Which explains why scientists often prefer to measure mass rather than weight? It is always the same.

It is affected by gravity.

scientists often prefer to measure mass rather than weight because weight is affected by gravity while mass is not. Mass is always the same.

Explanation:

Weight is a force rather than a measure of the amount of matter something contains. The same amount of matter can have different weights depending on where it is. Mass directly measures the amount of matter present and remains the same no matter where it's measured.

can be answered using momentum.

momentum = p

p1= mass1 * velocity1

p1 = p2

therefore

p1 = 6kg * 4m/s

p1 = 24kgm/s

p2=m2 * v2

24 kgm/s = 7kg * v2

v2 = 24 kgm/s / 7kg

v2= 3.42 m/s