Define the Principal
focus
of
Converging lens

When parallel rays of light pass through a convex lens(Converging lens) the refracted rays converge at one point called the principal focus.

ammonium chloride & tallow

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