I'm using Boost.Units in a program for school to make certain that my equations are consistent (and that I didn't screw them up entering them). In the next Velocity equation, first get assigned with no problems, second causes a compiler error that I believe is saying that the constructor for quantity<velocity> is explicit. Why does first work without a conversion and second doesn't? Also, how do I get second to work? I assume this will require some type of explicit conversion, but wrapping the right-hand side with quantity<velocity>(...) doesn't work. const quantity<dimensionless> pi = 3.14159265358979323846264338327950288419716939937510582097494459230781640628620899862803482534211706798; const quantity<time> deltaTime = .1 * second; const quantity<length> particleDiameter = .001 * meter; const quantity<dimensionless> dragCoefficient = 0.5; const quantity<dimensionless> specificGravityGold = 19.32; const quantity<acceleration> gravity = 9.80665 * meters_per_second_squared; const quantity<length> initialHeight = 0 * meters; const quantity<time> initialTime = 0 * seconds; const quantity<velocity> initialVelocity = root<2>(4.0/3 * specificGravityGold * gravity * particleDiameter / dragCoefficient); const quantity densityWater = 1000 * kilograms_per_cubic_meter; const quantity densityGold = 19300 * kilograms_per_cubic_meter; const quantity<mass> particleMass = 4.0 / 3.0 * pi * pow<3>(particleDiameter / 2.0) * densityGold; quantity<velocity> nextVelocity(quantity<velocity> old) { quantity<velocity> first = (pi / 6.0 * pow<3>(particleDiameter) * gravity * (densityWater - densityGold)) * deltaTime / particleMass; quantity<velocity> second = (pi / 8.0 * densityWater * dragCoefficient * old * root<2>(pow<2>(old))) * deltaTime / particleMass; ... }