Physics

Here's a little physics riddle. It's really meant as a moment of self-reflection for physics teachers (I invite you to compare what answers you'd give within Relativity Theory).

We're in the context of Newtonian mechanics.

There are three small bodies. In the inertial coordinate system (t, x, y, z), we know the following about the three bodies (at a given instant of time):

• The first has mass 3 kg
• The second has velocity (1, 0, 0) m/s
• The third has momentum (2, 0, 0) kg⋅m/s

Now consider a new coordinate system (t', x', y', z') related to the first by the following transformation (a Galileian boost):

t' = t, x' = x - u⋅t, y' = y, z' = z with u = 1 m/s

Questions:

• What is the mass of the first body in the new coordinate system?
• What is the velocity of the second body in the new coordinate system?
• What is the momentum of the third body in the new coordinate system?

Can you give definite answers to these three questions, and motivate your answers with simple physical principles? Note that by "definite answer" I don't necessarily mean an answer with a definite numerical value.

cross-posted from: https://slrpnk.net/post/15007841

Eating gamma radiation for breakfast

Some fungal species appear to be able to use strong radiation as an energy source for growth. Tom Ireland explores the exciting potential of these understudied organisms

In the heart of World War II, as the Nazis took control of Copenhagen, a peculiar situation took place at the Institute of Theoretical Physics, led by physicist Niels Bohr. Two Nobel laureates Max von Laue and James Franck, fearing the confiscation of their gold Nobel Prize medals by the Nazis, had sent their medals to Bohr for safekeeping.

On the day the Nazis arrived in Copenhagen, Hungarian chemist Georgy de Hevesy, who was working in Bohr's lab, devised a plan to prevent the discovery of the medals. Initially considering burying the medals, they quickly dismissed the idea, fearing the thorough searches the Nazis would conduct. Instead, de Hevesy proposed a chemical solution — literally. Utilizing a mixture known as "aqua regia" (a blend of hydrochloric and nitric acids), he set about dissolving the gold medals. This concoction is one of the few substances capable of dissolving gold, a notably unreactive element. As the Nazis marched outside, de Hevesy dissolved the precious medals, reducing them to a colorless solution that eventually turned bright orange. The liquid containing the dissolved gold was then placed on a high shelf in the laboratory, where it remained unnoticed throughout the Nazi occupation​.

Post World War II, upon returning to the laboratory after V-E Day, de Hevesy found the beaker undisturbed on the shelf. The gold was recovered from the solution and returned to the Nobel Prize committee, who then reminted the medals and presented them back to Laue and Franck in a ceremony in 1952.

Source: Fermat’s Library via LinkedIn

Hear me out. This thought process requires a bit of knowledge of physics/chemistry.

On the martian poles, there are vast quantities of frozes CO2. This frozen CO2 exerts a certain "vapor pressure" - in other words, a certain partial pressure of gaseous CO2.

Now, if we convert this CO2 into O2 by removing the carbon out of it, the concentration of O2 in the atmosphere increases. And therefore, the concentration (and partial pressure) of CO2 decreases.

But since the frozen CO2 on the poles causes a certain partial pressure of CO2, a bit of the frozen CO2 will go into gaseous phase to refill the CO2 partial pressure.

So, by converting CO2 into O2, the concentration of O2 increases, but the concentration of CO2 stays approximately the same. As such, the total pressure (and density) of the atmosphere increases. This would happen if large-scale biological photosynthesis/growth took place.

Any thoughts?

New video on Brady Haran's chemistry channel, but I thought it would be better suited for the physics community.

A 31 minute SciShow video on how lead ingots which were recovered from a Roman shipwreck enabled the CUORE experiment's search for neutrinoless double beta decay, and the challenges of ethically sourcing "low background material".

Has anyone set aside lead (and other materials) as a stockpile of low background material for future generations of physicists? Seems like something The Long Now Foundation might do.

Crossposted from: https://sh.itjust.works/comment/12976393

Source

I learned this week that many high speed CD-ROM drives used balancing balls on the spindle to stop discs from vibrating at 10Krpm.

Between the platter that supports the CD and the motor there is a puck with a toroidal void containing a few ball bearings. When an out of balance CD is spun up the spindle and disc together rotate around their common center of mass, some point between the spindle and the edge of the disk. This means that the void containing the balls no longer rotates around it's center, it spins like a hula-hoop around the spindle/DC center of mass. With the "lighter" side of the system being farther from the center of rotation the balls roll 'down hill' towards the side of the void that is experiencing more centrifugal force. Eventually enough balls will collect on the light side to perfectly cancel out the heavy side. If there are too many balls they will distribute themselves inside the void until they cancel out each other's weight!

The link leads to a scaled up demo of this using an empty water bottle and steel BBs.

I must admit that the details of how the fields are coupled went over my head, but I found the rest of the video quite accessible. Professor Copeland is a joy to listen to, and Brady asks great questions as usual.

I also learned that "inflation" and "the big bang" are not synonymous, and inflation occurred first.