What Would Happen if You Fell Down a Hole That Went Through the Centre of the Earth?

Created by Grok

Well you'd be incinerated as you pass through the hot and molten core! But imagine if the core wasn't there or you had a special suit that protected you from the heat—what would happen then? Would you suddenly stop when you got to Oz?

The scenario is an example of a simple harmonic oscillator—other examples being a swinging pendulum and a weight bobbing up down, suspended from a spring. Both of the latter rely on interchange of energy and its change in form, from kinetic energy due to motion of an object, to stored potential energy. In the example of a weight suspended from a spring, the potential energy is manifested as tension in the spring. Energy in physics is defined as "the ability to do work". And "work" in physics also has a specific meaning: work is done when a force moves a body through a distance. The stored potential energy in a spring can do work, which is how a clock spring can move the hands of the clock.

Simple harmonic oscillators as the name suggests oscillate or move backwards and forwards indefinitely. For an ideal spring-mass system in a vacuum, with a perfect spring and no friction,  the mass will bounce up and down forever, with energy being interchanged as described above. Simple harmonic oscillators have a natural frequency that depends on the magnitude of the mass and the stiffness of the spring (how hard it is to stretch or compress it). In real life, systems are lossy and due to mechanical friction in a spring and drag from air, oscillations eventually die out. Technically this is known as damping. Dampers are actually used as components in your washing machine's drum suspension and also vehicle suspension systems to smooth out oscillations.

Back to the hole in the Earth that goes all the way to Australia—if you jumped in and there was no air in the hole, you would fall and accelerate, your velocity increasing as you drop downwards. In fact, you'd free fall under the influence of gravity, being continually accelerated. This differs from the situation where you're falling in air, because in that scenario, you eventually reach a constant velocity, known as terminal velocity, when the force of gravity acting downwards is equalled by drag acting upwards, and there's no net accelerating force. As you continue to fall closer to the centre of the Earth, your velocity increases until you reach maximum velocity at the centre. Due to inertia however you overshoot and pass through the centre. However, from now on, there's "less Earth" in front of you and more behind you. So now you're being decelerated and the gravitational force of the Earth is having a braking effect. Your velocity decreases after you pass through the centre, and it keeps decreasing until it falls to zero as you emerge from the hole on the other side, just like the way a ball kicked up into the air pauses momentarily when it reaches the apex of its trajectory. Once your velocity reaches zero, you start to fall back down the hole and the process is repeated as you fall back to where you started from. Without any losses due to friction, you would keep moving from one side of the planet to the other indefinitely, and it works out that the transit time would be 42 minutes. In reality, if such a hole was a civil engineering possibility, which is highly unlikely and it was filled with air, your oscillations back and forth would be damped and you'd eventually settle and float at the centre of the Earth, weightless. 

Science Friday Podcast — Jim Lovell Interview

NASA astronaut Jim Lovell, photographed in 1969. Image credit: NASA, public domain, via Wikimedia Commons

The late Jim Lovell, commander of the ill-fated Apollo 13 mission talks with Ira Flatow of SF in an interview from 1995.
Apollo 13 was the seventh crewed mission and would have been the third operation to land humans on the Moon. However an explosion in an oxygen tank caused loss of electrical power and cut short the mission. The crew had to slingshot the spacecraft around the Moon to gain enough momentum to get back to Earth. In the interview, Jim explained the details of the event, we hear recordings of radio comms between flight control and the the spacecraft and Commander Lovell answers questions from callers. 

Another Few Stats on Poulaphouca Reservoir Water Levels and Volume

An ash tree growing on a trail beside the Poulaphouca Reservoir. © Eugene Brennan

In the period from 20th to 31st January, the volume of water in the Poulaphouca Reservoir increased by 52 million m³ of water, corresponding to a 2.3 m rise in level. In the same period, discharges from Golden Falls dam totalled 16 million m³. Since the level in the Golden Falls lake didn't change appreciably over those 12 days, and its surface area is much smaller than that of the Poulaphouca lake, the amount of water that flowed into Poulaphouca must have been equal to the increase in volume plus the amount that flowed out — 68 million m³. Over the 12 days, that gives an average inflow rate of 68/12 million or approximately 5.6 million m³/day. That's about 1.6 times the peak daily flow rate through Kilcullen last week. The Poulaphouca lake is fed by two rivers: The River Liffey and Kings River and so that's understandable. Some small streams also add to the water accumulated.
The ash tree above is on a nice section of path along the lake that leads to the car park on the N81, near the turn for Ballymore Eustace .
Stats are available on the ESB hydrometric website here.
 

