What happens if an astronaut hits a large boulder with a hammer in space?

Answer 1

That depends on where He/She is standing:

On the moon:

Not much different that earth.

In a space station:

Umm, why is there a boulder in the space station again?

Floating in space:

He/she would fly backwards and the boulder would move away very slowly.

Friction and gravity would keep the person in place on the Earth, moon or space station. However, when they are floating there is no external force. At that point the law of consevation of momentum is in full effect. ( p = m v )

Since the astronaut has a much smaller mass he/she moves more. A similar action-reaction can be seen when firing a standard gun. The bullet has a small mass compared to the person. The bullet moves very fast in one direction while the person only moves a little in the opposite direction.