An astronaut on a space walk floats a little too far away from the space station. Without air to push against, he cannot paddle back. However, the astronaut is holding a hammer. Explain how the astronaut could use the hammer to move and why this will get him back to the space station.

Answer:

The astronaut should throw the hammer in the direction opposite to the space station. According to the law of conservation of momentum, the initial momentum of the system (astronaut and hammer) is zero. When the hammer is thrown away from the station, it gains momentum in that direction. To keep the total momentum at zero, the astronaut must gain an equal amount of momentum in the opposite direction, which will push him back toward the space station.