ∂D/∂Z are all zero. That gives you three equations to solve simul...
by joe
Sun Dec 27, 2015 9:53 pm
Forum: Physics Questions
Topic: Question about ẍ = v.dv/dx .
Replies: 2
Views: 6957

Re: Question about ẍ = v.dv/dx .

I agree. when v goes to zero, dx goes to zero.

Happy new year
Joe
by joe
Sun Dec 27, 2015 9:50 pm
Forum: Physics Questions
Topic: Multi-variable systems?
Replies: 43
Views: 37540

Re: Multi-variable systems?

There doesn't appear to be a drawing.
If your surface is a parabola of rotation, then you need only a single integration:
the element of area will look something like 2*pi*r*ds
and you'll need to integrate along s.
by joe
Tue Dec 15, 2015 8:10 am
Forum: Physics Questions
Topic: Electricity and electric current?
Replies: 17
Views: 20876

Re: Electricity and electric current?

The spark is quite different from an incandescent bulb. In the bulb, ohmic losses in the conductor heat the filament up to a few thousand kelvin, and it then glows. To understand the spark, first let's note that there is no such thing as a perfect insulator: when an external electric field becomes c...
by joe
Fri Sep 25, 2015 12:56 pm
Forum: High School Physics
Topic: 2008 HSC
Replies: 1
Views: 13084

Re: 2008 HSC

0.5 s is the time interval on the clock on the ship. For an earth observer, that time is dilated. See http://newt.phys.unsw.edu.au/einsteinlight/ which explains time dilation with animations. However, there's a complication. 0.63 s is the answer you'd get if the ship happened to be travelling at rig...
by joe
Thu Sep 24, 2015 11:58 am
Forum: High School Physics
Topic: E=hv
Replies: 2
Views: 13314

Re: E=hv

I started to answer this, but the answer is too long, and it involves equations and links to further explanation. So I've added it to our FAQ in high school physics at
http://newt.phys.unsw.edu.au/~jw/FAQ.html
by joe
Wed Sep 23, 2015 10:31 am
Forum: High School Physics
Topic: Heat diffusion
Replies: 6
Views: 20388

Re: Heat diffusion

Incidentally, we always referred to such problems as Carslaw and Jaeger problems. If you are interested, I recommend "Conduction of heat in solids" by Carslaw and Jaeger (from NSW and Tas respectively). A very good book, and more generally useful, because the diffusion equation arises not ...
by joe
Mon Sep 14, 2015 11:57 am
Forum: Physics Questions
Topic: Multi-variable systems?
Replies: 43
Views: 37540

Re: Multi-variable systems?

F . dA can be set equal to three different expressions: i) F dA cos theta or ii) dA times the component of F parallel to dA or iii) F times the component of dA parallel to F You don't need to calculate that area, because you are not going to use (i) or (ii). In this case you use (iii), so the only ...