Making a Bit Of Space : Golden Falls Lake Level Has Risen

Water level in Golden Falls Lake, Ballymore Eustace. Image courtesy ESB hydrometrics

The Golden Falls lake rose 1.44 m since yesterday. That's a bit of a jump since the downwards trend over the last week or so, and equivalent to approximately 370,000 cubic metres (m³) of water. The Golden Falls lake doesn't really provide a large storage capacity compared to the much larger Poulaphouca Reservoir, which has a surface area of approximately 22.6 square kilometres (km²), 86 times that of the Golden Falls lake. According to Esbarchives.ie, the head of the Golden Falls dam is 17.4 m. If the walls of the 256,000 m² lake were vertical, that would give a storage capacity of about 4.4 million m³ of water. To put that into perspective, if it were empty, it could store only about a day and a quarter's worth of the water that passed through Kilcullen at peak discharge last week. So dropping the level isn't a huge advantage as regards overall storage and I guess the objective is to try and make more "headroom" available in the Poulaphouca Reservoir to cater for extended periods of heavy rain.

Image courtesy ESB hydrometrics.

Kilcullen Town Hall's Film Projector

Movie projector from Kilcullen Town Hall Cinema. © Eugene Brennan

I was examining the nameplate (rating plate) of Kilcullen Town Hall's former film projector the other day, now located in a corner of the porch of the heritage centre. The Peerless Magnarc carbon-arc lamphouse, paired-with the projector and  manufactured by the J. E. McAuley Manufacturing Company of Chicago, Illinois, ran on DC electricity. A rectifier may have been built into the projector to convert mains AC to DC, or possibly it was a separate piece of equipment.
Arc lamps were used as an intense white light source for projectors, essential for producing a bright image on the screen. They were also used of course in search lights during WWII, for spotting enemy aircraft. Arc lighting generates large amounts of harmful UV radiation and filters would presumably have been used in the optics to block this. Electric arcs also produce a lot of heat, as anyone who has ever done electric welding would know, and again, filtering would have been necessary to protect the film. Carbon rods were consumable items used in arc lamps, just like the rods used for arc welding and had to be replaced frequently. In the early days of movies, film stock was manufactured on a highly flammable nitrate base and fires were common. It's thought that a candle ignited a reel of nitrate film, resulting in the Drumcollogher tragedy of 1926, in which 46 people perished.

I'm not sure what the "motor" is, referred to in the spec. Since the lamp was an add-on unit to the projector, it was probably not the film feed motor, but rather a motor powering a cooling fan for the lamp or one that fed the electrodes as they were consumed.

The photo mounted on the projector shows actor Paul Newman standing in front of it on the occasion of his visit to Kilcullen over 20 years ago. At that stage, the projector was still located in the projection room.

In 2006, Brian Byrne published a more detailed article in the Kilcullen Diary which provided more information about the projector’s projection unit.

Arc volts 31 - 46
Arc amps 32 - 100
Motor volts 31 - 110
Motor amps 0.5 - 1.4

J.E. McAuley Mfg. Co.
Chicago, ILL 
United States of America

Rating plate attached to the projector. © Eugene Brennan

How Much Thrust?

SLS rocket. Image credit: NASA

8.8 million pounds, 4.0 million kilos or 17.3 million newtons (the SI unit of force). That's 15% more thrust than the Saturn V rocket, used for the Apollo missions to the Moon in the late '60s and early '70s.
The SLS or Space Launch System is the rocket that's being used for the Artemis missions that will once again carry astronauts to the Moon. (Artemis was Apollo's twin brother in Greek mythology).
Huge amounts of thrust are needed to accelerate spacecraft, so that they can escape Earth's gravity. The escape velocity of planet Earth is 11.2 km/s or 25,000 mph. We can't simply use rockets to travel point-to-point to the Moon as it would be inefficient and require continuous rocket firing and fuel use. By travelling fast enough initially, hence the massive engines and fuel tanks, we can "coast" there, like a car with one's foot on the clutch. The principle is based on Newton's first law of motion—so once an object starts moving, it keeps moving uniformly, unless another force acts on it, Earth's gravity in this case. (That's why the Voyager spacecraft are still moving out beyond the Solar System). Technically, a spacecraft is falling back to Earth under the influence of the latter's gravity, but in the case of lunar missions, the Moon and its gravity "take over" when the craft is close enough, allowing it to be injected into lunar orbit. 

