You didn't put a + charge on one of the Ag. It has to be on the left side
2H2O + MnO2 ==> MnO4^- + 3e + 4H^+
Ag^+ + e ==> Ag^+
Those are the two BALANCED half reactions. Multiply equation 1 by 1 and equation 2 by 3 and add them.
The car has greater momentum because it has more mass than a bike, even if they have the same speed. I hope this helped! :-)