Keep the NDH as a placeholder and draw a line with the DH index finger from the knuckle of the NDH to where the DH FT was. Don't sign FAST because velocity is a vector and speed is a scalar, as we will learn further in the book. Make sure to use space to show what affects what. For an animated gif of this sign, see A basic guide to ASL volcano. Click on this link, pull up frame to see sign model, click on right-pointing arrow in lower lefthand corner of picture box, and view sign.
Image credits The solar wind.
Interiors by Margaret Dolinsky.