Questions tagged [physics]

A puzzle relating to physical equations or physical principles such as energy, entropy, and classical/quantum mechanics. General physics questions are off-topic here but can usually be asked at Physics Stack Exchange.

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5
votes
6answers
4k views

If the second hand goes backwards, starting from 12:00, how long does it take to meet with the minute hand?

If the second hand of a clock goes backwards, starting from 12:00, how long does it take until it meets with the minute hand (which is going forwards)?
4
votes
1answer
192 views

Traffic lights in space

In a distant future, human beings are travelling between planets and even solar systems with ease. Their advanced technology allows them to drive their own space-cars, which have a cruising speed of ...
19
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6answers
3k views

The falling broom handle

As the reaction to my previous physics puzzle was somewhat mixed, I'll try again. I found this in an online physics book. You hold a broom as shown in the following picture: The head of the broom is ...
15
votes
3answers
2k views

A pencil in a beaker of water

You have a beaker containing 100ml of (slightly salty) water, standing on a digital scale. The scale has been calibrated to the beaker so that it shows only the weight of the water, which is exactly ...
-2
votes
1answer
93 views

Distance between two friends [closed]

Two friends are 200 meters apart in a concourse. They then both walk 100 meters each with their faces towards to each other. Yet after this 100 meter walk they are still 200 meters apart.
4
votes
0answers
110 views

Run to a point in a triangle in shortest time [closed]

This is a generalization of a puzzle that dealt with an equilateral triangle. Assume three runners with the following speeds - 4.5, 6.2, and 8.7 meters/sec. They are at the corners of a triangle with ...
3
votes
3answers
205 views

Two Identical Pistols Shot Simultaneously [closed]

Two identical pistols are shot at the same moment, one in Death Valley and the other the Schwarzwald Germany. Both of them are at targets 40 meters away. Which one hits its target earlier?
30
votes
8answers
4k views

Pulling the rope with one hand is as heavy as with two hands?

There is this box, having two holes, and the ropes going through each of them. I try to pull the left rope with my left hand... And it's heavy... Then I try to pull the right rope with my right ...
8
votes
2answers
375 views

How many times does the earth rotate per year?

Assuming there are exactly $365$ days per year. How many rotations about its axis does the earth make in one year?
38
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8answers
5k views

Who is our nearest planetary neighbor, on average?

Assume that the planets have circular orbits centered on the sun. Assume that the radius of the orbits is 0.39, 0.723, 1, and 1.524 for Mercury, Venus, Earth, and Mars. Assume also that there are no ...
4
votes
1answer
415 views

Is it Basic Physics?

Which number is next in the sequence? 1, 2, 3, 4, 5, 6, 7, 9, 10 ,11 ,12 ,13 ,14 ,15 ... Describe why and how your answer applies to the given problem.
1
vote
0answers
190 views

Is it possible to determine the original direction of water flow? [closed]

Take two beakers of water and use a spoon to stir water clockwise in one beaker and counterclockwise in the other beaker. Observe the beakers after a sufficiently long time when the water flow has ...
12
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2answers
311 views

Pucks in the arena

Two identical pucks of radius 10 cm are placed in a round arena of radius 1 m. They are positioned 50 cm away from the center of the arena on opposing sides. Assuming no energy losses during sliding ...
9
votes
4answers
453 views

Breaking Balance (Part C)

For a starting number of otherwise identical coins there are among them TWO IDENTICAL counterfeit coins which are either heavier or lighter than the rest. Using a three-pan balance (described in ...
2
votes
1answer
64 views

Breaking Balance (Part B)

If we assume the classic balance puzzle (Twelve balls and a scale) is phrased like this: You have 12 coins that are identical except for one counterfeit that is heavier or lighter. Can you use a ...
7
votes
5answers
337 views

Breaking Balance (Part A)

If we assume the classic balance puzzle (Twelve balls and a scale) is phrased like this: You have 12 coins that are identical except for one counterfeit that is heavier or lighter. Can you use a ...
9
votes
1answer
342 views

The Lazy Laser Physicist: Part 2

The Lazy Laser Physicist is quite shocked about what happened to his setup over night: Almost all of his mirrors were taken away, but some other things have been put on his table. Besides these there ...
15
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7answers
3k views

The Lazy Laser Physicist

You have a setup like in the image above. But it seems like detector A does some weird things. You should better check it with detector B. What is the minimum number of mirrors you have to move (...
10
votes
4answers
2k views

Where does the emperor sit and why the earplugs?

