James Frysinger wrote in USMA 10176: >I object to expressing torque in J/rad, as I would to expressing force >in J/m. > >Jim > >"Joseph B. Reid" wrote: >> >> Mike Payne wrote in USMA 10170: >> >> >Just visited South Africa. ....Engine power was expressed in >> >kW and Torque in N/m. >> >> Surely torque should be expressed in N.m. I prefer J/rad. A force expressed in newtons (N) acting through a distance expressed in metres (m) produces energy expressed in joules (N.m = J), a scalar quantity. A torque is a vector quantity. It has an axis. The force involved, expressed in newtons (N), acts at right angles to the axis and at a distance, expressed in metres (m), normal to both the axis and the force. No work is done if there is no movement. If there is movement it is expressed in joules (J) per radian (rad) of movement. Expressing the torque in joules per radian makes clear the distinction between energy and torque. The BIPM bible states in note (a) on page 100 "The radian and steradian may be used with advantage in expressions for derived units to distinguish between quantities of different nature but the same dimernsion. Some examples of their use in forming derived units are given in Table 4". In Table 4 are listed angular velocity rad/s = s-1, angular acceleration rad/s2 = s-2, The Canadian Metric Practice Guide adds radiant intensity = W/sr, radiance = W/sr.m2, luminous flux = lumen (lm) = W/683, luminous intensity = candela = lm/sr, luminance = lm/sr.m2. The use of the steradian makes explicit the distinction between radiant intensity (W/sr) and radiant power (W), between radiance (W/sr.m2) and radiant excitance (W/m2), between luminous intensity (candela) (lm/sr) and luminous flux (lumen) (lm), between luminance (lm/sr.m2) and luminous excitance (lm/m2)
