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QUESTION: Why is it that if I tie an apple to a thin piece of string and hold it in the air, the entire gravitational pull from the earth will not be strong enough to pull the apple to the ground, but yet the gravitational pull from the moon is strong enough to move the entire ocean?

ANSWER: The ocean is a lot heavier than an apple.  The Pacific alone has 660 million cubic kilometers of water, each of which has a mass of over a billion metric tons.  Gravitational force depends on mass.

---------- FOLLOW-UP ----------

QUESTION: Right, but the earth is a lot bigger than the moon. Do both objects have to be massive for gravity to have a strong effect? I was thinking "Earth big, apple small. Earth pull apple hard." v.s. "Moon small, ocean big. Why small moon pull big ocean?"

The moon is not small, and gravitation is a mutual force that depends on the mass of both objects.  The formula is F=G*m1*m2/r^2, where m1 and m2 are the masses of each object. The apple is still ridiculously small compared to the moon and the oceans.


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Dr. Stephen O. Nelson


I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.


I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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