Question

Two cars each of mass 1000kg are moving in a straight line but in opposite directions. the velocity of each car is 5m/s before the collision during which they stick together. what will be the velocity of the combined cars after collision

Answer 1

Given:
Mass of car 1 (m1) = 1000kg 
Mass of car 2 (m2) = 1000kg
Initial velocity of car 1 (u1) = 5m/s
Initial velocity of car 2 (u2) = - 5m/s (-ve sign for opposite direction)
After collision, final velocity (v) = 
m1u1 + m2u2 =m1v1 + m2v2   
1000 x 5 + 1000 x -5 =  v(1000 + 1000) ----------     (v1=v2 =v)
5000  - 5000 = 2000v
v = 0
So the velocity of the combined cars after collision is zero

Answer 2

given

m1=1000kg

m2=1000kg

u1=5m/s

u2=5m/s

combined velocity ( v)=?

according to law of conservation of momentum ,

m1u1 - m2u2 = (m1 + m2) v

(1000×5)-(1000×5)=(1000+1000)v

5000-5000=2000v

0=2000v

v=2000/0

v=0