Which of the following is true?
a. the earth exerts a gravitational force on the moon, but the moon does not exert a gravitational force on the earth.
b. the earth and moon both exert a gravitational force on each other.
c. the moon exerts a gravitational force on the earth, but the earth does not exert a gravitational force on the moon.
d. the earth and moon do not exert gravitational forces.

Asheet of steel 1.5 mm thick has nitrogen atmospheres on both sides at 1200oc and is permitted to achieve a steady-state diffusion condition. the diffusion coefficient for nitrogen in steel at this temperature is 6 x 10- 11 m2 /s, and the diffusion flux is found to be 1.2 x 10-7 kg/m2 -s. also, it is known that the concentration of nitrogen in the steel at the high-pressure surface is 4 kg/m3 . how far into the sheet from this high-pressure side will the concentration be 2.0 kg/m3 ? assume a linear concentration profile.