Sewage Treatment Plant Capacities in Kildare and Brannockstown Septic Tank

Wastewater treatment plant. AI image by ChatGPT.

This is Uisce Éireann's register of available capacity at waste water treatment plants (WWTPs), published in August 2025. It lists treatment plant capacity and "does not provide an indication of network capacity", i.e. the sewerage network involved in transferring sewage to the plants. Several treatment plants are in the red, including Timolin, Nurney and Rathangan WWTPs, meaning that "no spare capacity is available" at the moment.
"Brannockstown(Grangemore) WWTP" mentioned in the Uisce Éireann register is given a "green" rating indicating that "spare capacity is available." I'm not sure whether this is the same tank mentioned in this 82-page EPA licence-application document, which calls it the "Brannockstown Septic Tank" and says that "the tank was constructed in the 1970’s to serve approximately 18 properties in an adjacent Kildare County Council housing development." The tank appears to be located behind a fence in woodland at the back of the development.
Group sewage schemes or domestic wastewater treatment systems (DWTSs) are sometimes used in rural areas for groups of houses when mains sewer connections and a full-scale treatment plant are not feasible or economical. Meanwhile, on a related sewage matter, Cllr. Tracey O'Dwyer asked a question at a September 2025 meeting of the Kildare–Newbridge MD about whether the council can "confirm the total amount paid to waste companies for the removal of sewage from Moorhill Wood Estate, Brannockstown since the service commenced to date and confirm if this expenditure can be recouped from Uisce Éireann."

OSI Benchmarks

OSI benchmarks. Image reproduced with permission from Dr. Catherine Porter, author of this RTÉ article.

There are several of these located around the town. I'm not sure which are still in existence however. The marks, resembling an arrow, were chiselled into masonry by OSI surveyors when the country was surveyed in the early 19th century to create the first detailed and accurate maps. Typically, they're located on walls and gate pillars. There should be one located on the bridge in the town and a second one on a gate pillar on New Abbey Road. (At the gate on the left hand side of the road, just before the turn for McGarry's Lane when heading towards the cemetery.) A benchmark corresponded to an elevation in feet above a sea level datum (the reference point located at Poolbeg Lighthouse). The elevation was indicated on the 25" map with a crow's foot symbol. Surveyors inserted an angle iron into a socket located near the mark, and this was used as a bench for resting a levelling rod on during the elevation measurement process.
This recent article on the RTÉ website by geographer Dr Catherine Porter and Margaret Sullivan, University of Limerick, gives a background on benchmarks and their use use in the surveying process.

Image reproduced with permission from Dr. Catherine Porter, author of the RTÉ article.

Making a New Part For My Bicycle Tail Light

© Eugene Brennan

If possible I always repair rather than replace. I've always done this before it became a trendy green thing, simply because it saves money and I've never had lots of money to spare. I remember my father fixing a hole in the silencer on the car, with a section of tin plate, cut from a Cow & Gate baby formula tin, and fixing it to the silencer with coat hanger wire. So I didn't lick it up off the ground, as the saying goes. I've got slagged off by Millennials and successive demographic cohorts, for doing such things, and asked why I don't "just but a new one". Anyway, today I was fixing the tail light for my bike.

Poulaphouca Water Level Stats Updated: Level up 30 cm

Water level in Poulaphouca reservoir

Water level has increased by 30 cm from the same time yesterday, equivalent to 6.78 million cubic metres (m³) for the 22.6 square kilometre lake. That's despite the 2.94 million m³ of water discharged in the past day from Golden Falls dam. Although discharges for Golden Falls were forecast to be 45 m³/s at four points in the day yesterday, flow stats showed that they were 34 m³/s, averaged over the whole day. Possibly this was due to lower-than-forecast discharges in the evening or at midnight today. Water level under the bridge in Kilcullen circa 8 pm last night had dropped by about 4 inches.
The ESB press office has just contacted me with details about "theoretical highest level possible" at the dams. More info later.
Screenshots courtesy ESB hydrometrics.

