If 2001: A Space Odyssey followed real world technology, as it said it did, it seems to me that it would be impossible for Bowman and the EVA pod with Frank Poole in tow to make it back to the Discovery One. The EVA pod would have to be able to fly faster in space then the Discovery One.
As you can see, the Discovery isn't using its main engines at the point that Bowman exits the ship
That being the case, the two objects in question; Bowman and the Discovery are traveling at the same relative velocity, presumably tens of thousands of miles per hour.
To put it another way, when Bowman leaves he uses the pod's thruster to increase his speed from 50,000(ish) mph to 50,005 mph. When he returns to the Discovery, he uses the pod's thrusters to slow him down to 49,995 mph, allowing the Discovery, traveling at a constant 50,000 mph to slowly catch up with him.