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Opinion: When Pro Scientists Explain Using Pop Science

Abstract There is so much to say about the many endeavors by professional scientists to explain to us the world. The list is long: Carl Sagan, Harald Lesch, Neil deGrasse Tyson, Sabine Hossenfelder, Michio Kaku, and I even saw Roger Penrose and Steven Hawking on tv. The list is – of course – considerably longer…

How to Measure Internal Resistance of a Battery

Introduction A commonly encountered school-level Physics practical is the determination of the internal resistance of a battery – typically an AA or D cell. Typically this is based around a simple model of such a cell as a source emf in series with a small resistor. The cell is connected to a resistive load and…

Why There Are Maximum Mass Limits for Compact Objects

In this article, we will look at why there are maximum mass limits for objects that are supported against gravity by degeneracy pressure instead of kinetic pressure. We will look at the two known cases of this, white dwarfs and neutron stars; but it should be noted that similar arguments will apply to any postulated…

Oppenheimer-Snyder Model of Gravitational Collapse: Implications

Part 1: Overview Part 2: Mathematical Details Part 3: Implications In the last article in this series, we finished up with a metric for the Oppenheimer-Snyder collapse: $$ ds^2 = – d\tau^2 + A^2 \left( \eta \right) \left( \frac{dR^2}{1 – 2M \frac{R_-^2}{R_b^2} \frac{1}{R_+}} + R^2 d\Omega^2 \right) $$ Now we will look at some of…

Oppenheimer-Snyder Model of Gravitational Collapse: Mathematical Details

Part 1: Overview Part 2: Mathematical Details Part 3: Implications In a previous article, I described in general terms the model of gravitational collapse of a spherically symmetric massive object, first published by Oppenheimer and Snyder in their classic 1939 paper. In this follow-up article, I will give further mathematical details about the model, using…

When Discussing the Twin Paradox: Read This First

This article is intended for anyone who wants to start a thread here at Physics Forums on the twin paradox. There are already many, many threads here on this topic, and they tend to cover the same ground over and over again, so it seems useful to put a description of that ground into an…

The Oppenheimer-Snyder Model of Gravitational Collapse: An Overview

Part 1: Overview Part 2: Mathematical Details Part 3: Implications Most people who have spent any time at all studying GR are familiar with the Schwarzschild solution. (A series of Insights articles discusses the key properties of that solution.) Much of that familiarity probably derives from the fact that the Schwarzschild solution describes a black…

Subtleties Overlooked in Friction Questions: Object Slides Down Ramp

Problem statement (simplified) An object slides down a ramp at angle θ to encounter level ground. Both surfaces have kinetic friction: μ’ on the ramp, μ on the level. The object reaches the ground at speed u. What is its speed when first fully on the level? (Original is at https://www.physicsforums.com/threads/distance-a-block-slides-along-a-surface-with-friction-given-with-an-initial-velocity.1047556/.) There are several missing…

Yardsticks to Metric Tensor Fields

I asked myself why different scientists understand the same thing seemingly differently, especially the concept of a metric tensor. If we ask a topologist, a classical geometer, an algebraist, a differential geometer, and a physicist “What is a metric?” then we get five different answers. I mean it is all about distances, isn’t it? “Yes”…

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