The momentum formula is M = P - Px where  P = Latest price and Px = closing price "x" events ago.

To construct a 10 day Rate of Change oscillator (ROC) the latest price is divided by the close 10 days ago, i.e 

ROC = 100 x (P/Px).

The lines obtained by either method look very similar and are interpreted in the same way.

The crossing of the 'zero' line (or the '1' line in the case of the ROC) is used by some technicians for generating trading signals but momentum signals have to be co-ordinated with the existing trend, for example the crossing of the "zero (1) line" should be taken as a sell signal only in a downtrend.