A 3 kg slab of putty moving at 8 m/s strikes, and sticks to, a 5 kg slab of stationary putty. What is the final speed of the combined mass?

(3kg)(8m/s)+ (5kg)(0m/s)

24kg*m/s
=
(3kg+5kg)vf
(8kg)vf

24kg*m/s=(8kg)vf
vf=3m/s