A member of the crew of a SPACESHIP experiences weightlessness in a space when they

Analysis of the Question

The question asks for the physical condition that leads to the sensation of "weightlessness" for an astronaut or crew member in a spaceship.

Concept Explanation

Weightlessness does not mean that gravity has disappeared. Instead, it occurs when there is no normal reaction (the upward force exerted by a surface) acting on an object.

In physics, we define weight as the force with which a body presses against a surface. If you are standing on a scale, the scale measures the normal reaction. When you are in a state of "free fall," both you and the container (the spaceship) are accelerating towards the center of the earth at the same rate (acceleration due to gravity, $g$). Because you are falling at the same rate as the floor, you do not press against the floor, and the floor does not push back on you. Therefore, your apparent weight becomes zero.

Step-by-Step Solution

1. Evaluate Option B: Walking on a planet provides a surface that exerts a normal reaction force on your feet. You feel your weight clearly here.
2. Evaluate Option C: Being in between the sun and the earth does not automatically guarantee weightlessness; you would still be subject to gravitational pulls from both bodies.
3. Evaluate Option D: This statement is physically nonsensical in the context of gravitational mechanics.
4. Evaluate Option A: When an object is in orbit or traveling through space, it is essentially in a state of continuous free fall around the Earth. Because the spaceship and the crew are falling together at the same acceleration, the crew experiences no contact force from the walls or floor of the ship. This state of "free fall" is the definition of weightlessness.

Conclusion

The sensation of weightlessness experienced by astronauts is due to the fact that they are in a state of free fall within the gravitational field.

Exam Tip: Always remember that "weightlessness" is a state of apparent weightlessness. Gravity is still acting on the astronaut (which is why the ship stays in orbit), but because the astronaut and the ship are accelerating at the same rate, the astronaut feels no weight.