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I'm a software developer, and I need to calculate the estimated amount of force expended typing stored text. Preferrably in some interesting way. (i.e. the force exerted on keys thus far is enough to push a car 5 miles) (or: equivalent to 100 kg of TNT)

Assumptions:

  • We're not going to worry about deletes or moving the cursor or anything, I'm just counting characters of stored text.
  • I don't really care if the space requires more force or not, this is more of a "fun fact" than anything.
  • From what I've found online, the mean force required for a keystroke is about 12.9N (source).
  • Hundreds of millions of characters have been typed.

Questions

  • What is a good way to make this something people can relate to?
  • How can I calculate it?

Thank you all in advance for your valuable time and input.

EDIT: I thought my original post would make this pretty clear: I realize I only have the force required to push each keystroke. I'm looking for a way to demonstrate that force applied to something to help people quantify it, hypothetical energy in terms people can understand.

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it depends on how old is your keyboard.. (put your keyboard over a weighing scale, hold down a key, and voila!) – HDE Jun 2 '11 at 14:59
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"From what I've found online, the mean force required for a keystroke is about 12.9N." The source for this number would be good, please edit to add it. You should distinguish between force and energy. The daily calorie requirements of an office worker who enters N keystrokes per day would be a more inclusive measure than what you are asking for. – Peter Morgan Jun 2 '11 at 15:04
The force at which a the keys will register is often listed on the specs (and the value that Peter lists agrees with my memory), but the force you actually apply depends a lot on the user, the amount and reliability of the tactile feedback you get from your keyboard. My favorite keyboards have a short throw, a light to medium touch and really crisp feedback. The work done of course depends also on the length of the throw and on the style with which you type, the layout of your keyboard, etc, etc, etc. And how much energy you expend is a matter of biophysics on top of that. – dmckee♦ Jun 2 '11 at 15:20
One rarely types in piano pianissimo (the force needed to register. The IBM AT keyb mentioned below made this impossible). I find theres at least a few emails per day that requires me to type my reply in fortissimo; sometimes even leaving my fingertips sore. – Captain Giraffe Jun 2 '11 at 17:39
I've added my source for the mean force required to actuate a keyswitch. I thought I'd distinguished between Force and Energy, but I suppose I wasn't explicit enough. I can calculate the Force, I'm interested in a hypothetical way to apply that as energy in terms people can understand. – blesh Jun 2 '11 at 20:27
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2 Answers

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@HDE's comment's experimental approach done. Answer between about 1 and 2 ounces = 1/4 to 1/2 Newtons (http://www.wolframalpha.com/input/?i=convert+2+ounces+to+newtons). Key travel 3mm, say, = 1.5 milliJoules = $3.59×10^{-7}$ dietary Calories (http://www.wolframalpha.com/input/?i=convert+0.5+newtons+times+3+millimeters+to+Calories). So, for every 3,000,000 keystrokes, say, that's one Calorie. Not a large proportion of the daily Calorie consumption of the person who's pressing the keys.

As this shows, Wolfram alpha is your good friend for this kind of Question.

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Note that the usual business with biophysics and work applies here the "useful work done" is not the calories expended. You can burn a lot of calories just holding a barbell still at arms length. Zero $\int \vec{F} \dot d\vec{s}$, but very hard "work". – dmckee♦ Jun 2 '11 at 16:24
@dmckee, Absolutely. Also this is definitely order-of-magnitude stuff. I nonetheless think this is a more-or-less real-world comparison. A few billion keystrokes is loosely comparable as an amount of energy to the energy of a person's daily Calorie intake. – Peter Morgan Jun 2 '11 at 16:54
If it's on that kind of scale, then maybe the calories expended on thinking are comparable. – Emilio Pisanty Dec 19 '12 at 18:34
up vote 6 down vote

You seem to be more interested in energy than force. It is not possible to say "the force exerted on the keys thus far is enough to push a car five miles, or is equivalent to 100 kg of TNT" because pushing a car over a certain distance and exploding a certain amount of TNT aren't examples of a set force. The TNT has a certain amount of energy. Pushing the car also requires energy, but the amount of energy required depends on the car and on how fast you push it. There is a certain minimum force to push the car at a given speed, but once you apply that force, you can push the car one foot or one thousand miles, all while applying the same force.

Force also doesn't add in the way your question implies. If I exert one Newton of force on something for one hour, that is not 3600 times more force than if I exert one Newton of force for one second. Instead, the momentum that I impart to the object will be 3600 times as much. The force is the same.

Similarly, if I exert five Newtons of force on a key, release, and exert five Newtons again, that is not ten total Newtons. It's just five, then zero, then five Newtons.

You might want to calculate energy by the formula

$$E = Fd$$

$E$ is the energy expended. $F$ is the force. $d$ is the distance over which the object travels while the force is exerted. So, if a keystroke usually involves 10 Newtons and the depth of the keystroke is 1cm, then you are exerting $10 N * 1cm = 0.1 J$ per keystroke.

However, the 10 N (or 12.9N) figure is almost certainly wrong. That's the weight of a heavy textbook, not a computer keystroke. This ergonomics paper discusses the force of keystrokes, but I don't have access to it and no force measurements are cited in the abstract. This page cites forces in the 0.25 - 1.5 N range. The total energy per keystroke might be about $0.5N * 0.5 cm = 2.5*10^{-3}J$. That means that in typing this response, I expended around 5J of energy on the keyboard.

Typing out the complete works of Shakespeare would take about as one Calorie (one one-hundredth the energy you get from a banana).

As dmckee pointed out in a comment, this is the work done on the keyboard, so it's a minimum amount of energy expended assuming the springs in the keyboard don't return any energy to you. The actual human user is less efficient.

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I should have specified that I know this, and I was looking for some way to "apply" the estimate force exerted on the keys. (e.g. "Enough force has been used to push keys to move a [llama] [30 cubits]" – blesh Jun 2 '11 at 20:14
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@blesh: the point is that moving a llama 30 cubits could take any amount of force, depending on how quickly you move it. Heck, you could move the llama to Pluto with less force than it takes to push one key on a keyboard. – David Zaslavsky♦ Jun 2 '11 at 22:01

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