 Jan 2016

markbetnel.com markbetnel.com

This is an NPR piece that talks about the pros and cons. Two articles about projects in California, one opinion piece from the LA Times arguing against desalination plants and then an article talking about the plans for a plant in the Catalina Islands. Finally, this article described the measures Australia took to deal with its water crisis and how the reduction measures made their desal plans unnecessary.
None of these pieces really present positives about desalination  can you find some more resources that explain how desalination works and in what circumstances it makes sense to do it?


web.hypothes.is web.hypothes.is

by URL
Is there a way to filter by domain? I'm trying to use hypothes.is in a high school class to get students to annotate pages on my own site. I can't count on them to use consistent tagging, but if I could filter all annotations from my entire domain I could get a useful stream of the students' work.


markbetnel.com markbetnel.com

What's the kinetic energy associated with that center of mass velocity? It's the standard KE formula, using the total mass and the center of mass velocity  KECM=12mtotv2cmKECM=12mtotvcm2KE_{CM} = \frac{1}{2} m_{tot} v^2_{cm}. That energy must remain as kinetic energy during the collision, so this is the nonconvertible part of the total energy.
Why is there a nonconvertible kinetic energy?
Because momentum is conserved. If there was a net momentum before an interaction, there must still be a net momentum after, and that implies that there will still be some kinetic energy after, no matter what happened during the collision.


markbetnel.com markbetnel.com

$$\begin{eqnarray} v_f & = & v_i + at \\ t & = & \frac{v_f  v_i}{a}\\ \Delta x & = & v_i (\frac{v_f  v_i}{a}) + \frac{1}{2}a(\frac{v_f  v_i}{a})^2 \\ a\Delta x & = & v_i v_f  v_i^2 + \frac{1}{2}(v_f^2  2v_fv_i + v_i^2)\\ a\Delta x & = & \frac{1}{2}v_f^2 \frac{1}{2}v_i^2 \\ v_f^2  v_i^2 & = & 2a\Delta x \end{eqnarray} $$

What was the velocity of the block of wood, just before it impacts the nail?
The experiment in question involved dropping a block of wood onto a nail that was partially driven into a block of styrofoam. We manipulated the mass of the block and the height of the block, and measured the depth the nail was driven into the block as the responding variable.
We found, in general, that the response was linear for both manipulated variables, though the data was quite noisy. Here we are trying to understand what we should expect the result to be.
