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An 800 kg car moving at 80 km hr1 collides with a...

An 800 kg car moving at 80 km hr1 collides with a 1200 kg car moving at 40 km hr1 in the same direction. If the cars stick together, calculate their common velocity.

A.

60 km hr1

B.

8 km hr1

C.

40 km hr1

D.

56 km hr1

Correct answer is B

From momentum conservation 

(800 x 80) - (1200 x 40) 

= (800 + 1200) V

= 64000 - 48000 = 2000V

16000 = 2000V

V = 160002000

= 8kmh1 

The common velocity = 8kmhr1