A body of mass 20kg moving with a velocity of 80ms\(^{-1}\) collides with another body of mass 30kg moving with a velocity of 50ms\(^{-1}\). If they both moved in the same direction after collision, find their common velocity if they moved in the :
(a) same direction before collision ; (b) opposite direction before collision.
Explanation
(a) 
Momentum before collision = \((20 \times 80) + (30 \times 50) = 1600 + 1500\)
= \(3100 kgm/s\)
Momentum after collision = 50v kgm/s
Momentum before collision = Momentum after collision
\(3100 = 50v\)
\(v = \frac{3100}{50} = 62m/s\)
(b) 
Momentum before collision = \((20 \times 80) - (30 \times 50) = 1600 - 1500\)
= \(100 kgm/s\)
Momentum after collision = \(50v kgm/s\)
\(\therefore 100 = 50v\)
\(v = 2m/s\)