How would a newtons cradle act in zero gravity? I imagine that the first ball would strike the rest as normal, but what would the last ball do, without gravity to swing it back? 
 A: It would rotate around through most of a 360${}^{\circ}$ arc, until it collided with the second to last ball on its way back around.  Then the whole thing would end up mucked up.
A: My assumption is that it would react the exact same way as it would on Earth (in a vacuum) or any similar instance where the pull of gravity is relatively uniform (also in a vacuum). By uniform, I mean a gravitational pull whose net force is in the direction of an arrow perpendicular to the balls.
If it were suspended in space, the nearest/strongest gravitational pull to the last ball would probably be the other balls (unless the strings and poles on which the balls are suspended are much denser/massive), in which case it would end up all jumbled.
A: The Newtons Cradle would not work in space because it would have to be pulled down by gravity. Even if you threw the first ball, it would just float freely along the side of the cradle. However even if it succeeded in hitting the second ball, it would just slowly float off. The energy could not make its way through. If you wanted to move all the balls, you would have to push them manually.
