Joy in Physics

I have been studying or working on Physics continuously for about 15 years now. When I was in college, I had no intention of studying Physics, but discovered the subject my sophomore year and found that I really loved it. I had a number of other interests as well, especially in languages and cultures, and divided my time pretty equally. But then, when I started graduate school, I needed to focus all my time and energy on Physics. My time in industry, as a researcher, and as a teacher were all the same. I missed spending time on other things and tried whenever possible to spend my out of work time on non-Physics related things (knitting, rock climbing, traveling, etc.).

I have always been surrounded by other Physicists who seem to love it so much more than I do. I hear them talking about trying to figure out how things work and calculating physical equations in their daily lives, thinking about research problems as they fall asleep, coming up with ideas for Physics problems on the way to and from work, and generally thinking about Physics for much of their day. I have always assumed that I just did not really like Physics as much as I ‘should.’ I have always firmly believed (and told my students on numerous occasions), that Physics is all around us in our daily lives, but honestly, I never wanted to think that much about it outside the classroom. I think I would be a better teacher if I did.

So why am I writing a blog on Physics?

At the end of the spring semester last year, I decided to take a break. I took much of the summer off, never working more than 20 hours a week on Physics-related tasks, and instead concentrated on my soon-to-be-born baby. Since my daughter has been born, there has been no time or energy for anything but thinking about her. And sleeping whenever possible (which is not very often it turns out).

Ever since my daughter was a couple of months old, I have found myself really thinking about the Physics of every day life. How loud does my daughter scream? How is the frequency content of her tired vs. hungry cries different? What’s the best way to heat a bottle of milk? Why should I breastfeed her when flying? What are the forces involved in her learning to roll over? Why is one way easier than the other? What cool images can we make with her play mirror by bending it? How does my breast pump work?

For the first time in 12 or so years, I really want to understand how everything around me works and think about how I can teach people these things and show them how interesting and fascinating Physics is.

I needed a break. Not a vacation by any means – caring for a baby is by far the hardest thing I have every done! But a break from Physics. A mental change of scenery.

It really is nice to have found that Joy of Physics again. The fascination with the world and how it works. I am glad it returned in time to share it with my daughter. Or maybe watching her explore the world around her with wide eyes has re-awakened that in me as well. It is definitely going to make me a better teacher when I get back to teaching Physics to students learning it for the first time. I hope I can share that joy and fascination with them. Not to mention all the excellent homework and example problems I am coming up with in the process of writing this blog!

Side note: While all the above is true and I have been thinking about it for a while, in the interest of complete honesty, I must note that this past week or so my daughter has not been sleeping much at all. I have been completely exhausted and haven’t cared the slightest bit about anything related to Physics…or really anything not related to me sleeping. Sleep and time to think seem to be important for enjoying much of anything.

Flying and diving

Recently, my husband and I took our daughter on her first plane trip. This was something I had been dreading for many reasons, one of which is detailed in my post on sound. Fortunately, we are seasoned air travelers and so handled security and boarding and such just fine, the screaming was at a minimum, and overall it was a pretty successful trip.

I had been told to make sure that I feed my daughter on take off and landing to help with the ear popping issue because babies do not know how to pop their ears and get very upset when the airplane makes big altitude changes.  As adults, we automatically pop our ears when we feel like we need to, but babies do not know how to do that on their own.

So why do we need to ‘pop our ears,’ what are the ways of doing that and why is feeding such an effective method for babies?

The reason our ears feel so uncomfortable when doing these activities is due to the pressure changes that result in forces on our ear drums. When the pressure changes slowly, it can be equalized through the Eustachian tube – a tube that connects our ear drum to our nasal passages. When we swallow, yawn, or open our mouths wide, this opens the tube to let the air pressure equalize. So, this is not a problem we usually have when hiking up a mountain since we do it so gradually, but when we take off in an airplane, the pressure changes very quickly and our bodies do not naturally equalize the pressure fast enough, so we need to manually do it with the activities listed above.

I was thinking about all this when we were on the airplane (and our daughter was not screaming), and was wondering how big of a pressure change causes us to need to pop our ears and what types of forces this pressure change causes on our eardrums. Naturally, being a scientifically-minded person, I decided to figure it out.

First, let’s think about airplanes taking off. Because airplanes travel so high, the temperature, pressure, density (and lots of other characteristics) of the atmosphere change significantly. There are equations that describe the pressure in the atmosphere as a function of height, but I am an experimentalist, so I decided to get the actual data.

Weather balloons, with instruments called radiosondes, measure characteristics of the atmosphere all around the world. They are released every twelve hours and send back data on the atmosphere until the balloon pops and falls to earth. The University of Wyoming has a website where you can download data from these weather balloons for stations across the US.  Below is a graph of atmospheric pressure vs. height above Atlanta for January 20, 2012.

I have labeled some points on the graph for reference. The ground level is not at 0 ft because Atlanta is not at sea level. The Atlanta airport has an elevation of 1026 ft (according to  Wikipedia). When flying, the cabin is pressurized to match atmospheric pressure at 10,000 ft, so your ears never experience the pressure of the higher altitudes, despite having cruising altitudes around 30,000 feet. Also, for reference, my brother tells me that normally he skydives from heights of about 10,000 to 14,000 ft, but he has jumped from as high as 23,000 feet.

If we assume that the air pressure inside your ear is the pressure of the ground level, since you have had a lot of time to equalize to that level (that’s where we live), and at 10,000 ft, the air pressure outside our ears is given by the graph above, then we can see that there is a difference in air pressure across our ear drum of about 4 lb/in2. The force on our eardrum depends on this pressure change as well as the area of our ear according to the following relationship:

I do not know how big my daughter’s eardrums are, but I estimate that they are approximately the size of the opening in her ears, which is about a quarter inch. If they are roughly circular, the area is

That gives us an area of approximately 0.049 in2. So, the force on her eardrums is a little less than 0.2 lbs. That does not sound like a huge force, but still uncomfortable for tiny little eardrums. Of course, this is the total force if she did not pop her ears at all on the way up to 10,000 ft. By regularly popping her ears by swallowing, she equalizes the air pressure before the force builds up too much.

The force on our adult eardrums will be larger, since the area is larger, perhaps closer to 0.6 lbs (my rough estimate). And for my brother jumping out of a plane at 23,000 ft, the force will be even larger. We should definitely pop our ears regularly when flying (or falling). Normally this is not a problem for adults, but as many of you know, when we get congested, it is much more difficult to equalize the air pressure because our nasal passages get clogged. This makes flying (and skydiving apparently) very uncomfortable.

Some other activities where ear popping is necessary include going up a mountain in a cable car and diving into a pool. The effect is much more pronounced under water, because the pressure changes much more quickly. Below is a estimate of the force on little eardrums due to pressure changes in a pool.

Fortunately, my daughter is unlikely to go diving underwater anytime soon. = )

There are a variety of methods of equalizing the air pressure on both sides of our ear drums to remove the force we feel on our ears. We can hold our nose and blow out, open our jaws wide, or swallow. Does anyone know any other good tricks that work?

I tend to either open up my jaw wide or swallow. The latter is what is used for babies. By giving a baby a bottle or breastfeeding the baby on the plane during take off and landing, you get the baby to swallow continuously, which removes the pressure change and resulting force on their tiny little ear drums that makes them so uncomfortable. The baby does not understand any of this, of course, but knows that they are getting nice warm cozy milk and cuddling with mommy, so they’re as happy as can be. Of course, this is a perfect opportunity to teach your infants some Physics. Maybe some of it will stick and they will remember it later when they encounter forces and pressure in high school and/or college.

Is breastfeeding the only thing that works? Of course not – bottle feeding works just as well. Also, a pacifier is a good tool since when the baby sucks, they generate saliva, which causes them to swallow. Breastfeeding (or bottle I assume) is also soothing, and helps calm a fussy baby who doesn’t want to be stuck on an airplane, so it’s a good multipurpose tool.

Feeding a baby while taking off or landing in an airplane seems like sound advice, and definitely worked well for us with our daughter on her first plane trip.  However, while the air pressure problem on your ears is also relevant for diving in a pool and skydiving, I would not recommend breastfeeding a baby while doing either of those activities.

Citation note: Scientists (and much of the world) use standard metric units for calculations, but I thought that English units would be useful for the majority of the people reading this blog. Conversions were made with the help of Google, a pressure conversion site, and a force conversion site.

Nazis, Nobel Prizes, and Bedtime Stories

Back when my daughter was about two months old, she needed to be held constantly. I couldn’t put her down when she was a awake or sleeping. I tried slings, carriers, etc. to no avail, so I carried her at all times. She wanted to be rocked or walked to sleep and was generally not tolerant of me ever sitting down until she was fast asleep. This was both physically exhausting (she may have only been about 10 lbs, but carrying a squirming 10 lb mass around all day long gets tiring and tough on the wrist joints!), and mentally exhausting – I felt like I never really had a break to read email or the news or do anything but carry around this little girl. I certainly had expected her to require all of my attention but had not really realized how hard it would be to not have any intellectual stimulation during the day.

One day, she was being more tolerant than usual with sitting down and I was sitting in front of my computer as she was drifting off to sleep. I had been singing/talking to her but wanted to read an article that a friend had posted on Facebook. The minute I stopped talking, she started to fuss and cry. I was a bit exhausted and frustrated after a long day (and a long night of very little sleep). Then I figured that maybe I could do both things at once. So I started reading the article out loud. It was a very interesting article on an NPR blog about how Niels Bohr and Georgy de Hevesy managed to hide two Nobel Prizes from the Nazis when they came to Copenhagen: Dissolve My Nobel Prize! Fast! (A True Story).

My daughter immediately stopped fussing and looked to be listening intently to the story. By the end, she had fallen fast asleep. Of course, I am sure she had no idea what I was reading to her and just wanted to hear my voice to help her feel safe and soothe her to sleep, but I thought it was a little funny that she fell asleep to a story about Niels Bohr.

I have always been fascinated with Niels Bohr as he was one of the main players in the discovery of Atomic Physics (how atoms work and what they’re made of), which I find really interesting. I have taught a course in Modern Physics, an introduction to this area of Physics for several years. The people involved are a fascinating group and it is interesting to see how Physics evolved through two world wars with a number of collaborating scientists who ended up on opposing sides. Maybe my daughter will be interested in learning more about this part of history or the science they were discovering when she is older. Or perhaps not.

She has since become more discerning about her bedtime stories. There are two determining factors on whether or not the book is interesting – what the pages look like and what sounds the book makes her parents imitate.

One main factor is the book pages. Now that she is getting more motor control, she prefers the board books where she can turn the pages on her own – or at least try to ‘help’ turn the pages –  she doesn’t have that much motor control yet. There should be baby board books on Nobel Prizes and Modern Physics I think. Brian Green does have one book, Icarus at the Edge of Time, but it does not quite look like a baby book. Many of the board books we have are silly and nonsensical. While these will be fun when she is a toddler, it seems like it would be better to teach infants about language and reading with real words and pretty pictures of interesting (scientific?) topics.

However, the real determining factor between good books and bad books at this point in her life seems to be whether or not they have interesting sound effects. As long as we add interesting sounds effects (lions roaring, birds chirping, etc.) to the story, she’s happy with pretty much any story. Maybe I should start reading her more stories about Bohr with interesting sound effects and she can learn all about the history of Modern Physics before she’s a year old. Whirring sounds for electrons orbiting atoms, Booming noises for atomic explosions, Bloop! when light energy is absorbed and electrons jump to a higher energy level and, of course, Pew Pew Pew when electrons jump to a lower energy level and emit light (as in lasers).

If you are interested in an animation of the various energy levels of a hydrogen atom (really, who wouldn’t be interested in such a thing?), the University of Colorado’s PhET page is amazing.

How Loud Can Your Baby Cry?

Babies cry. Sometimes they cry a LOT. And loudly. As a new mom, my baby’s cry was a horrible sound that made me upset and sick to my stomach and not being able to stop it was (and still is) the worst feeling in the world (at least it feels that way in a hormonal, sleep-deprived state). There’s a good physical reason for this, but more on that some other time. Today I want to talk about volume.

It seemed that our daughter cried louder than other babies. ‘Everyone’ told us that newborns do not cry loudly and that it just seemed that way to us as parents. Then friends and family started commenting on “what an extraordinarily healthy set of lungs she has!” Being the scientifically minded folk that we are, my husband and I decided to measure the volume of her cries.

At about two months old, we measured our daughter’s cry to be 122 dB at about 4″ from her mouth.

Okay, what does that actually mean? What is a dB and how loud is 122 of them and why did I feel the need to include the distance?

The sounds that we hear are waves of energy that travel through the air by changing the air pressure by small amounts. This vibration of the air vibrates our ear drums and causes us to hear a sound. For more on how hearing works, there is an interesting article on this at the How Stuff Works website: How Hearing Works.

Today I’m interested in how we measure sounds. Normally, we would measure how much energy reaches the area of our ear in a certain amount of time, or the intensity of the sound, which is usually measured in Watts/(meter)2. To give you an idea of some common intensity values, the threshold of hearing (smallest sound we can hear) is 1 x 10-12 W/m2 and the threshold of pain is 1 W/m2. That means that we feel pain at an intensity level that is 1012 times larger than the smallest sound we can hear – that’s a trillion (a million million) times larger. But of course, sounds that hurt my ears don’t sound a trillion times louder than the faintest whispers. Why not?

Interestingly, our ears do not work on a linear scale – that is to say that if twice as much energy hits our ear in a given time, it does not sound twice as loud to us. Our ears interpret sounds logarithmically, so we have a unit of measurement that describes how loud a sound is, or the sound level. This is measured in decibels, or dB for short. For those of you who like equations, the formal definition of sound level for a given intensity is

For those of you who don’t like equations, this table might be more useful in understanding what different values of intensity and sound level mean to you.

Okay, so what does that mean about my daughter? Her loudest screams are above the pain threshold. Ow! The Occupational Safety & Health Administration (OSHA) recommends that that you not be exposed to that level of noise at all in the workplace. Too bad my daughter does not abide by their recommendations. It really does hurt my ears when she screams and I am holding her.

There is one more piece to this loudness puzzle, though. I mentioned that we measured her scream at 4” from her mouth. Why does this matter? Sound spreads out in all directions as it travels away from my daughter’s mouth, so the energy gets spread out over a sphere (approximately) as it travels away from her mouth (see silly cartoon at right). The farther away you get, the lower the intensity (and sound level) of the cry. Good news for everyone else, not such good news for me holding my daughter on my shoulder so that her mouth is only a couple of inches away from my ear. To better see how the sound level decreases with distance, a graph of sound level vs. distance for her cry is below:

Does this have an effect on the theory that parents think their children are louder? Is it just that the parent’s are so much closer? Probably not.  We are conditioned as parent’s to hear and respond to our children’s cries because they need us.

Then what does this all mean, other than being a good example for sound level problems for my introductory Physics students? Not much, I suppose. If my daughter is screaming at full volume, I should be wearing earplugs, but if she’s screaming that loud for an extended period of time, I’m going to be less worried about my ears than about what is wrong with her to make her scream so loud. Fortunately, she doesn’t  scream like that often anymore. It does mean that you can ignore all those people who tell you that your child only sounds loud to you. Tell them that some babies scream above the pain threshold.

If you’re curious about you baby’s cry (or other sound levels), my husband and I used a dedicated sound meter, which most normal people don’t own, but, as with everything these days, there is an App for this! Several in fact. With a small financial (~$1) and time commitment, you too could measure your baby’s screams!


I am a new mother with an amazing and frequently overwhelming little girl. I was also until very recently a Physics professor, but have decided to shift my focus to being a full time mom for a while. This blog is about Physics, and being a mom.

Physics is perceived by many to be an abstract, complicated, mathematical science that has little relevance to the real world. At least, many of my students seemed to feel that way. Nothing could be further from the truth. Physics is the study of the world around us – how we move, how our toaster and coffee maker work, why we see ourselves in a mirror and how rainbows form after it rains. I see Physics in many of the tasks and events of the day, even (or especially) in caring for my daughter.

I want to share the Physics of the every day. Since most of my days right now consist of caring for a small child, this will mostly be Physics related to babies, but may also include Physics in other things I do and people with whom I interact. Or interesting Physics in the news that catches my eye – anything having to do with light and lasers usually catches my eye, because it’s just so cool.

I am also a teacher and very interested in ways to help students learn and become interested in Physics. Raising a child is the ultimate in ‘active learning’ techniques and I plan to spend some time thinking about these ideas to help make me a better teacher when I get back to Physics teaching as well as while I am helping my daughter learn.

I am new to blog writing as well as motherhood. I discover new things about bring a parent every day and need to adapt my plans accordingly. I imagine this blog will reflect that and morph as time goes by.

I plan to write weekly. I hope you enjoy and please share any constructive comments and feedback you have. I am open to suggestions for future topics though it may take me a while to get to them.