When does initial and final speed come into play when calculating momentum?

[daygobaker]

im having trouble find out when exactly is the intial speed and final speed when dealing with momentum...for example: i bounce ball...im using the conservation momentum equation and understanding what the initial and final is making me confused...

 

[Doc Al]

Please provide a specific example so you can point out exactly where the trouble is. (It's not clear to me why you'd apply conservation of momentum to a bounced ball.)