I was actually asked this question yesterday by one of my physics students. He was surprised to see a figure of 32 in his pre-calculus book associated with the acceleration due to gravity on earth, rather than the 9.8 we had been discussing in class. Of course, he missed the units (feet/second^2 for the first, versus meters/second^2 in our physics class). However, it begs the question, why would we possibly be talking about scientific applications of math in English units of measure?
Want to know what happens when scientists use US customary measures? They blow up multi-million dollar spacecraft.
I know that here in the States, we're the last holdouts in the world still using units derived from Imperial measures. To the rest of the world, I apologize for our arrogance. However, I just can't understand why a math textbook would still be talking about trajectories and freefall would fail to use the metric system. We're not talking about a problem where little Johnny needs to find the area of his 3 foot by 4 foot sandbox. We're talking about the acceleration of gravity in a high school mathematics textbook. It's not even that old (although since the rest of the world had largely adopted the metric system by the 70's, it would have to be really old for this to be excusable).
My students know that the metric system (or its lack of use here in the States) is a remarkable pet peeve of mine. However, I'm a physics and math teacher; I think I'm allowed to get irritated about this. I'm educating future scientists, engineers, astronauts, and citizens in a global economy. If one of them blows up another multi-million dollar spacecraft because he or she is thinking in pounds, I'm moving to Canada, where they have the sense to think in kilograms like the rest of the world.