Science - Puzzle Prime

Airplane on a Conveyor Belt

Section: Brain Teasers Category: Science

Difficulty: Advanced

A Boeing 747 is sitting on a conveyor belt, as wide and long as a runway. The conveyor belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Will the plane take off?

SOLUTION

Solution coming soon.

Unknown Author March 19, 2024

2 Comments

Clean Death

Section: Brain Teasers Category: Science

Difficulty: Hard

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?

SOLUTION

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:

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

Unknown Author March 25, 2023

3 Comments

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The Bicycle Problem

Section: Brain Teasers Category: Science

Difficulty: Hard Tags: Featured

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?

SOLUTION

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:

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

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.

Unknown Author January 1, 2023

2 Comments

Water with Ice

Section: Brain Teasers Category: Science

Difficulty: Advanced

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?

SOLUTION

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.

Unknown Author February 10, 2022

6 Comments

Canaries in Space

Section: Brain Teasers Category: Science

Difficulty: Hard

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?

SOLUTION

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

Unknown Author November 4, 2021

0 Comments

The Frozen Lake

Section: Brain Teasers Category: Science

Difficulty: Medium

You wake up on a frozen lake in an isolated region, a hundred meters away from the shore. 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?

SOLUTION

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.

Unknown Author May 4, 2021

8 Comments

Under a Microscope

Section: Brain Teasers Category: Science

Difficulty: Easy

How much will a 12° angle measure when looked at under a microscope that magnifies 8 times?

SOLUTION

The angle will measure the same, 12°.

Unknown Author February 3, 2021

0 Comments

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Balloon in a Car

Section: Brain Teasers Category: Science

Difficulty: Hard Tags: Featured

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?

SOLUTION

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.

Unknown Author September 16, 2019

13 Comments

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Front Wheels

Section: Brain Teasers Category: Science

Difficulty: Advanced Tags: Featured

When a car is making a turn, are its front wheels parallel to each other?

SOLUTION

The front wheels are not parallel to each other. The reason is that when steering, all wheels are turning along arks that have a common center. Otherwise, the car would be drifting.

Unknown Author February 21, 2019

1 Comment

The Golden Cylinder

Section: Brain Teasers Category: Science

Difficulty: Advanced

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?

SOLUTION

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.

Unknown Author January 25, 2019

5 Comments