Why do you need soap?

I'm sure all of us has had the day where we're in too much of a hurry to shower/bathe properly. In the hustle of the mornings, we don't shower with soap(if we're in a hurry).


It feels clean at first, but on your way to work, you'll most probably feel 'unclean', although you could have sworn that you'd washed your entire body properly, without soap. Then, you'd think would soap (or not using it) be the main trigger for the uncomfortable feeling of dirtiness, and would probably wonder how did the ancients survive without soap.


We are here to answer part one of the question.

That's a 'bar' of soap.

Besides the obvious fact that the-

Before that, let me tell how how does soap work. Soap is made of two chemicals: a fatty acid and a base, that are attached to each other. The fatty acid end is hydrophobic, it latches on to anything else but water, so it sticks to the dirt on your skin. The base end is highly hydrophilic, and when you rinse your body with water, the base end pulls the fatty-acid with it, and all dirt with it, leaving you with clean skin.

Let's get back to where we were. Besides the obvious fact that soap washes away dirt particles away, the surfactant nature of soap actually makes water wetter.

You ask: how can water be wetter when it is already wet?

Surface tension.

Surface tension is the term for the attractive force that results from the attraction of molecules in the liquid to each other. Such a force must of course be present - otherwise the molecules would fly away and return to the gaseous state. For most liquids, this force is a relatively weak electrostatic force (van der Waals attraction). This force is not too strong, for the liquid molecules must be able to move past each other and occupy the volume of a container whereas a solid would not.

A surfactant is a long, skinny molecule with different chemical groups at either end. Electrostatic interactions result in one end of this molecule sticking to water, while the other end is repelled(read the above illustration in the paragraph below the soap-controller). The molecule is fairly unyielding, and a large collection of such molecules will orient themselves so that all of the regions that are repelled by water are pointing in one direction.

This interferes with the water-water bonding at the surface of the water layer, and reduces the cohesive force between the molecules that was the source of the surface tension.

TL;DR: Soap helps one clean up by reducing the surface tension of water so that it can make direct contact with the dirt.


References: 
The Physics of Superheroes, ISBN 9780715639115
Wikipedia.org http://en.wikipedia.org/wiki/Micelles

Jumping

What determines how high you can leap? Two things: your mass and the force your leg muscles supply to the ground. Taking the Second Law of Motion into account, they determine how much acceleration you can achieve as you lift off the ground. Once you are no longer in contact with the surface, the only force acting on you is Gravity, which decreases your acceleration as you ascend. So there are two vectors we have to concern ourselves with: the initial force that gets you airborne, and the deceleration of gravity that eventually halts your ascension.

v^2 = 2gh gives the height h you will climb, and g represents the deceleration due to gravity.

wheee

 

As you might, or might not have noticed, nowhere does the final height that you achieve depend on your mass. Big or small, if you start off with a velocity v and the only thing pulling you back to earth is gravity, then your jump height depends only on the deceleration due to gravity g and your total velocity v. Of course, there is another acceleration that enters into the leap - that provided by your leg muscles at the start of the jump. And this acceleration depends on the mass of the jumper. Using the second law of motion, that force equals mass times acceleration [F=ma] it is clear that for a given force, F, the heavier a person is, the less of an initial velocity he will achieve.
Which means a lower height h that said person will be able to attain.

I was bitten by a radioactive spider. I can jump as far as a spider can.

It's not that spiders are such great leapers that they can jump many times their body length. Rather, it's that small insects have tiny muscles (small force), but they only have to lift an equally tiny mass to leap one meter, which just turns out to be many times larger than their sizes. Humans have much bigger muscles than insects, and can output much greater forces, however they also have to lift much heavier masses, so the net effect is that the range/height that they can jump is also about one meter.

TL;DR - F=ma demonstrates how Spiderman's jumping abilities are pure fiction.

Legacy: Humans?

The world is not immortal. Nothing is.

What do you think would happen if humanity were to disappear?

Let's imagine a future where there are no more humans on Planet Earth. A future where all humans are dead, or missing. A possible dimension where the world is free, without mankind.

I went out, one day, taking a stroll on the streets of human infrastructure. My mission was to observe all signboards, the buildings and the sidewalks, looking at everything as though all humans were to disappear. There must be another sentient, intelligent civilisation out there. What if they were to come across our deserted planet, one that had no signs of life?

I picture them to land and gather information regarding the human civilisation, by looking at the relics we left behind. What would they conclude?

What would you conclude?

Strolling down the streets, I see signboards and advertisements of people laughing, having fun. Flyers of insurance and bank loans are all over the pavement. The graffiti on the walls would probably make no sense to the lifeforms that would learn our history. I assume myself to be one of them, seeing everything in a different angle.

I see a group of people laughing at random people passing by. I remind myself that they are not there in this particular future, the future where no humans remain. I stroll on.

Electronic paraphernalia and screens flash and flare. Would they still work once humans are gone? I suppose so, since electricity cycle is automated. Probably not for long. I observe the images on the television screen. Bands perform, newsanchors drone on, citizens lament...

My brain stubbornly resisted my efforts to be an alien landing on Earth for the first time.

I reassure myself that the laws of the universe are constant throughout the cosmos. Aliens might not recognise Newton, but they'll surely know his laws of Motion. They might not recognise our concepts of science, but they will know Hydrogen has only one proton. They won't know Einstein, but they must know about relativity.

With this 'knowledge' in my mind, I transform myself into an individual that is no longer a part of the world. I continue my walk, taking each step forward with a brain utterly devoid of direction. Thoughts fill my mind and several voices in my head argue.

My efforts as an alien chronicler were valiant, but futile. I will try again in the future.

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