What happens if we made a tunnel through the earth and throw a ball in it?

Answer:-

as we all know that the center of earth is attracting us towards itself. if we make a tunnel of about 2x6380=12760 km through the earth. so it will cross the center of earth and meets at the other point on the earth's surface. 

let us create a scenario that we threw a ball inside of that tunnel, what will happen to it?

the answer is it will oscillate. because of the force of attraction towards the center of earth, it will move towards the center but due to inertia it will never stop at the center of earth, it will move forward but in that case the force and the displacement both are opposite to each other. therefore, the restoring force comes in action and  tends to oppose the motion of the body against the force of attraction. hence, it will stop at any point and move again towards the center of earth. In this way, it will oscillate until its energy dies out.