What If? Rejects #7.2: Kisses

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Q: If you saved a whole life’s worth of kissing and used all of that suction on one single kiss, how much suction force would that single kiss have?

Randall’s response: No response.

My response: Um… I’m not entirely sure what this means, much less why. And how do you measure a kiss? Does duration count? Are we counting all kisses or just kisses on the lips?

Okay, Fermi problem time! We’ll have to make a few assumptions, but I think we can compute a “suction force.”

First, let’s say that we’re taking the average duration of an ordinary kiss for our one super-kiss instead of adding them one after the other. That sounds more in the spirit of the question. We’ll include kisses anywhere on the body, not just the mouth, but based on the wording, that is specifically kisses given, not received. Next, the question said “suction force”. There are two factors that go into the force exerted by a kiss: pressure and area of contact. An open-mouthed kiss will have both greater pressure and greater area of contact than a simple peck, so we’ll have to consider a wide range of both.

Now, how many kisses does the average person give over the course of their life? For that matter, how many kisses does the average person give per day? There’s going to be a big range there, too. For parents with babies, I’m sure it’s a lot. For tweens who are embarrassed by their parents, very few. Hormonal teenagers might not give that many, but they’ll probably rack up a lot of “suction force” when they do.

It will vary with the person, too. Does gender affect the number? Are husbands more likely to kiss their wives, or vice versa? Are mothers more likely to kiss their children than fathers? Are daughters of any age more likely to kiss their parents? Or alternatively, what about personality? Do extraverts give more kisses than introverts? What about nationality? If you’re in Europe, where kissing is a greeting, you might give several times as many kisses as here in America, where we like our personal space (although those are frequently air kisses, which probably shouldn’t count).

Let’s say the average human gives 100,000 kisses in their lifetime. That’s an average of 3.4 kisses per day over 80 years, which is probably good for an order of magnitude estimate. The majority of those will probably be closed-mouthed, so let’s say 10 square centimeters for the average area of contact. And let’s give them an average duration of 1 second. These are very rough numbers, but for a Fermi problem, they only need to be within a factor of 10, so they’re good enough.

Now, what about suction pressure? That’s a bit trickier and can also vary a lot. One important fact to keep in mind is that pressures in the human body rarely go above a couple of PSI, generally less than blood pressure. Any more than that, and you start to tear stuff that you don’t want torn. Since the large majority of kisses probably don’t put a lot of effort into suction, I’m going to lowball it. Let’s say 1 kPa, or about 1/7th of a PSI.

Let’s multiply these numbers together!

100,000 kisses x 0.001 m2 x 1 kPa x 1 s = 100,000 N*s

That’s 100,000 newton-seconds—that is, a suction force of 100,000 newtons (about 20,000 pounds) delivered for a period of 1 second.

Newton-seconds are a measure of momentum and, similarly, a measure of impulse (which happens to be how rocket engines are rated). 100,000 newton-seconds represents the momentum (note that this is different from kinetic energy) of a large car barreling down the highway at 100 miles per hour. It’s also the amount of impulse needed to launch a 10 kilogram satellite into space. Both of those are done by thrusting force, not suction force, and it neglects air resistance, but it gets the idea across. A lifetime of kissing represents a surprisingly large amount of suction. Use it wisely.


About Alex R. Howe

I'm a full-time astrophysicist and a part-time science fiction writer.
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