Dig a hole through the Earth. Jump in. How long until you reach the other side?
If you dug a hole through the Earth and jumped in, you would accelerate toward the center. At the center, you would be moving at about 8 kilometers per second. You would overshoot. You would slow down as you climbed to the other side. You would arrive at the surface on the other side, momentarily weightless, and then fall back in. You would oscillate forever.
The time to fall to the center is 21 minutes. The time to reach the other side is 42 minutes. That is it. 42 minutes. This is not a guess. It is a physics result. The time depends only on the density of the planet, not its radius. A hole through the Moon takes 53 minutes. A hole through Mars takes 38 minutes. A hole through Jupiter takes 24 minutes. Different planets. Different sizes. But the math always works out.
This assumes no air resistance, no friction, and a perfectly smooth hole. In reality, you would burn up. You would hit the sides. The Earth's core is molten. You would not survive. But the math says 42 minutes. The math is beautiful. The reality is not.
The strangest part: the time to fall through any uniform sphere is the same as the orbital period of a satellite skimming its surface. A satellite orbiting Earth at sea level takes 84 minutes. Half of that is 42 minutes. You falling through the Earth and a satellite orbiting the Earth are the same motion. Just different directions. Physics is weird like that.
In a perfect vacuum with no friction, yes. You would fall from one side, reach the other side, and fall back. Forever. Like a pendulum. Like a yo-yo. No one has ever tested this. The math says it works.
Because the time to fall through a uniform sphere depends only on its density. Earth's density gives 42 minutes. A denser planet takes less time. A less dense planet takes more time. But the radius cancels out. Bigger planets have more distance but more gravity. The two cancel.
The Earth rotates. As you fall, the walls of the hole would move sideways. You would hit the walls. This is a real problem. To avoid it, the hole would need to go from pole to pole. At the poles, there is no Coriolis effect. You could fall straight through.
At the center of the Earth, you would be moving at about 8 kilometers per second. That is 28,000 kilometers per hour. Faster than a bullet. Faster than the International Space Station. You would be screaming through the core at orbital speed.