We make a hole (white color) through the earth.
From the top of the hole we drop a particle (red color) with initial speed equal zero.
The stone will fall through the hole with increasing speed until the center of the earth.
Then the speed will decrease and the speed will be zero when the particle reach the other end of the hole.
Then it will start returning back to the original position.
The movement of the particle will be a simple harmonic motion.
In this simulation the default speed of the particle will be 400 times the real speed.
Mark the checkbox 'Sphere' to see the sphere enclosing the part of the earth that attrack the particle.
By Gauss's Law it can be shown that the resultant force acting on the particle comes from the part of the Earth
that is inside the sphere with center at the center of the earth and radius equal to the distance from the center of the earth to the particle.
Mark the checkbox 'Satellite' to see the satellite falling around the earth with the same period as the particle.