Final answer:
The net gravitational force on an astronaut exactly halfway between Earth and the Moon is not zero, as the point where Earth's and Moon's gravitational forces cancel out is not the halfway mark but at the Lagrange point L1, closer to the Moon. The correct answer is d. Equal and opposite, in line with Newton's third law, though the forces will not result in zero net force on the astronaut.
Step-by-step explanation:
If an astronaut were exactly halfway between Earth and the Moon, the net gravitational force exerted on the astronaut by these two objects would not necessarily be zero. The reason for this is that the point at which the gravitational forces of the Earth and Moon cancel each other out is not exactly at the halfway mark but at the Lagrange point L1, which is closer to the Moon. This is due to the Moon's smaller mass compared to Earth. Gravitational force is a function of both mass and distance, so the Lagrange point is located where the forces due to gravity from the Earth and the Moon are equal in magnitude but opposite in direction.
To calculate the exact location, one would set the gravitational forces exerted by Earth and Moon equal to each other and solve for the distance. For an astronaut exactly halfway between, the gravitational pull from both bodies would be significant but not equal, leading to a non-zero net force. The correct choice would therefore be d. Equal and opposite, as the forces are always equal in magnitude and opposite in direction due to Newton's third law, though their effect on the astronaut will not cancel out at the halfway mark.