The author is estimating. The whole point to an exercise like this is to be able to draw conclusions without having to conduct serious tests or rely on research. He says "suppose that a housefly weighs about the same as a grain of rice. It could be half as much, or twice as much, but it is unlikely to be either a tenth, or ten times as much." He is clearly trying to get an answer that is accurate within an order of magnitude. g, therefore, might as well be 10.
The author is estimating. The whole point to an exercise like this is to be able to draw conclusions without having to conduct serious tests or rely on research. He says "suppose that a housefly weighs about the same as a grain of rice. It could be half as much, or twice as much, but it is unlikely to be either a tenth, or ten times as much." He is clearly trying to get an answer that is accurate within an order of magnitude. g, therefore, might as well be 10.