Higher-than-normal water levels under one of the arches of Kilcullen Bridge. © Eugene Brennan

Reduction in Poulaphouca Lake Levels

Poulaphouca Reservoir viewed from above Valleymount. © Eugene Brennan

The water level in Poulaphouca lake has dropped from yesterday's 186.64 m to 186.52 m a drop of 12 cm. It may seem small, but it corresponds to a discharge of 2.7 million cubic metres (m³) of water for the 22.6 square kilometre lake. This reduction in volume of the lake is simply due to the difference between "water in" minus "water out". The River Liffey and Kings River are feeding water into the lake, and water is discharged through the Poulaphouca dam into the Golden Falls Compensatory lake. In a previous article, we discovered the reason why there's a lake between the two dams. Some of the drop in level is accounted for by water purified into drinking water by the Ballymore Eustace Water Treatment Plant. According to the EPA, the BWTP produces 340 million litres of drinking water a day. That's equivalent to 340,000 cubic metres. So most of the reduction in water volume in the lake is accounted for by Poulaphouca discharges. Liffey releases through the Golden Falls dam today are forecast to be 45 cubic metres per second (m³/s), all day. If that rate of discharge is sustained for the day (the ESB say "flows are estimates"), it corresponds to 3.9 million m³ of water discharge versus the 2.7 million m³ reduction in volume of the Poulaphouca lake. So at the moment, the total discharge from the Golden Falls lake is greater than the rate at which the Poulaphouca lake is discharging into it, making room for high flows on rivers and lakes feeding the lakes in the coming days.

Water level stats courtesy ESB hydrometrics. 

 

Water level stats, courtesy ESB hydrometrics. 

Related Reading

• Press Release From the ESB About Potential Releases From the Poulaphouca Reservoir (this explains the setup at Golden Falls and the buffering of flow)
• What Volume Does a 1 m Change in Water Level Represent in the Golden Falls Lake? 
•  Golden Falls Headrace Level Up a Metre, Flywheels, Capacitors and Air Compressor Tanks
•  Let's Do Some Calculations: How Much Water?
•  High Volume Releases at Golden Falls Dam
•  Golden Falls Generator Assembly
•  Erosion of the River Bank in Kilcullen
•  Ballymore 5 km Loop 

 

  

What Volume Does a 1 m Change in Water Level Represent in the Golden Falls Lake?

Golden Falls lake. Image courtesy Google Maps, Airbus and Maxtar Techmologies

Distance can easily be measured on Google Maps on desktop by right clicking and selecting "Measure distance" from the context menu. Then simply click the start point and the end point, or draw a series of line segments by repeatedly left clicking and following an outline until the end point is reached. This is useful for measuring distances, but you may not have known that it's also possible to measure area by drawing a series of line segments around for instance a field, and then clicking on the start point again. You can read a complete article I wrote about how to do this here.

Types of Plastics Used in the Home: How to Identify PVC, PET, Polythene and Others

© Eugene Brennan

What Are Plastics?

"Plastic" is a term applied to any material that can be moulded or bent into shape; for instance, soft toffee, modelling clay, or red-hot iron is plastic because it can be reshaped. In general though, when we speak of plastic, we mean polymers or synthetic materials made from petrochemicals.

Earth Speeding Through the Universe and How is Speed Measured in Spacecraft

AI image generated by ChatGPT

Did you know that we're travelling at 600,000 km/hr on planet Earth as our home galaxy, the Milky Way, orbits the Local Group, a group of neighbouring galaxies? That's somewhat faster than the mere 1000 km/hr velocity of a point on the island of Ireland, due to the Earth turning on its axis. We don't experience the effects of constant velocity on our bodies, only acceleration and deceleration. So we can't tell whether we're stationary or moving.

