2. (15 points) A dust speck (m = 1 ng) travels horizon- tally at speed u until it hits an ice cube of mass M = 1g
that is sitting at rest on a horizontal frictionless surface.
Having absorbed the dust speck, the ice cube proceeds
to travel horizontally with speed v = 3 ms-1.
What was the initial speed, u, of the dust speck?

Hints: (1) 1 ng = 10-9g. (2) See if you can figure out
why Newton would have gotten this question wrong.
What is the relativistic momentum? Do you need it for
the ice cube? How about the speck of dust?
U