Invent and compare spaceships (draw them in the blank space above) that are driven according to the following (ideal) criteria. The actual source of power is e.g. a small fusion plant onboard the spaceship.
a) Suppose a spaceship is powered by a laser that emits 1000 Watts in a
beam 1 cm
in cross-sectional area. What is the recoil force (per
KW) exerted by the laser?
b) Suppose instead the spaceship is powered by throwing mass. If it
throws 1000 small beads per second, each with mass 
gram and with
a kinetic energy of 1 Joule per bead (so the power required to operate
it is still 1000 Watts), what is the average force (per KW) exerted by
the mass-driver?