The Emperor is annoyed that the crowd routinely chant out of step at the Empire's largest circular amphitheatre. For example, they are supposed to shout phrases in unison such as "Hail to the Emperor, ...
14
votes
4answers
587 views

Balancing Balls

I have a disk with 6 equally spaced dents around the edge. The disk balances on the center point. I want to place marbles around the edge so that it stays balanced. There are four ways that this can ...
8
votes
7answers
1k views

Perpetual Motion Machine [closed]

An engineer tests a new generator by attaching it to a motor-powered cart. The generator operates such that the cart's batteries charge when the cart's wheels turn. Assume the following true ...
20
votes
7answers
2k views

Making a physicist

Materials: CV step-up transformer with coil ratio 1:5 Method: Apply the CV to the primary coil and spin it. Result: A physicist is output. Who? Hint 1: Hint 2:
4
votes
1answer
159 views

An Incorrect Summary of a Twenty-Two Day Tour

After a long but rewarding life in an office cubicle, Sally the office worker became Sally the retiree. Feeling unfulfilled, and with ancestors who had served admirably in the Navy, Sally the retiree ...
0
votes
1answer
249 views

Fuzzy's Fizzy Dilemma [closed]

Fuzzy the Fuzzball's friends was going to the Fuzzball Festival, and of course he went along. While there, he saw the most curious display. The booth was displaying brand new types of Fuzz. There was ...
10
votes
6answers
2k views

In which direction does the water flow?

There is a hard metallic pipe in your dark wine chamber. You hear the water flowing slowly in the pipe but you cannot understand in which direction that water flows since it is a closed system. ...
23
votes
15answers
11k views

Separate the milk!

You have a jug of milk and a jug of water. You must empty both jugs into a nine gallon barrel. You must be able to: Differentiate between the milk and the water Separate the milk from the water You ...
3
votes
2answers
258 views

Packing (cooking equipment) for Mars

Heading to Mars tomorrow, and I want to make sure I can still make my famous bean soup. Ordinarily I just cook it in a pot, but I remembered that beans are hard to cook at high altitudes, and Mars is ...
12
votes
5answers
779 views

More than equilibrium

Assume you have 2 identical metal cubes, the only difference being their temperatures $T_1 = 0 \, \text{°C}$, $T_2 = 100 \, \text{°C}$: The task is now to transfer as much heat as possible to the ...
-1
votes
3answers
236 views

Untwisting a long rope

Have you noticed that if you take your mouse and hang it from the end it will start to twist until it becomes untwisted? My puzzle is this: If you have a really long rope and not enough vertical ...
13
votes
10answers
2k views

Frequentists be damned! Design an evil coin to prove a point [closed]

The frequentists' definition of a "fair coin" is a coin that, tossed a million times, converges toward an equal number of heads and tails. But what is meant by converges? We might imagine a plot of ...
19
votes
4answers
2k views

Martin Gardner's “The Twiddled Bolts”

This is a puzzle of Martin Gardner, taken verbatim from My Best Mathematical and Logic Puzzles: Two identical bolts are placed together so their grooves intermesh. If you move the bolts around each ...
9
votes
2answers
443 views

A big logical mess

I found a drawing under a stack of books in my house. It looked like a logical circuit diagram, so I decided to make it. I assumed that I was allowed to put anything in the boxes with the question ...
3
votes
1answer
239 views

Figure out the ? in the figure below

Here is the link to the puzzle
-3
votes
1answer
213 views

Everyone is attracted to me! [closed]

