## Thursday, April 9, 2015

### Physics Links

The recent episode of Skeptics Guide to the Universe podcast had two interesting "basic" physics stories.

1. A new twist to how long it takes to "fall through" a tunnel that stretches from one end of the earth to the other.

The standard answer models the earth is a sphere of uniform density, and resulting trajectory is simple harmonic oscillation, with time period $T = 2 \pi \sqrt{\frac{R}{g}},$ where R is the radius of the earth, and g=9.8 m/s^2 is the gravitational acceleration on the surface.

The required time is T/2 which is approximately 42 minutes.

2. Does a half dead battery bounce more than a full battery when dropped? Neat videos here.
However, “maximum bounce” is reached when the battery is down to about half its charge, at which point the amount of bounce levels off despite the fact that more zinc oxide is still forming. So the bounce technique can reveal that a battery is not fresh, but it is not an indicator that it’s entirely flat. Still, it’s an easy and instant way of checking the profusion of batteries filling our drawers – no multimeter required.