# Feedback Comments on Gravitation

by Ron Kurtus

A total of 64 comments and questions have been sent in on Gravitation. They are listed according to date.

You can read them to further your understanding of the subject.

## List of next 10 letters

### Country

Law of Universal Gravitation Why is distance squared in gravitation law? Pakistan

Gravitational Escape Velocity for a Black Hole When two black holes merge USA

Gravitational Escape Velocity for a Black Hole What is the real escape velocity? India

Force Between Two Objects Whty is force of attraction is less on Earth? India

Force Between Two Objects May have an error in calculation USA

Force Between Two Objects Movement of static objects in Relativity Gravitation India

Gravitational Escape Velocity Derivation Why is velocity at infinity zero? India

Similarity Between Gravitation and Electrostatic F Why aren't we aware of Coulomb forces? Nigeria

Force Between Two Objects How does gravitation cause tides? India

Gravitation and Center of Mass What defines the center of gravity? Ghana

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First 10 letters

## Why is distance squared in gravitation law?

Topic: Law of Universal Gravitation

### Question

September 19, 2013

why d in newton law of gravitation is squared

shajobe - Pakistan

24028

### Answer

The formula for the law of gravitation is F = GMm/R^2, where R is the distance between the two objects.

Just as waves spread out from a source, gravitation also spreads out and is diminished with distance. This is known as the inverse-square law.

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## When two black holes merge

Topic: Gravitational Escape Velocity for a Black Hole

### Question

September 7, 2013

If nothing can escape from a black hole after crossing the event horizon. If two black holes were to merge by by the gravitational waves. It would consist of two separate dence object each with an event horizon combining together into one dence object so the gravitational pull was overcome by the gravitational pull of the other thus condensing the smaller into the larger. So the gravitational pull of the larger black hole overcame the pull from the smaller black hole.

So our physics claims we know it cannot over one the gravitational pull but a larger black hole does over overcome the pull of a smaller one or does the individual black holes just merge into 2 separate holes with an event horizon inside an event horizon or the two holes struggling for matter and which side approached from would determine which one an object would be condenced in but we would never know because if the breakdown in physics and these laws change so all the matter from the first is now the matter of the second.

Wouldn't this create an escape velocity of the smaller one. Thus their is an escape velocity we just know it? Based on our understanding of physics

Mike - USA

24001

### Answer

The gravitational equations for the event horizon assume that there are no large objects nearby. If there was a black hole or even galaxy relatively close to the first black hole's event horizon, its gravitational field would distort that horizon inwardly, such that matter or radiation close to the original event horizon would be pulled toward the second object and be able to escape.

With two black holes near each other, the effect of gravitation on the other would cause both event horizons to distort and allow some matter or radiation to go from one black hole to the other until the black holes combined.

Also, within black holes, there may be a different set of physics laws beyond quantum field mechanics.

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## What is the real escape velocity?

Topic: Gravitational Escape Velocity for a Black Hole

### Question

August 5, 2013

So,what is the real escape velosity of black hole?

Subhajit - India

23907

### Answer

An object would have to travel at a velocity greater than the speed of light in order to escape a Black Hole. According to the General Theory of Relativity, this is impossible. However, if an object is beyond the Schwarzschild radius of R = GM/c^2, objects can escape at speeds less than that of light.

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## Whty is force of attraction is less on Earth?

Topic: Force Between Two Objects

### Question

May 27, 2013

why force of attraction is less between two bodies which are present on the earth?

sneha - India

23722

### Answer

The gravitational force between two objects is dependent on the masses of the objects. Two objects on Earth do not attract each other as much as the Moon attracts one of those objects, because the two have small mass compared to the mass of the Moon.

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## May have an error in calculation

Topic: Force Between Two Objects

### Question

April 21, 2013

Hi,
you have some error in girl attracting the moon.. Her weight in Newtons is incorrect by factor 10..

- USA

23631

### Answer

Thanks for pointing it out. I checked over the calculation, and they seem correct.

The factor of 10 comes into play because the girl's weight is 50 kg-force. Weight must be converted to kg-mass by dividing by 9.8.

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## Movement of static objects in Relativity Gravitation

Topic: Force Between Two Objects

### Question

March 22, 2013

i just wanted to know how to explain the gravitation between two static
objects using the theory of space curvature theory? for example the gravitation on a moving body can be explained as it is traveling in a curved space where the observed straight line is curved making it to move along a curved line.but what about a body at rest? why should it move towards other if space around each other is curved? give an explanation.

rajesh kumar - India

23556

### Answer

Two static object are actually moving at the same speed and direction in space, such that each seems stationary to the other.

Since matter curves space, the path of object A moves on a curved path toward object B. But also, object B is moving on a curved path toward object A, caused by object A. Thus, they move in unison toward each other.

See General Relativity Theory of Gravitation for a basic description of gravitation. I plan to add this specific problem to the lessons, since it has puzzle me, too.

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## Why is velocity at infinity zero?

Topic: Gravitational Escape Velocity Derivation

### Question

February 25, 2013

i cant understand why the velocity at infinity equals zero?

adarsh - India

23488

### Answer

Escape velocity is the initial velocity of an object that will allow it to escape the gravitational field of a large mass.

Gravitational fields hypothetically extend to infinity. As an object moves away from the source of the gravitational field, it slows down. In theory, at an infinite distance from the source, the object's energy and thus its velocity become zero.

But this is in theory only and assumes that there are no other objects in space. Since an object would be affected by other masses, it can escape and not need to reach zero velocity. It could be accelerated by other masses.

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## Why aren't we aware of Coulomb forces?

Topic: Similarity Between Gravitation and Electrostatic F

### Question

February 13, 2013

Coulomb's forces in nature are enomous when compared with the gravity force. Why are we not all that conscious of these enomous coulomb's forces in every day live?

Simon - Nigeria

23199

### Answer

Coulomb's forces come from tiny subatomic particles, so the actual force is small. Also, since there are both positive and negative electrical charges involved, much of the Coulomb forces are neutralized.

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## How does gravitation cause tides?

Topic: Force Between Two Objects

### Question

November 3, 2012

Sir,then how the gravitational force b/w earth and moon results earth quakes cyclones,and tides....etc is it really effects?????????

k.lachiram - India

23021

### Answer

The gravitational force from the Moon, as well as the Sun, pulls the ocean water to it, resulting in the tides. The pull is not noticeable on a small amount of water, but it is on the large mass of the oceans.

The pull has a very small effect on the creation of cyclones. Most of the effect comes from the heating of the ocean by the Sun and the circulation of the water due to the spin of the Earth.

The inside of the Earth is molten materials, causing expansion and contraction of the Earth's surface. The effect of the Earth's rotation, as well as the gravitational pull of the Moon, can cause the ground to shift, causing an earthquake.

See Gravitation Causes Tides on Earth for more information.

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## What defines the center of gravity?

Topic: Gravitation and Center of Mass

### Question

October 28, 2012

What defines the center of gravity of a rigid body, and how is it related to the center of mass?

Joshua - Ghana

23010

### Answer

There is a difference between center of mass and center of gravity. Center of mass is the mass distribution throughout the object, while center of gravity requires that the object is under the influence of gravity. Often--but not always--they are in the same location.

Take a look at our Center of Gravity lesson to see how to find the center of gravity.

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## Summary

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