Recent content by Philip Wood

We define ##\mathbf D## as ##\mathbf D=\epsilon_0\mathbf E + \mathbf P##, in which ##\mathbf E## is the electric field strength and ##\mathbf P## is the polarisation. Would it not be more convenient for ##\mathbf D## to be defined in such a way that it had the same units as ##\mathbf E##. In...

Problems solved. Thank you so much for your help.

Sorry – I had technical problems. Please see amended post.

Sorry: I had problems using the site. Please see amended post.

Thank you for your suggestion. I'm not writing multiple lines of LaTex between double dollar and double dollar. I am enclosing between double hash and double hash stuff that I want to appear in lines of text. I haven't omitted any $$ or ## delimiters, nor nested any. I'm completely baffled...

I know, but I can't find an 'edit' button, so I can't do anything about them.

I know this is the wrong place to ask, but I need help with this site and don't know where to find it... (a) I can't find the 'edit' button on my longish post above with red error messages. (b) I can't understand the error messages (c) I can't find the latex guide, the one that uses a quadratic...

"The portion of the chain that is sliding across the table will not be accelerating." Why can't it have the same |acceleration| as the rest of the chain? I should have extended the ramp in a straight line across the deck. But I now agree that I was wrong to argue that the end-speed found for...

This is a re-write of my original post – of which I am ashamed. Mechanical energy is lost, because of the inelastic collision between the chain and the floor. This makes it unlikely that methods based on energy will be of any use. This prediction is confirmed by the form of the answer. Instead...

Many thanks. I'd better look into the concept of a second order tensor.

Many thanks, renormalize. So Jeevanjee's being too restrictive? No wonder he doesn't use the Cauchy stress tensor as an example.

I've just started learning about tensors from Jeevanjee's highly praised 'An Introduction to Tensors and Group Theory for Physicists'. He defines a tensor as a function, linear in each of its arguments, that takes some vectors (maybe only 2) and produces a number. [The components of the tensor...

Yes – which means that this re-arrangement is not much use for calculation purposes, but I thought it bore an interesting resemblance to x=\frac{1}{2}a_{co-ord}\ t^2. I'm thinking of t^2 - \left(\frac{x}{c} \right)^2 as the space-time interval between (0. 0) and (ct, x).

I do agree. Just a foonote… I find that the equation for x under constant proper acceleration, a, can be cast into the form x=\frac{1}{2} a \left(t^2 - \left(\frac{x}{c}\right) ^2 \right). Quite pretty, I thought.

Yes, I've just derived "pervect's equation" for x from constancy of proper acceleration. Yes indeed – if constant co-ordinate acceleration could be maintained – but it can't. But I'm being a bit of a devil's advocate; although I still believe that x=\frac{1}{2}at^2 isn't actually wrong for...