When measuring radioactive things why is it important not to change the distance between the shielded readings and the unshielded readings?

There are 6.022 x 10^23 atoms of hg in 1 mole of hg. the number of atoms in 4.5 moles of hg can be found by multiplying 4.5 by 6.022 x 10^23 a. 2.7 x 10^24 b. 27 x 10^23 c. 2.71 x10^24 d. 27.099 x 10^23

Ajet plane is speeding down the runway during takeoff. air resistance is not negligible. identify the forces on the jet.

What is the most common reason for matter changing its state?