# B Gravity can't be so strong

#### nichos

If gravity of the earth is so powerful to hold the moon in orbit, why does she not drag in & squash to smitherings all on its surface?

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#### PeroK

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If gravity of the earth is so powerful to hold the moon in orbit, why does she not drag in & squash to smitherings all on its surface?

The gravitational force (due to the Earth) is proportional to the mass of the other object. In particular, this means that the force on the Moon is proportional to the mass of the Moon; and the force on a person on the surface is proportional to the mass of the person.

When you hear that "gravity is constant", it means the acceleration due to gravity is constant. The force is, therefore, greater for larger objects in proportion to their mass.

#### ZapperZ

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If gravity of the earth is so powerful to hold the moon in orbit, why does she not drag in & squash to smitherings all on its surface?

There appears to be two factors here that you do not seem to understand. And this comes from not knowing the description of gravitational force.

$$F = \frac{GMm}{r^{2}}$$

First, this force depends on the two masses (M and m) involved, such as Earth-Moon, or Earth-person. The force on the moon due to the earth is different than the force on a person due to the earth, because the mass of a person is significantly less than the mass of the moon.

Secondly, it depends on the distance between the two entities (r). The further away the two entities, the weaker the gravitational force.

So to make the comparison between the two, you actually have to sit down and calculate the force of gravity for those two situations. You are not comparing apples to apples here because of those two variations.

This is a clear example where physics involves not just a qualitative, conceptual understanding, but also a quantitative aspect (i.e. numbers).

Zz.

#### unusually_wrong

Also, the moon travels away from earth at about 4cm per year.

#### nichos

Thanx both for making the effort.

Mass is the point. ..........nick

#### Drakkith

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One thing to note is that the acceleration of any object at a distance from the Earth of 384,000 km (semi-major axis of the Moon's orbit) is only 0.0028 m/s2. This includes the Moon. The acceleration at the surface of Earth is 9.81 m/s2. This fact is why a spacecraft in low earth orbit has to travel at roughly 7-8 km/s to stay in orbit while the Moon only travels at about 1 km/s. One way to think about it is that the slow speed gives gravity much more time to act on an object. If you're moving at 8 km/s out near the Moon, gravity simply can't change your direction of motion fast enough to prevent you from shooting off into interplanetary space. You move away from the Earth so fast that the force of gravity falls off too quickly to keep you in orbit.

So one could say that the strength of Earth's gravity isn't that powerful when you're 380,000 km away. In fact it's quite weak. It's just that the Moon is traveling relatively slow, giving this weak gravity plenty of time to act, keeping the Moon in its orbit.

#### nichos

Thanx,

How interesting, I suppose this is comparatively easy compared with the complexities of utilising gravity to atract & shoot off to another & another...... encounter to get you to where you want to go. Mind bogling to a mear mortal how they calculate all these things.

Similarly, is how much were doing when computers had less power that a small mobile.

Also when they extract info from objects thou. or billions of light years away & already dead just as long in earth time. I can't get over that what I see when I look up is not real but, only the sparks of long ago.

My N.Geogr. chart of the universe on the wall blows my mind away each time I look at it, & that's what only we thing there is. .........nick

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#### Eric Bretschneider

The moon isn't stationary. If it were then the earth's gravity would pull it down to a spectacular collision (which we wouldn't survive).

Roughly speaking the moon is moving sideways. The combination of the moon's motion and the earth's gravity means that the moon essentially falls forever in a circle around the earth.

#### nichos

Nature is full of large coincidences.

#### PeroK

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Nature is full of large coincidences.
It's not a coincidence, as such, that the Earth has a Moon. Some planets have no moons and some have many. There is a range of velocities that would lead an object to be held in orbit. The Sun, for example, has everything from close planets to distant planets, an asteroid belt, and comets that travel in very large orbits over hundreds or thousands of years.

#### nitsuj

Also when they extract info from objects thou. or billions of light years away & already dead just as long in earth time. I can't get over that what I see when I look up is not real but, only the sparks of long ago.
.........nick
Note that from a physics perspective it's real in every sense of the word. What you envision beyond those "sparks of long ago" is just imagination.

#### StandardsGuy

The moon isn't stationary. If it were then the earth's gravity would pull it down to a spectacular collision (which we wouldn't survive).

Roughly speaking the moon is moving sideways. The combination of the moon's motion and the earth's gravity means that the moon essentially falls forever in a circle around the earth.
I agree except it isn't a circle, it's an ellipse. The same mass that the gravity acts on also affects the moon's momentum. It wants to travel straight, but the earth's gravity makes it curve, so it falls around the earth.

#### CWatters

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Gravity is quite weak. A human can overcome the gravity of an entire planet.

#### phinds

Gold Member
Gravity is quite weak. A human can overcome the gravity of an entire planet.
Yeah but it gets harder and harder as the years go by.

#### rootone

Yeah, eventually quite a lot of humans sink below the surface of the planet.