energy and motion at our scale

The battery in Sukie's laptop has seen better days, so I started looking for a replacement.  It turns out to store 65 watt-hours of energy.  You buy energy delivered as electricity by the kilowatt-hour - usually for about fifteen cents a kWh.  The average home in the US uses about a kilowatt of this form of energy every hour, so 65 watt-hours is nothing - right?

 

After thinking about placing an order I decided it was time for my morning exercise session on the rowing machine.  I grabbed the iPod loaded with music and podcasts so I can keep my sanity for the eighty minute sessions and headed for the basement to begin the grind.  

 

I aim for about 150 watts delivered to the machines's flywheel.  Fifteen minutes into the workout my tshirt has to come off and thirty minutes in I'm covered with sweat and have to reach for the water bottle.

 

This is physically hard work!  At 80:06 I give up.  I've averaged 152.5 watts for the session - a bit over 200 watt hours.

 

Great - if I had the machine hooked up to a perfectly efficient generator I could keep a 100 watt bulb going for a couple of hours.  Suddenly 15 cents per kilowatt-hour looks really cheap and if I was providing light I'd be very interested in compact fluorescents or LEDs...

 

Cheap energy was the core product of the Industrial Revolution.  An adult male in good shape can provide about one kilowatt-hour of work a day.¹ All of a sudden people didn't have to spend the entire day working to produce enough food for their families use and we could work on larger projects.  We were tapping energy stored over many thousands, even millions, of years for millions of years, and were quickly using what we believed was an enormous supply.

 

Even Sukie's laptop uses more energy that I would like to supply - now if I could put a generator on my rowing machine I could use some of that work to change the battery - wait - I know someone who did that.

 

Some of you knew Lynn and Al.  A very clever couple who didn't allow television in their house.²  When the Olympics came in 1992 Lynn, who had been an alternate in the US kayak team a dozen years before, decided they might allow a screen in the house so the two kids could watch.  

 

Al had an idea

 

He found a generator and converted an old junker bike into a stationary exercise bicycle that put out 12V DC.  The kids were in their early teens and, with the inefficiency of the generator and mechanical assembly (my guess is it was about 70% efficient), a kid could provide enough power to run the small DC black and white television he found between commercials.

 

The kids alternated and, by the end of the games, had developed very healthy leg muscles and a fantastic sense of guessing how long the commercials would last.  Al didn't install a flywheel on purpose, so when they stopped pedaling the TV quit.³

 

I had been thinking about the Olympics anyway.  Last night was the opening ceremony and I was up at 4am to watch a beach volleyball match.  It reminded me of my friend Colleen who was a professional beach volleyball player.  She had dreamed of the Olympics, but her career timing and partner selection was not what was needed.  But she was an amazing example of energy conversion in a human - we "compete" with our training and I detail some of the energy involved in an earlier post.⁴ It is really impressive seeing someone who needs to eat at least 5,000 Calories a day work out.

 

It is fun to watch an athlete jump.  They leap into the air following a parabolic path.  In Colleen's case she would adjust the position of her body near the peak of the arc to give herself more "hang time" to be well positioned.  It was perhaps three tenths of a second, but it looks impressive.  You see basketball players do this when they dunk.

 

But this is a diversion - but a fascinating one.

 

Since the Olympics are on it makes sense to talk about sport and the physics of motion.  This is a wonderful way to talk about energy at a level we can appreciate - when you have a notion of what the physical effort might feel like in your own muscles, these things make much more sense.  Last night I mentioned to Om that I might veer off along this path for a few post and now it looks like I've committed myself.

 

Let's table the physics of hang time until a bit later and we can go into the high jump.  There are so many other great subjects.  If you have any specific sports you'd like to see covered, let me know and we can work out what is taking place (or not - there are some things that would require large amounts of effort - the physics of walking and running, for example, are not completely understood at a fundamental level).

 

But there is still Sukie's laptop to worry about.

 

Imagine an apple (the fruit, not the computer) sitting on a dining table.  It has the potential to fall to the floor.  If it falls this potential energy is converted into motion until it strikes the floor and something else happens.  It turns out an average sized apple on a table of average height has about a joule of potential energy.⁵ If the table was covered with apples and you arranged for one to fall every second the power involved would be a watt - a watt is just a joule per second.  The other apple - er Apple ..  Sukie's MacBook - requires about 20 watts.  So if I could arrange for 20 apples to fall every second and could convert all of that energy to electricity I could run it.⁶  The battery stores 65 watt-hours, so I would have to do this for about three hours.

 

Twenty apples a second for three hours - 

a boatload of apples!

 

I'll be looking for a Lithium-ion battery and using the house's electrical outlet rather than relying on gravitational energy conversion or rigging my rowing machine for powering the laptop.

 

Enjoy the Olympics and we'll take a deeper look at things like the motion and energy associated with jumping and perhaps explain how an athlete - or a dancer - creates measurable hang time. And then from there who knows!

__________

¹ Note that I use the term work here.  Energy is a deep and fundamental concept - deep enough that physicists, who deal with it constantly, don't understand exactly what it is fully.  It can't be created or destroyed, but can change forms.  Energy stored in the chemical bonds of molecules and changed into other forms by rearranging those bonds.  Combine oil and oxygen under the right conditions and you get some new compounds - notably carbon dioxide and water (lots of other stuff too, but ...).  You also get a lot of heat.  The energy stored in  the bonds becomes heat energy.  Now boil some water with this heat and you get steam.  The steam can be converted into a mechanical energy by a steam engine.  This engine can cause something to happen - like spinning a generator which converts the mechanical energy into electrical energy.  And so on...  

Anything that happens in the Universe is associated with energy changing from one form to another.  The process causes stuff to happen.  If something moves we usually say work has been done.  

Think of work as the application of a conversion of energy.  It is measured in the units of energy.  So when I row I'm converting chemical energy in my body into a mechanical energy in my muscles which is converted into another form of mechanical energy which spins the flywheel ... and so on..   I have done work on the flywheel.

Power is just the rate at which energy is converted...  I have learned that people do not have a natural feeling for energy, but they do for power.  

² One of the family activities was boat building. Every year they built a new boat for their place on Lake George.  This sometimes involved homemade steam engines.  Al was a lefty and even machined his own bolts - left handed bolts held his engines together.  Amazing people...

³ An important point.  Electrical energy has to be used as it is created.  Systems that have to keep running usually require some form of storage.

⁴ I add that we handicap my exercise compared to hers.  She is much stronger.

⁵ It just works out to that number.  The potential energy is just E = mgh, where mass is the mass of the apple, g is the acceleration of gravity and h is the distance the apple can fall.

⁶ There is this pesky issue of efficiently converting the energy of the falling apple into electrical energy.  I could probably build a contraption good for about 90%, so I'd had to drop 22 apples a second to get 20 watts of electricity out .. best case.

____________

Recipe corner - two quick ones  and a quick little experiment

Really good!  It is a bit thrown together, but use it as a guide and then barbecue some corn.  This should be enough for about a dozen ears of corn.

There are many techniques for barbecuing corn on the cob.  I like stripped the ears completely and just throwing them on the grill.  I tend to go until the kernels are nearly charred - it gives a very nutty taste.

 

Masala Dip for Barbecued Corn on the Cob 

Ingredients

° 3 tablespoons garam masala

° 5 tablespoons of a good extra virgin olive oil ( though you could use clarified  butter)

° 4 cloves of garlic

° 150 grams of cilantro (' kept the stems)

° Maldon salt (or any finishing salt)

° lime juice 

° honey

 

Technique

° mix the garam masala , olive oil, garlic and cilantro and the purée

° salt to taste and add a bit of lime juice to taste and bring up the sweetness a bit with honey

 

Roasted Tomato Salad

 

This is just a quick guide - be creative and do your own thing.  The trick is to take advantage of the great tomatoes that are currently in season.  Heirloom tomatoes almost went out of fashion as their yield is poor, but the taste is incredible.  You can only find great ones for about a month in most areas so don't worry about spending more for them.  They are a rare treat!   You could do this with chunks of fresh mozzarella - there are any number of other things you might do.

 

Ingredients

 

° 1 kg of a mixture of halved smallish heirloom and cherry tomatoes.  Find local produce to make sure they are fresh and perfectly ripe. 

° about 50 ml extra virgin olive oil - use a good one

° tablespoon maple syrup

° a bit of a good course finishing salt (Malden is what I use)

° 30-40 grams sliced or roughly chopped toasted pecans

° 2 tablespoons capers fried in a bit of olive oil

° a handful of torn lettuce leaves

° 15 ml lemon infused olive oil

° finely chopped chives

 

Technique

 

° gently toss half of the tomatoes with the olive oil, salt and maple syrup.  place on a rimmed baking sheet cut side up and roast in a 350°F oven for about an hour. They should have started to caramelize a bit.  Remove and cool

° Just before serving toss the roasted and remaining tomatoes, the lettuce, capers and most of the pecans with the lemon infused olive oil.

° serve topped with the remaining pecans and chives

Finally the experiment

When you have a hot grill try grilling a slice of eggplant with a smear of honey followed by some good goat cheese.  There are a thousand ways to take it.  

Have fun!

 

Comments

[Jheri]

Awesome Steve;)

I vote for running and cycling

[Howard Greenstein]

I biked 50 miles today but since I didn't have a pulse monitor my phone app just estimated that I burned about 2700 calories. What is the best way to measure this with all the tech toys out today?

[Steve]

It depends on a lot of things - your speed relative to the wind, frontal area, coefficient of drag, your weight, the bikes weight, accelerations along the way are the big ones. I've measured a person who was presenting a lot of frontal area on a level path with few accelerations and a constant relative wind speed of 14 mph (her real speed - no wind). She was close to 500 kcal/hr - it worked out to a bit more than 1000 mpg(e).

http://tingilinde.typepad.com/omenti/2011/07/how-to-get-1.html

A good way is in a wind tunnel with a dyno - or a good calibrated load dyno while you are measuring the rider's CO2 output. but this is overkill for non- athlete applications.

There are some tables that show speed ranges and weight - I find they over estimate by about 20%, but most people probably aren't at ideal speed for their gear, so perhaps it is ok.

[Nancy White]

Years ago when I was running a little internet start up, a guy came by with a prototype recumbant bike to "sit" at your desk. It was clumsy and I could not keep my head still enough to efficiently read my computer screen (old style, big boxes!) but I loved the idea and asked if it could be set up to power the computer. This was 1997!!!

[Steve]

You could certainly power a laptop - current laptops are in the 20 watt region dropping to perhaps half that with MacBook Airs. A nice way to do it would be to charge the battery - then you could decouple the pedaling with the operation so you could have meetings and so on. Come to think of it, why not put these units in meeting rooms to power the machines in your office - at least there would be some productivity;)