Momentum is measured by continually taking price differences for a fixed time period. To create a 10 day period momentum line you would subtract the closing price from 10 days ago from the last closing price. This result is then plotted around a zero line.
What are good momentum stocks?
Ticker Price 10 days ago Momentum
------ ----------------- --------
BLL 90.59 107.19
TTWO 152.58 107.05
HAL 28.74 106.96
AXP 168.40 106.78