## How long would it take to fall through the Earth?

This is a classic calculation, and one that’s surprisingly tricky: suppose you could bore a hole all the way through the Earth. If you jumped in, how long would it take you to fall through and come out of the other side?

If you’re taking A-level physics you know pretty much everything you need to do the calculation for yourself, but there are a few fiddly little problems you need to deal with along the way. Most importantly: if you’re falling through the Earth, only some of it is below your feet. The rest of it is above your head, and the force of gravity from that part is pulling you not downwards but upwards. So working out the forces isn’t as simple as you might hope, but you do get something back: when you start picking away at the maths, you’ll find that what happens when you reach the centre of the planet is pretty important.

Have a think about how you’d tackle the problem yourself, then take a look at this video from MinutePhysics.

Tip of the hat to Alom Shaha for pointing us to this.

## Gravitational waves – The Think Physics Guide

As a project with ‘Physics’ in our title, it hardly seems possible not to be talking about gravitational waves in the office this morning. We read the reports avidly, we got all excited, and we also realised that we’re hardly the experts on this. So here’s our brief run-down of the really useful stuff we’ve found from better journalists than ourselves and more informed cosmologists:

First up, an excellent film from the New York Times, which sets out what the LIGO experiment in Louisiana and Washington has done:

The rest of the Times’ report is a good solid overview of what’s happened. Through the arms-length reporting you can glimpse the level of excitement and the significance of the work.

If animation is more your style, this primer from PhD Comics will spin you through the bumpy landscape of gravitational waves:

Hooked? Fascinated? Excited? The New Yorker has an outstanding long article about the inside story of the discovery:

“Space and time became distorted, like water at a rolling boil. In the fraction of a second that it took for the black holes to finally merge, they radiated a hundred times more energy than all the stars in the universe combined. They formed a new black hole…

The waves rippled outward in every direction, weakening as they went. On Earth, dinosaurs arose, evolved, and went extinct. The waves kept going. About fifty thousand years ago, they entered our own Milky Way galaxy, just as Homo sapiens were beginning to replace our Neanderthal cousins as the planet’s dominant species of ape. A hundred years ago, Albert Einstein, one of the more advanced members of the species, predicted the waves’ existence, inspiring decades of speculation and fruitless searching.”

It’s a beautifully-written piece, and it really captures the human aspect of this – of hundreds of physicists around the world experiencing that moment of discovery. It’s an image that’s ingrained in popular conceptions of how science works, of Archimedes leaping out of his bathtub and exclaiming ‘Eureka!’ The reality, of course, is usually very different. Science tends to proceed in small steps, miniature breakthroughs in labs and desks and computers around the world, inching forwards piece by piece. But the LIGO work appears to be a genuine breakthrough, and the excitement is both real and hard-earned.

That’s also the theme of yesterday’s BBC Radio 4 Inside Science Special:

“It is the cleanest signal you can imagine… you have to feel fantastic for those 800 scientists, who have been spending – some of them – decades of their careers working towards this first detection.”
— Dr. Andrew Pontzen, UCL

The programme also hears from the leading UK scientist on the project, Prof. Sheila Rowan of the University of Glasgow. You can get a good sense of how giddy everyone is about this by listening to her impression of the signal ‘chirp.’

Do take a look at the LIGO experiment website, but for now, the final words:

## Measuring g via Free Fall

Four different approaches to measuring g: try several and prompt your students to think about how they differ in terms of accuracy and repeatability.

## Roller Coaster Design Workshop

Our roller coaster workshop looks at the range of specialisms involved in building rides… by building rides.

## 9th December: More Rollercoasters

This morning, I was back at Kenton for some more K’nex rollercoaster building. There’s a lot that goes into building a rollercoaster, and we only just scratched the surface. For more details, head over to our workshop notes page where you’ll find videos, games, and more information than you can shake a (K’nex) stick at. Now if you’ll excuse me, I have six rollercoasters to dismantle and put back into the correct boxes. Sigh.

## Physics things to make you go ‘Hmmm’

This morning 22 year 9 students from Kenton visited Think Lab.  They tried out a range of simple activities, all of which encouraged them to ‘Do Physics!’  Some of the explanations used physics ideas that they had met before, but some required them to think about topics which they won’t meet until A-level physics (or beyond).  All were chosen to make the students go ‘Hmmm’ and think hard about what they had observed.

One of the activities was dropping different balls to see which reached the ground first.  They had two tennis balls, one of which was filled with water.  Lots of the students predicted that the heavier ball would fall faster.

It didn’t.  Although it was hard to spot visually, when we listed for the balls hitting the ground, they bounced at the same time.  This was quite surprising!  Part of the reason for this is that ‘common sense’ tells us that heavier things fall faster.  And if air resistance becomes a significant factor, then we do find that heavy things fall faster.  The tennis balls take air resistance out of the experiment, and then the balls fall at the same rate.

A slightly larger demonstration of this phenomenon was done by Professor Brian Cox using a giant vacuum chamber.

Another very popular activity was looking at water beads, or hydrogel beads.  These are a water absorbing polymer.  When they are dropped into water, they disappear.