What would happen if someone dug a hole completely through the earth?
The acceleration due to gravity at the surface of earth is g and is pointed toward the center of earth. At a point diametrically opposite side, the acceleration is again g but is oppositely directed. That is to say, it is again pointed toward the center. However, the acceleration due to gravity decreases as the distance from the center decreases from the surface of earth. Thus the acceleration due to gravity is zero at the center. Any object whose acceleration varies directly with distance and pointed toward a fixed point is in simple harmonic motion. Hence any object which falls from the surface of earth will be moving toward the center of earth with increase in speed and attains a maximum speed at the center. By virtue of its speed it goes to the other side of earth. But acceleration is now against the direction of motion and hence its speed decreases and is zero when it reaches the opposite surface. Then it returns to the center with increasing speed and hence the process is repeate