Everyone in the world is attracted to me even though I'm the most abhorrent and repulsive creature to roam the earth. Though I am human, exploit no human weakness, and there are much more suitable ...
0
votes
2answers
197 views

First I'm the parent and then I'm the daughter

A simple, one sentence riddle which I'm confident you'll know when you have the correct answer. First I'm the parent, but I lose He and some energy, then become the daughter. What does this riddle ...
1
vote
2answers
353 views

Colliding Hang Balls [closed]

Two identical balls are attached at the ends of inelastic and very light string. Hanging on a fixed friction less pulley, one is pulled higher so that it is at right angle with the other ball. If ...
1
vote
2answers
200 views

Identifying Ancient Artifact

You are a group of archaeologists and scientists investigating an ancient cave. You set up and verify a presurization system at the entrance, which you can use to suck all the air out of the room, or ...
11
votes
1answer
699 views

Meeting of 8 flies [closed]

In the following diagram 8 flies are located at the corners of a cubic room, 10 feet on each side. At time 0 each fly starts to fly towards its neighbor fly as indicated in the diagram. Their speed of ...
4
votes
4answers
1k views

The largest and smallest value with 4 resistors

In physics, a resistor is an electrical component that inhibits current. We represent its ability to inhibit current with a value called its resistance, represented with $R$. ($R$ is always positive.) ...
6
votes
6answers
550 views

Misguided testing

Tom's job was to test light sensors for the Navy. He would mount 4 sensors at a time in a test circuit, then shine a light at them and measure the output. "Sensor #157 is clearly the best," Tom ...
0
votes
2answers
150 views

The driver and the chickens

In a sealed container van, there are 5 chicken cages with a chicken each. The driver has to cross a dilapidated bridge by jumping over it. If he can minimize the weight of his truck by even just 3 ...
5
votes
1answer
258 views

The Random Rube Goldberg Messup

You just finished making pretty cool Rube Goldberg machine. You test it. In a part of the machine that utilizes dominos, (where each domino is 1.65 times bigger than the one before) one domino didn't ...
4
votes
1answer
259 views

Game of Billiards: Kill the Planets Too

Below is a $8 \enspace \text{ft} \times 4 \enspace \text{ft}$ $\enspace (=243.84 \enspace \text{cm} \times 121.92 \enspace \text{cm})$ billiard table, with a perfectly flat playing surface. The ...
8
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4answers
2k views

Use a circuit to multiply two resistances

Given two resistors of unknown resistances and an infinite supply of wires and other resistors, create a circuit using multiple series and parallel combinations such that the effective resistance of ...
6
votes
9answers
6k views

Color of the bear

A bear fell from a height of $10$ metres from ground. It reached ground in $\sqrt2$ seconds. But somehow, it didn’t get hurt. What is the colour of the bear? Note : There is no ...
1
vote
4answers
2k views

Sudden braking on the highway [closed]

Have you ever noticed how, on a highway in conditions of heavy traffic, whenever you have to brake for any reason, the braking turns out to be sudden and hard? The same does not seem to happen on ...
3
votes
2answers
352 views

Hot Tea and an Unexpected Visitor

You're enjoying a Saturday morning, making a cup of tea to start the day. Just as you've finished pouring the boiling water into your teacup, you hear a knock at the door. If you like your tea with ...
14
votes
3answers
2k views

Bars in the river

There is a brick of gold and a brick of iron in a boat on a lake (both 10 inch blocks), if they are both dropped into the water which will make the water level higher? Edit we can assume its a very ...
5
votes
4answers
846 views

Dungeon Crawler - Final Level

You are the protagonist in a game designed by NeedAName Inc. Proceed through the levels using your video game knowledge to solve the puzzles therein. Solve all levels and beat the game! Level 4: ...
-2
votes
1answer
208 views

What is heavier?

Remark: Because of the comments I decided to drop the made up story, to remove any ambiguities. You have a balance scale. On one side of the scale you put an $1kg$ cube made of iron, on the other ...