A colleague mentioned today that the April Fools tradition of pranking unsuspecting people into believing something false can be very unwelcome on the receiving side. That made me think of better ways to observe April Fools and I think I've found a slightly Discordian one that I wish I'd thought of years ago.

Let's share puzzles/riddles that often leave the listener very confused and help them realize that something they might have believed about the world is inaccurate. I think it's much better, because it's educational, there's no temptation not to ask the recipient whether they wish to take part beforehand, and I don't expect recipients to feel like they're being made fun of.

Let me start with a physics puzzle I'm fond of:

Consider a car that travels northward with speed v. Assume there are no losses (no rolling friction, vacuum, etc.) so the car travels at constant speed with engine off. At a point in time, the car engages its engine and speeds up to 2*v northward. How much work did the car engine do?

Well, we can compute the increase of car's kinetic energy: m/2*((2v)^2-v^2)=m/2*3v^2

Alas, let us consider a different (inertial) reference frame: one that moves northward with speed v (note that it's not tied to the car, even though it starts stationary in it). In that reference frame the car sped up from 0 to v, so the increase in car's kinetic energy is m/2*v^2.

What gives? How much energy did the engine actually have to use to speed the car up?

h/t to Ryszard Zapała, my HS physics teacher

Forgot to ask: Do you have similarly confusing riddles (maybe about something other than physics)? I would love to hear some.

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@robryk I know a bit a silly one. ;)

You enter a completely dark room. You strike a match to see what's in there.

You see: A candle, a flashlight, and a desk lamp.

What gets lit up first in this room?

@trinsec I think I have trouble parsing. By "lit up" do you mean illuminated (presumably by the light from my match)? Or maybe do you mean that the light from that objects reaches my eyes?

@robryk You're thinking way too sciency here. ;) Just what gives light first?

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@robryk Yeah. ;) When told right it's a nice one to catch your buddies unaware. It doesn't convey as well in text I suppose. :D

@trinsec Not necessarily. It's probable that I just had a nonmatching preconception.

@robryk
It had to be matching, you figured out it was a match. ;p
@trinsec

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