Tuesday, April 9, 2013

On accuracy and the suspension of disbelief

Science teachers are a weird bunch, to be sure. But there has to be a method to our so-called madness. A couple of months ago, a fellow Physics teacher came under severe parent fire for assigning the following word problem:

“A northbound car with a velocity of 100 m/s ran over a baby with a momentum of 800 kg m/s, what is the mass of the car?”

I’m sorry, but that problem is an absolute ignominy. For one thing, it should have been broken up into two sentences. For another, the solution is 8 kg. Eight kilograms!!! That’s 17.6 lbs. A seventeen/eighteen-pound car??? This guy is a disgrace to the profession.

A word problem has to be realistic. Here, for example, are a couple of the momentum problems I assigned during midterms:

“Cyclops” is a one-eyed cat with a mass of 6.8 kg. Cyclops’ owner is tired of looking into Cyclops’ sad little eye, and throws it out the door, where it strikes a stationary three-legged cat, “Stumpy,” that has a mass of 1.6 kg.  Stumpy flies forward with a velocity of 6.8 m/s, and Cyclops continues forward at 4.5 m/s.  What was Cyclops’ initial velocity?

An astronaut with a mass of 81 kg is outside a space capsule when the tether line breaks. Luckily, said astronaut has his pet cat “Litter” with him. To return to the capsule, the astronaut throws the 2.5 kg cat away from the capsule at a speed of 16 m/s, thus being able to return to the capsule safely. As for Litter... well, better luck in the next of its nine lives. At what speed does the astronaut move toward the capsule?

An 8-kg car simply won’t do.

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