Final answer:
During a spacewalk, astronauts appear to be weightless because they are in a state of free fall, continuously falling towards Earth but missing it due to their horizontal velocity. However, the gravitational force acting on them is only slightly weaker than on Earth. They experience weightlessness because they are counteracting gravity with the force of their fall.
Step-by-step explanation:
During a spacewalk, astronauts experience a gravitational acceleration that is very similar to the acceleration experienced on Earth. However, they appear to be weightless because they are in a state of free fall. Just like passengers in a falling elevator or airplane, astronauts are continuously falling towards the Earth, but their horizontal velocity allows them to continuously miss the Earth and remain in orbit.
Since astronauts are in a state of free fall, they do not feel the gravitational force acting on them. This is why they appear to be weightless and why objects float inside the spacecraft. Gravity is still present in space, but it is counteracted by the force of the astronauts' fall. This is why they experience the sensation of weightlessness.
It's important to note that this weightlessness is relative to the force of gravity on Earth. In reality, the gravitational force acting on astronauts during a spacewalk is only slightly weaker than on the surface of the Earth. They still have about 88% of their weight while inside the International Space Station.