"Sam Wormley" wrote in message
news:u3wxe.111934$_o.106703@attbi_s71...
cool_cat wrote:
Something is bugging me... Consider a situation in space... an astronaut
mass m, and a rock m (no other objects anywhere). The astronaut exerts a
force F on the rock (let's say along the x axis) for some time t. Let's
say the mass acquires a velocity v, so the astronaut acquires
velocity -v.


Their momenta* will be equal and opposite, but not their velocities.

*with respect to the original reference frame where they were both
at rest.

In my example I'm using equal masses... hence equal and opposite momenta =
the velocities are equal and opposite.