Slug feet/sec is the unit of measure for an object's momentum.
MOMENTUM: The unit of measurement for momentum is slug-feet per second. A slug is a portion of the subset of coherent units known as the gravitational foot-pound-second system. The physical weight of one slug of mass equals 32.174 pounds. One slug of mass will acquire an acceleration of one foot per second per second when acted on by a one pound force (at sea level).
Here's the simplified formula for an arrow:
Momentum = Arrow Mass in grains x Arrow Velocity in feet per second, divided by 225218
In other words, momentum equals the arrow’s mass, measured in grains, multiplied by the arrow’s velocity, expressed in feet per second, and then divided by 225218. The resultant answer will be the momentum, expressed in slug-feet per second.
Just FYI, 225218 is the product of 7000 x 32.174. There are 7000 grains per pound (that converts the arrow's mass in grains to pounds) multiplied by the gravitational constant 32.174 feet per second per second.
Hope that helps,
Ed