Question

what does it mean by "A particle of unit mass..."

Answer 1

it mean in the equation you are about to use substitute
m=1

Answer 2

maybe some more context would help

Answer 3

In physics you need to determine the mass so, you should you the International System.

Answer 4

See more here. http://hyperphysics.phy-astr.gsu.edu/hbase/mass.html

Answer 5

@UnkleRhaukus , I think you are right,

When I use m=1 it made sense,
I even worked out that m=1

thanks.