Practice with the Laws of Physics
Gain some practice with the Laws of Physics by applying them to some puzzles and movie plots.
Let's put into practice the theory we learned in Laws of Physics.
The Fridge Puzzle
I have placed a refrigerator in the middle of a well insulated room and plugged it in. In a fit of pique I leave the door of fridge open and cool air pours into the room. Does (i) the average temperature in the room rise (ii) the average temperature stay the same (iii) the average temperature drop?
Answer:
Although cool air flows out of the open fridge, warm air comes off of the back of the fridge. Is it a draw
or does one win the temperature battle? The answer is provided by Conservation of Energy: the fridge is
plugged in because it uses electrical energy to run. This energy is effectively being pumped into the
room by the fridge and must be dissipated into it (energy is neither destroyed nor created), thereby
raising the temperature of the room.
The Frozen Lake Puzzle
You have been transported by Darth Maul to the ice planet Hoth where you find yourself in the middle of a large frozen lake. You quickly discover that (i) the ice is completely frictionless (ii) you have your cell phone (iii) you are naked. Point (i) is serious because it means that you cannot get traction when you try to walk -- you are stuck in the middle of the lake, doomed to freeze to death. You try calling 911 but there is no reception! How do you escape?
Answer:
The ice is frictionless so if you can get moving you will stay moving (neglecting air resistance) until
you reach the edge of the lake. Conservation of Momentum to the rescue! If you can throw your cell phone
forward at 100 mph (good luck) and if it weighs 1/100 of your weight, you will move backwards at 1 mph,
eventually reaching the shore.
North by Northwest
In one of the most famous scenes in movie history, the bad guys try to off Cary Grant by chasing him with a crop duster
(why they don't just drive up and shoot him is beyond me). He avoids death by diving in rows of corn and running
like heck. Eventually a truck stops on a nearby road and Cary jumps underneath it. Watch carefully now ... the plane
slams into the truck and explodes -- but the truck doesn't move a whit! This is a blatant violation of the Conservation
of Momentum. In fact we can estimate that if the crop duster were flying at 150 mph and the truck weighed about four times
as much as the plane, the truck plus plane wreckage should have plowed into Cary at about 30 mph. Thanks
Alfred Hitchcock!
Soylent Green
Edward G. Robinson and Charlton Heston are cops investigating a brutal murder in a dystopian future.
Things are so bad, that people are reduced to eating wafers produced by the Soylent Corporation
(presumably the
successor to Exxon-Mobil), and soylent green is the best of the wafers. The climax occurs when Heston
realizes that soylent green is made of people. Yum! The problem is that if soylent green is the only
source of nutrients in the future people will starve. See The Matrix for an explanation.
The Matrix
Machines have taken over the world! Dude, what if we're the computer program! Take these
two stock premises, throw in the acting of Keanu Reeves and the direction of the Wachowski brothers
and you have
the Matrix franchise. The kicker is the Soylent Green moment when our heroes realize that the matrix
is a virtual reality set up to keep humans happy while they float in giant test tubes. Why bother? To
extract human heat and electrical activity as an energy source of course!
Sadly, the entire premise of the movie violates the Laws of Physics. Humans need
energy (its called food) to operate, even if all they do is float in a test tube. Conservation of Energy
tells us that the total energy the machines take out from the humans must equal that put into them.
To make things worse, there will be losses in growing the human's food, transporting it to the facility,
and extracting the electrical activity
. Alternatively the Second Law of Thermodynamics tells us
immediately that the whole operation is futile. Stupid machines.
