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# Feedback Comments on Gravitation

by Ron Kurtus

A total of **59** 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 most recent 10 letters

## Topic |
## Title |
## Country |

Force Between Two Objects |
What do the signs mean | Australia |

Force Between Two Objects |
Gravitational force | USA |

Force Between Two Objects |
Correction on exponent | UK |

Gravitational Escape Velocity Derivation |
Escape velocity equation seems wrong | Nigeria |

Force Between Two Objects |
Gravitation miscalculation | USA |

Tides |
Tidal acceleration equation | USA |

Force Between Two Objects |
Graviational pull on a girl | USA |

Force Between Two Objects |
Should the force be inifinte is the separation is zero? | USA |

Gravitational Escape Velocity for a Black Hole |
What is the equation for a Black Hole? | USA |

Force Between Two Objects |
Force of the earth on a girl | Australia |

## What do the signs mean

Topic: **Force Between Two Objects**

### Question

February 19, 2017

Hey I'm Aric and I'm trying to work out the gravitational attraction between the Sun and the Earth for homework

I just wanna know what the times, divide, plus and minus mean

Like as in;

* = Times?

/ = Divide/Fraction

- = Minus

and so on

Thanks

Aric - **Australia**

27888

### Answer

The equation F = GMm/R^2 means G times M times m divided by R squared or R times R.

To answer your question:

a*b means a times b or multiply a times b

a/b is a divided by b

a - b is a minus b or subtract b from a

The big thing is to find the mass of the Sun and of the Earth, as well as the distance between them.

I hope that helps. Best wishes in getting the problem solved.

## Gravitational force

Topic: **Force Between Two Objects**

### Question

January 28, 2017

1. Two objects attract each other with a force of 1×10^-8 N when separated by 20 cm. If the total mass of the objects is 5 kg what is the mass of each?

2. A spacecraft is on a journey to the moon. At what point between these two bodies does the gravitational force exerted on the spacecraft by the earth exactly balance gravitational force divided by the moon? At this point what is the net gravitational force on the spacecraft? assuming no other gravitational forces act.

I'd really love some help figuring these out!! Thank you!

Emily - **USA**

27831

### Answer

In the gravitational force equation, let "m" equal the mass of one object and "5 - m" be the mass of the other object. Thus, the equation is:

F = G(m)(m-5)/R^2

Thus: F(R^2)/G = m^2 - 5m

Use the quadratic formula to solve for m.

Note that R must be in meters.

See Law of Universal Gravitation for more information.

The second problem is solved by using the forces from the Earth and from the Moon on the spacecraft: GmM/r^2 = GmE/R^2

where m is the mass of the spacecraft, M is the mass of the Moon, r is the distance from the Moon to the spacecraft, E is the mass of the Earth, R is the distance from the Earth to the spacecraft. Also D = r + R the distance between the Earth and the Moon. That involves a lot of Algebra.

The net force is zero, since the gravitational forces cancel out.

I hope that helps.

## Correction on exponent

Topic: **Force Between Two Objects**

### Question

February 25, 2016

Hi, Great site, but your page with the calc of the gravitational force between two people is wrong at the end when you convert to standard scientific notation. It should be 0.9x10-6, not -16

- **UK**

26809

### Answer

Thanks for pointing that out to me. I made a goof on handling the exponents. I corrected the Gravitational Force Between Two Objects page.

## Escape velocity equation seems wrong

Topic: **Gravitational Escape Velocity Derivation**

### Question

January 18, 2016

you wrote that ve = ??(2GM/Ri) but in my textbook I saw that ve = ?2gR

valentine - **Nigeria**

26708

### Answer

The equation ve = SQRT(2GM/Ri) is the escape velocity from the gravitation of any very large object, such as the Sun.

However, you can simplify the equation for the escape from the Earth as v = SQRT(2gR), where g is the acceleration due to gravity and R is the radius of the Earth. See Escape Velocity from Gravity - Common Escape Velocities paragraph.

I hope that clarifies things.

## Gravitation miscalculation

Topic: **Force Between Two Objects**

### Question

January 4, 2016

The boy is 165lbs has a weight of about 740 Newtons and a mass of 70-kg not 7-kg.

Steve - **USA**

26661

### Answer

Thanks for noticing the miscalculation..

There is so much confusing between kg-mass and kg-force, that I put everything in terms of mass in the Gravitational Force Between Two Objects calculations.

## Tidal acceleration equation

Topic: **Tides**

### Question

November 5, 2015

Ron,

I bought a copy of your Gravity and Gravitation book. Please answer this question:

In NASA Science News, Frank Reed said the following:

I calculated the tidal influence of each planet at the Earth over the course of several centuries using standard almanac algorithms (only a few decades are presented here). The tidal acceleration is proportional to the mass of the planet, inversely proportional to the distance cubed, and also depends on direction.

Ron, my question is, "Why is the tidal acceleration inversely proportional to the distance cubed, instead of to the distance squared?"

Thank you very much!

Dick - **USA**

26500

### Answer

The whole subject of tides can get quite complex. The tidal acceleration is defined as the gradient or change in force per change in distance.

If you take the derivative of the gravitational force with respect to distance, you get: dF/dR = d(GMm/R^2) = 2GMm/R^3.

I hope that clears things up.

## Graviational pull on a girl

Topic: **Force Between Two Objects**

### Question

October 27, 2015

Um when You say like the gravitational pull with a girl and moon don't you mean a mermaid when u say that?

Summer - **USA**

26483

### Answer

The idea there is that the gravitational attraction from the Moon pulls on all objects and people on the Earth. That pull is also the reason for the tides.

## Should the force be inifinte is the separation is zero?

Topic: **Force Between Two Objects**

### Question

September 28, 2015

How is it that if the distance between the 2 objects is zero that you can pull the objects apart. Hypothetically speaking the force between the 2 objects should be infinite if the distance between them is zero.

Karim - **USA**

26412

### Answer

The distance between objects is measured from their centers. Thus, since they have some size, and if they are touching, there still would be a separation.

If the particles were points, and they were touching, there separation would be zero and the force would be infinite.

One thing to note is that when particles approach an extremely small size, Quantum Wave Mechanics takes hold, and the particles become considered as waveforms. Different rules then apply.

## What is the equation for a Black Hole?

Topic: **Gravitational Escape Velocity for a Black Hole**

### Question

July 15, 2015

i know light can't escape a black hole but can you give me an example of what the speed would have to be to escape a black hole say if our sun was compressed to the size necessary to create a black hole. I am not a physicist. I saw your equations but I don't know how to calculate the theoretical speed to escape a black hole. Thanks.

William - **USA**

26174

### Answer

The gravitational force of a Black Hole is so great that not even light or some particle traveling at or near the speed of light can escape. According to the theory of Relativity, nothing can exceed the speed of light.

The escape velocity equation is s = ?(2GM/R). If the value of s > c (s is greater than the speed of light) the object is a Black Hole and nothing can escape it.

## Force of the earth on a girl

Topic: **Force Between Two Objects**

### Question

May 23, 2015

The force of the earth on the girl is mg = 9.8 x 50 = 490N

David - **Australia**

26026

### Answer

Although you can calculate the gravitational force between a person and the Earth, it is easier to use the equation F = mg, which actually gives the weight of the person on Earth.

See: Mass, Weight and Gravity for more information.

## Summary

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