COME ON (1), LOOK BACK

come on! =  do it! (we encourage somebody to do something)

examples:

• Come on! Call him! I know he will be very happy if you do.
• Come on! Help him! He is your brother.
• Come on! Stop crying! It wasn 't that terrible!
• Come on! Hurry up!
• Come on! Tell us the truth!
• You can do it now! Come on!
• Quickly! Come on! The train is about to leave.

look back (on something) = remember, bring to mind an event of the past
examples:

• When I look back on those days, I can 't help laughing! We had so much fun!
• Don 't look back on the past. It will only make you sad. 
• Looking back, I realise I was wrong.
• Looking back now I cannot believe I made such big mistakes.
• Looking back on it now, maybe I was wrong after all.
• I look back on my childhood with joy.
• When I look back, I 'm surprised myself. How could I do such a thing? 
• I look back on those years with nostalgia.