Clean Death

A man was going to bleach his socks because they had gotten muddy the day before. As he was pouring the bleach into the washing machine, he spilled some on the floor. He got some cleaning fluid and mopped it up with a rag. Minutes later he was dead. What killed him?

When the ammonia (NH3), found in cleaning fluids, is mixed with bleach (a dilute solution of sodium hypochlorite, NaOCl), a deadly gas (monochloramine, NH2Cl) is produced that can kill a person instantly. The resulting chemical reactions are the following:

  1. Bleach decomposes, forming hydrochloric acid and subsequently chlorine gas:
    NaOCl → NaOH + HOCl
    HOCl → HCl + O
    NaOCl + 2HClCl2 + NaCl + H2O
  2. The chlorine gas reacts with the sodium hypochlorite in the bleach, releasing chloramine as a vapor:
    2NH3 + Cl2 → 2NH2Cl
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The Bicycle Problem

If you pull straight back on a pedal of a bicycle when it is at its lowest position, will the bicycle move forward or backward?

The surprising answer is that (usually) the bicycle will move backward.

When a bicycle moves forward, the trajectory its pedal traces with respect to the ground is called a trochoid. Depending on the selected gear of the bicycle, that trochoid could be:

  1. Curtate trochoid (for almost all gears of most bicycles)
  2. Prolate trochoid (if the gear is very low and the bicycle moves slowly)
  3. Common trochoid, a.k.a. cycloid (if the wheels of the bicycle and the pedal spin at identical speeds, practically never happens)
curtate trochoid
prolate trochoid
common trochoid (cycloid)

Since we are fixed with respect to the ground, by pulling the pedal backward, we are causing it to move leftward along the trochoid and therefore the bicycle will be moving backward. We note that despite that, the pedal will be moving forward with respect to the bicycle (but not with respect to the ground).

You can see a visual explanation of this puzzle in the video below.

Water with Ice

You have a glass of water and an ice cube floating in it. When the ice cube melts, will the water level increase, decrease or remain the same?

It will remain the same. The amount of water that the ice cube displaces is equal to its mass. Since the mass does not change and the density of water is equal to 1, the extra water after melting will be the same amount as the displaced water before that.

Canaries in Space

NASA was considering sending canaries into space to study them under zero gravity. The project was scrapped when someone realized that in spite of having sufficient water supplies, they could die of dehydration within a few hours. Why?

Unlike humans, birds need gravity in order to swallow. Thus, in space they wouldn’t be able to drink and will die of dehydration.

The Frozen Lake

You wake up on a frozen lake in an isolated region. The surface of the lake is frictionless and no grip of any kind can be attained over it. You find just your mobile phone in your pocket, but when you take it out to call for help, you realize there is no reception.

If there is no wind force to help you escape, what are you going to do to avoid freezing to death?

Throw your phone as hard as you can. Thanks to Newton’s third law of motion and the frictionless lake, you will start sliding away.

Balloon in a Car

You are sitting in a motionless car, which is tightly sealed, i.e. no open windows, holes in the car, etc. A helium balloon on a string is tied to the floor. If you start accelerating the car, is the balloon going to move back, forward, or stay in place?

The reason the balloon floats up in the air is that the helium has a lower density, so when gravity pulls the denser air around down, the balloon gets pushed up. Similarly, when the car accelerates, the air around gets drawn to the back of the car, which makes the helium balloon go forward.

The Golden Cylinder

You are shown two cylinders – one of them is made of gold and hollow, the other one is made of alloy and solid. Both of them have the same weight, shape, and are colored in black. How can you figure out which cylinder is the golden one, without scratching the paint?

Simply roll the two cylinders on the ground. The hollow one will roll farther than the solid one since it will have a higher moment of inertia.