Golden Falls Headrace Level Up a Metre, Flywheels, Capacitors and Air Compressor Tanks

Image courtesy the ESB

Even with the large discharges from the golden Falls dam, the level in the lake has risen a metre in the last two days. This would be due to significant discharges from the Poulaphouca dam. I'm not sure what the max allowed level is for the lake. I'll see if I can find out. Both the Golden Falls and Poulaphouca lakes act as buffers to smooth out flow. In fact they're analogous to three types of systems, both mechanical and electrical: an air compressor tank, a flywheel on a machine such as an engine or punching machine and the filter capacitors in a DC power supply. In signal theory, all these systems act as low pass filters, storing energy and smoothing out "bumps", converting pulses into smooth ripples—ideally small ripples. If the ripples are too big in an electrical power supply, you get mains hum, which you may have heard in a bad PA system. Back to the lakes, they act as buffers. Once they fill, they no longer smooth out the bumps and the full flow of the River Liffey upstream of Poulaphouca feeds through the system.

Image courtesy ESB.

Erosion of the River Bank in Kilcullen

Crop of a Lawrence Collection photo of the River Liffey in Kilcullen. Image courtesy the National Library of Ireland.

A close-up crop from a Lawrence Collection photo, circa 1900, showing a section of the river bank that had slipped into the river near what's now the entrance through the bushes onto the mass path. It's probably taken from the vantage point of the bridge or the slope where the Valley Park is now located. The slippage could have occurred decades or hundreds of years before. Interestingly, as far as I remember it, there was a ditch just before the fence in the photo (not sure whether it was the same fence in the 70s), which we used to have to step over to walk further along the path. I noticed recently, from hearing water running, that the ditch, if it's the same one, is now maybe 20 feet further into the bushes and in a culvert/pipes. So the bushes must have spread outwards over the last 50 years or so into the field.

Press Release From the ESB About Potential Releases From the Poulaphouca Reservoir

Poulaphouca Lake. © Eugene Brennan

Water isn't released directly into the River Liffey south of Ballymore Eustace from the Poulaphouca reservoir — it's discharged indirectly from the Poulaphouca dam into a compensatory lake in Ballymore. This acts as a buffer, protecting the river from huge surges and flooding. Although it's not related to surges on the river, a surge tank adjacent to the Liffey bridge over the gorge on the N81 protects the pipes, or penstocks, that deliver water to the turbines in the generating house from hydraulic shock.

Devils Tower, the Hill of Allen, the Valley Park and Close Encounters of the Third Kind

Devil's Tower in Wyoming. Ben Stephenson from Cleveland, OH, CC BY 2.0 , via Wikimedia Commons

If you were watching Close Encounters of the Third Kind this afternoon (I hadn't seen it for maybe 15 years), you would have seen the towering rock formation that's used as a plot device and features in the end of the movie when the huge spaceship appears and the aliens emerge. This is Devils Tower in Wyoming, classed as a butte and possibly a laccolithic formation, composed of igneous rock. A butte is an isolated hill with steep, often vertical sides and a flat top. Igneous rock began as molten rock or magma that originated from inside the Earth, and then cooled.

Top 100 Cool Science Facts for Kids!

© Eugene Brennan

Why is the sky blue? What is air made of? How many stars are there?
World of wonder fun science facts that every child should know! This article covers space, nature, technology, engineering, elementary mathematica, chemistry, physics and biology.
Science is fascinating and tries to explain how everything in the world and outer space works. Science gives us the answers to questions like "What is electricity?" and "How does an aeroplane fly?" Read on and learn 100 more cool science facts! 

Super Human Strength and an Injury on Church Mountain

Fence on Church Mountain, Co. Wicklow. © Eugene Brennan

In 2019 I was making my way down a gully on Church Mountain near Hollywood with my bike slung over my shoulder. I had cycled half-way up on the trails at the back of the mountain and was going to go further, but it was getting late in the day and I decided it would be wise to head home before it got dark. The gully was peppered with lots of rocks, covered in wet, slippery moss and algae and difficult to navigate. I was distracted by an annoying off-lead dog belonging to walkers behind me which kept barking at me. I lost my concentration, slipped and started to fall backwards, my bike pulling me down (which is why it's never a good idea to carry a ladder over your shoulder on a stairs).