Understanding Astronaut Weightlessness on the ISS

The question asks us to explain why an astronaut, weighing 600 N on Earth, experiences weightlessness while on the International Space Station (ISS) orbiting the Earth. Weightlessness is a common observation for astronauts in orbit.

Analyzing Forces and Motion in Orbit

Let's examine the situation and the given options to understand the physics behind weightlessness:

• The Role of Gravity: The Earth exerts a gravitational force on the ISS and the astronaut. This force is what keeps the ISS in its orbit. Therefore, the gravitational pull of the Earth on the astronaut is not zero; it's crucial for maintaining the orbit. Option 3 is incorrect.
• Acceleration in Orbit: Objects in orbit are continuously falling towards the Earth due to gravity, but they are also moving sideways fast enough that they continuously miss the Earth. This means they are constantly accelerating towards the Earth. The ISS and the astronaut inside it accelerate together. Thus, the acceleration of the astronaut is not zero. Option 1 is incorrect.
• Defining Weight: On Earth, our 'weight' is the force of gravity pulling us down, but we also feel the ground pushing up on us. This upward push is called the normal reaction force. Our perceived weight is often related to this normal force. The astronaut's weight on Earth (600 N) is the force exerted by Earth's gravity on their mass.
• Weightlessness Explained: On the ISS, the astronaut and the space station are both accelerating downwards due to gravity at the same rate. They are essentially in a state of continuous freefall together. Because the astronaut is falling at the same rate as the space station, they don't press against the floor or any other surface inside the station.

The Significance of Normal Reaction

Weightlessness is the sensation experienced when the normal reaction force supporting your body becomes zero. In the ISS, since the astronaut is freely falling along with the station, the floor doesn't need to push up on the astronaut to support them. The normal reaction force from the floor on the astronaut is effectively zero. This lack of a supporting force leads to the feeling of weightlessness.

The astronaut's mass (m) can be found using their weight on Earth (\(W_{Earth} = 600 \text{ N}\)) and the acceleration due to gravity on Earth (\(g_{Earth}\)). The formula is \(W_{Earth} = m \times g_{Earth}\). While the mass remains constant, the apparent weight (related to the normal force) changes in orbit.

Option 4 is misleading. While orbital motion involves concepts related to circular motion, attributing weightlessness solely to a 'centrifugal force' applied by the station is not the standard explanation. The primary reason is the state of freefall and the resulting absence of a normal reaction force.

Therefore, the most accurate explanation for the astronaut experiencing weightlessness is that the normal reaction of the space-station floor on the astronaut is zero.