Three (hopefully simple) physics puzzlers

[AxiomOfChoice]

(1) I was walking with a friend of mine over a bridge spanning the Chicago river the other day. It was FREEZING outside; probably 15 without windchill, subzero with windchill. He asked me how much I'd like to jump into that water. I said: "I wouldn't mind at all; it's got to be warmer than it is where we are. After all, if it were colder than 32 degrees, it'd be frozen." He bought my answer, and we both walked on...but I'm not quite sure it makes sense. Am I *really* right to say what I said?

(2) Suppose you're driving down the road at 65 MPH. There's a fly in the car with you, resting on the side of the passenger door. He flies from the passenger's side to the driver's side and lands on the driver's side door. Does this mean that the fly has to fly 65 MPH in the perpendicular (to the door and the car's velocity vector) direction? That doesn't seem quite right. After all, isn't there air in the car that's traveling at 65 MPH that kind of "carries" the fly along (even though an observer on the side of the road would say that the fly was moving at 65 MPH in the aforementioned direction).

(3) Suppose my friend has a stick of length 3 x 10^8 m, and we're separated by exactly that distance. He pokes me. The instant he moves the stick, I feel it. Therefore, information traveled from him to me at faster than the speed of light. Therefore, faster-than-c travel is possible.

 

[NobodySpecial]

He asked me how much I'd like to jump into that water. I said: "I wouldn't mind at all; it's got to be warmer than it is where we are. After all, if it were colder than 32 degrees, it'd be frozen."

Still wouldn't recommend it - temperature isn't as important as heat flow

(2) Suppose you're driving down the road at 65 MPH. There's a fly in the car with you, resting on the side of the passenger door. He flies from the passenger's side to the driver's side and lands on the driver's side door. Does this mean that the fly has to fly 65 MPH in the perpendicular (to the door and the car's velocity vector) direction?

Yes - and italso has to do upto 900mph to handle the Earth's rotation, and 66,000mph of the Earth around the sun.
Relative velocity isn't just how fast your kids approach at christmas.

He pokes me. The instant he moves the stick, I feel it.

No you don't - you feel it after the pulse has traveled at the speed of sound alomng the stick

 

[alxm]

(1) I was walking with a friend of mine over a bridge spanning the Chicago river the other day. It was FREEZING outside; probably 15 without windchill, subzero with windchill. He asked me how much I'd like to jump into that water. I said: "I wouldn't mind at all; it's got to be warmer than it is where we are. After all, if it were colder than 32 degrees, it'd be frozen." He bought my answer, and we both walked on...but I'm not quite sure it makes sense. Am I *really* right to say what I said?

Not quite. What we perceive as 'hot' or 'cold', isn't actually temperature. From the perceptual perspective, hot is what warms you up, and cold is what cools you down. That's not dependent on temperature alone, but also the rate of heat transfer. For instance, if you cool a piece of metal and a piece of cork down to the same temperature, the piece of metal will still feel colder, since it is a better thermal conductor, and so is cooling you down faster. From the biological perspective, that's of course the more sensible way to do things; maintaining your body temperature is what's important, not what the actual temperature difference might be.

So while the water may be warmer than the air, it's also a better conductor of heat by an order of magnitude (about 20x). So that ends up meaning a lot more than the relatively small difference in relative temperatures. Which we all know really, since you'll obviously get hypothermia faster by jumping in some cold water than standing around in the cold air, even if the air is quite a bit colder. (Also, the water effectively negates the insulation of your clothes as well)

So if by "warmer" you mean "feels warmer" and "heats you up faster", then the water wouldn't be warmer. Only in the relatively narrow sense of temperature would the water be colder.

 

[ko_kyi]

(1) I was walking with a friend of mine over a bridge spanning the Chicago river the other day. It was FREEZING outside; probably 15 without windchill, subzero with windchill. He asked me how much I'd like to jump into that water. I said: "I wouldn't mind at all; it's got to be warmer than it is where we are. After all, if it were colder than 32 degrees, it'd be frozen." He bought my answer, and we both walked on...but I'm not quite sure it makes sense. Am I *really* right to say what I said?


(2) Suppose you're driving down the road at 65 MPH. There's a fly in the car with you, resting on the side of the passenger door. He flies from the passenger's side to the driver's side and lands on the driver's side door. Does this mean that the fly has to fly 65 MPH in the perpendicular (to the door and the car's velocity vector) direction? That doesn't seem quite right. After all, isn't there air in the car that's traveling at 65 MPH that kind of "carries" the fly along (even though an observer on the side of the road would say that the fly was moving at 65 MPH in the aforementioned direction).

(3) Suppose my friend has a stick of length 3 x 10^8 m, and we're separated by exactly that distance. He pokes me. The instant he moves the stick, I feel it. Therefore, information traveled from him to me at faster than the speed of light. Therefore, faster-than-c travel is possible.

1. No, water conducts heat and can store more heat than air, so you would die in minutes in near-freezing water. Remember, when the Titanic sank, the people in the lifeboats survived, the people in the water died quickly.

2. In a closed system moving at a steady speed, everything in the system is at rest relative to each other. The fly no more needs to go 65 mph than it needs to fly hundreds of mph to match the Earth's rotation at your latitude.

3. I've heard variants of this for years. The speed of sound in whatever the stick is made of determines how fast movement is transmitted through it. Except for maybe "Spooky interaction at a distance" pretty much NOTHING is actually instantaneous. Measure carefully enough, and everything is separated by time and space from everything else.

 

[NobodySpecial]

Except for maybe "Spooky interaction at a distance" pretty much NOTHING is actually instantaneous

Except monarchy.
When the king dies the heir instantly becomes king - this has led to attempts to communicate faster than light by torturing spare monarchs. (By the great philosopher T Pratchett)