# Gravitational force

#### jaan

This question was asked by a Std 9 student, and has me foxed.
Since the earth is so much more massive than the moon, and its centre much closer to the surface, isn't the earth's pull on the water much stronger than the moon's? Then how is it that the moon can cause earth's water to rise to form tides, even overcoming the earth's much larger force of attraction, which should prevent the waters from rising?

Related General Physics News on Phys.org

#### Mk

The tides are caused because the moon pulls on the water, but the Earth pulls stronger, that's why the water stays on the Earth. I understand what you are confused about, but don't know how to unconfuse you!

#### Doc Al

Mentor
The lunar tides are caused by difference in the moon's gravitational pull on the earth at various points. (The force per unit mass is stronger nearest the moon, weakest away from the moon.) That variation in gravitational strength causes a stretching force called the tidal force; the effect of which is most readily seen in the oceans, which can flow.

Yes, the earth's pull on the water is great, so the tidal force doesn't just strip off the oceans into space. But the water does get pulled away from the earth a bit at the ends furthest and nearest to the moon, creating the tides.

#### Gir

jaan said:
This question was asked by a Std 9 student, and has me foxed.
Since the earth is so much more massive than the moon, and its centre much closer to the surface, isn't the earth's pull on the water much stronger than the moon's? Then how is it that the moon can cause earth's water to rise to form tides, even overcoming the earth's much larger force of attraction, which should prevent the waters from rising?
think of it like this, the moon doesnt actually cause the water levels to go up and down, it causes more of a sloshing effect with the ocean.

#### tony873004

Gold Member
The Earth's gravity does win. The water does not shoot up into space.

#### OnTheCuttingEdge2005

Here's one to chew on.

If the Earth and the Moon both have gravitional attraction then why is the Moon getting further away from the Earth every time it orbits, the appogee and paragee is expanding its diameter every orbit around the Earth.

Why isn't the Moon falling into Earth's Gravity well instead of leaving it?

#### mykingdomforapurpose

gravity causes the moon to roll into the earth. but the moon is also travling sideways at just a slightly higher pace than it falls. so it keeps missing the earth. and slowly but surly it will icrease the diffrence as the sideways distance extends.

#### Doc Al

Mentor
OnTheCuttingEdge2005 said:
If the Earth and the Moon both have gravitional attraction then why is the Moon getting further away from the Earth every time it orbits, the appogee and paragee is expanding its diameter every orbit around the Earth.
Due to the Earth's rotation, the tidal bulges are not directly aligned with the Moon-Earth axis. The Moon's gravity acting on those bulges creates a torque on the Earth, slowing it; the pull of those bulges on the Moon speeds the Moon's orbit, causing it to spiral outward.

"Gravitational force"

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving