## Physics

An archive of questions and answers that may be of interest to puzzle enthusiasts.
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Question 1 - balloon:
A helium-filled balloon is tied to the floor of a car that makes a sharp right turn. Does the balloon tilt while the turn is made? If so, which way? The windows are closed so there is no connection with the outside air. Show Answer

Question 2 - brick:
What is the maximum overhang you can create with an infinite supply of bricks? Show Answer

Question 3 - bubbles:
In a universe with the same physical laws, but which is mostly water with little bubbles in it, do the bubbles attract, repel, or what? Show Answer

Question 4 - cannonball:
A person in a boat drops a cannonball overboard; does the water level change? Show Answer

Question 5 - magnets:
You have two bars of iron. One is magnetized along its length, the other is not. Without using any other instrument (thread, filings, other magnets, etc.), find out which is which. Show Answer

Question 6 - milk.and.coffee:
You are just served a hot cup of coffee and want it to be as hot as possible later. If you like milk in your coffee, should you add it when you get the cup or just before you drink it? Show Answer

Question 7 - mirror:
Why does a mirror appear to invert the left-right directions, but not up-down? Show Answer

Question 8 - monkey:
Hanging over a pulley there is a rope, with a weight at one end.
At the other end hangs a monkey of equal weight. What happens if the monkey starts to ascend the rope? Assume that the mass of the rope and pulley are negligible, and the pulley is frictionless. Show Answer

Question 9 - pole.in.barn:
Accelerate a pole of length l to a constant speed of 90% of the speed of light (.9c). Move this pole towards an open barn of length .9l (90% the length of the pole). Then, as soon as the pole is fully inside the barn, close the door. What do you see and what actually happens? Show Answer

Question 10 - resistors:
What is the resistance between various pairs of vertices on a lattice of unit resistors in the shape of a
1. Cube,
2. Platonic solid,
3. N dimensional Hypercube,
4. Infinite square lattice,
and
5. between two small terminals on a continuous sheet? Show Answer

Question 11 - sail:
A sailor is in a sailboat on a river. The current is 3 knots with respect to the land. The wind (air velocity) is zero, with respect to the land. The sailor wants to proceed downriver as quickly as possible, maximizing his downstream speed with respect to the land.

Should he raise the sail, or not? Show Answer

Question 12 - shoot.sun:
If you are standing at the equator at sunrise, where must you point a laser cannon to hit the Sun dead center? Assume that the Sun is stationary and that the Earth's orbit around it is circular. Show Answer

Question 13 - skid:
What is the fastest way to make a 90 degree turn on a slippery road? Show Answer

Question 14 - spheres:
Two spheres are the same size and weight, but one is hollow. They are each made of uniform material, though of course not the same material. With a minimum of apparatus, how can I tell which is hollow? Show Answer

Question 15 - wind:
Is a round-trip by airplane longer or shorter if there is wind blowing? Show Answer