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Can someone explain the answers to these questions?
The answers are q12)A
EDIT- q9 can be ignored as I found the answer.
Let the mass of these "things " be m.
You must use the principle of linear momentum here, i.e, Sum Of Linear Momentum Before = Sum Of Linear Momentum After
Sum Of Linear Momentum Before = Sum Of Linear Momentum After
(m x 60) + (m x -30) = (2m x v)
30 m = 2 mv
v = 15 m/s
Note: I used -30 since the velocity is in the opposite direction. As for the 2m, it's quite obvious. Since they stick together on impact, thier total mass is m + m = 2m
Hope It Helps.