ASK A SCIENTIST
Question: What makes the world go round?
Answer: Have you ever watched an ice skater do a spin? The skater starts out with his or her legs and arms apart and then brings them in closer to their body. As this happens, the skater spins faster and faster. To slow down, the skater pushes his or her legs and arms away from their body. Scientists have an explanation for this speeding up and slowing down during a spin. It is due to conservation of angular momentum.
Angular momentum is a big term that simply means if you spread the mass (the stuff of which the object is made) of a spinning object out while it is spinning, its spin will slow down. This allows the angular momentum value to stay the same. When the value stays the same, we say it is conserved. Pull the mass inward, and the spin will speed up.
The solar system has a certain amount of angular momentum, and so do each of the planets. Nature says the angular momentum amount must stay the same as long as nothing happens to change it. When the solar system was forming, every chunk of material was in motion. As the chunks came together to form the planets, so did the motions. When the Earth was finally formed it had a certain motion we call rotation. Unless the Earth shrunk or expanded the speed of the rotation should have remained constant. But it didn't remain constant.
The reason it hasn't stayed the same is because the Earth's moon formed (and the Earth also did shrink a little after it was formed). The Earth and the Moon affect each other and the Moon has caused the rotation speed of the Earth to slow down over time. Scientists estimate that back around the time that the Moon formed, the Earth was rotating about 400 times a year instead of the 365.25 times a year today.
If the Earth had formed out of material that had no motion, and the process of formation did not produce any motion, then the Earth would not have had a rotation when it was first formed. You should think about how things would be different on Earth if it did not rotate.
What could change the angular momentum of the solar system? If a wandering planet- sized object came into our solar system and starting orbiting the Sun. Or, if that planet-sized object collided with a few planets as it passed through our solar system. Either of these events would cause the value of the angular momentum to change.
If you ask your grandparents what makes the world go 'round, they may have a different answer.
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