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# Feedback Comments on Gravity

by Ron Kurtus

A total of **78** comments and questions have been sent in on Gravity. 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 |

Artificial Gravity |
Problem with spinning spaceship | USA |

Center of Gravity |
Center of gravity when standing on a ball | UK |

Work Against Gravity |
Would the Gravity book be helpful? | USA |

Horizontal Motion Unaffected by Gravity |
Finding the horizontal velocity | USA |

Artificial Gravity |
Idea to use Lenz"s Law for artificial gravity | Peru |

Velocity Equations for Objects Projected Downward |
Is the number in the equation squared? | Kenya |

Velocity Equations for Objects Projected Upward |
Projection at a 45 degree angle | USA |

General |
Why doesn't book crash through table? | South Africa |

Center of Gravity |
What is a point mass? | South Africa |

Mass and Weight |
Speed on Earth and on the Moon | Pakistan |

## Problem with spinning spaceship

Topic: **Artificial Gravity**

### Question

October 11, 2017

I can see a big problem where a spaceship has a spinning part as spaceship tries to make a turn. The spinning part want to go straight unfortunately the spaceship part wants to turn. It may be too much for the air seals.

a p - **USA**

28280

### Answer

If a spaceship was a rotating wheel or had part of it rotating, and it made a turn to change directions, it would affect the artificial gravity inside the wheel. It could be possible for one side to increase the artificial gravity, while another part lose the gravity, such that people on that dies would start floating.

The best thing for a rotating spaceship would be to stay in a straight line or be in orbit around the Earth.

Air seals would not be a factor.

## Center of gravity when standing on a ball

Topic: **Center of Gravity**

### Question

September 20, 2017

If we are standing upright on the ball of one foot where would our centre of gravity be on our foot?

Athos - **UK**

28251

### Answer

The centre of gravity would be somewhere in the centre of the person. However, there would be a ling going from the COG to the point where the foot meets the ball.

## Would the Gravity book be helpful?

Topic: **Work Against Gravity**

### Question

September 1, 2017

Hi, I will be working on a physics project this fall semester and was wondering if your book would be helpful in assisting with the project and/or project ideas.

Thank you,

Ashley - **USA**

28207

### Answer

The advantage of the book is that everything is in one place, such that it is easier to navigate. However, all the material on Gravity and Gravitation is on the website. You can pick and choose what you want to see.

Best wishes for success in your Physics project.

## Finding the horizontal velocity

Topic: **Horizontal Motion Unaffected by Gravity**

### Question

January 11, 2016

Is there a way to find an objects horizontal velocity of a falling projectile using conservation of momentum, given the mass of the object, distance traveled and original height? If there is could you explain how to use such an equation?

Thank you

- **USA**

26682

### Answer

The equation for the horizontal velocity as a function of mass, height, and distance traveled is v = x/SQRT(2y/g) where

v is the velocity

x is the distance traveled

y is the height

g is the acceleration due to gravity

SQRT is the square root sign.

See Effect of Gravity on Sideways Motion for details.

## Idea to use Lenz"s Law for artificial gravity

Topic: **Artificial Gravity**

### Question

December 15, 2015

If An astronaut use a magnetic suit within a copper's chamber, He needs to do a job for displacement due lenz's law. Read more:

Artificial gravity by lenz's effect

jorge - **Peru**

26630

### Answer

According to Lenz's Law, a moving magnet in a copper chamber will experience a resistance to that movement, due to the electric fields produced. That means that an astronaut in a magnetic suit would experience resistance to his movement in a copper chamber.

Artificial gravity would be a force pulling him toward the floor, whether he moved or not. Thus, I don't think Lenz's Law would apply.

## Is the number in the equation squared?

Topic: **Velocity Equations for Objects Projected Downward**

### Question

December 15, 2015

from the equation v=±?[2gy+vi2],is it vi squared or vi multiplied by 2?

patrick - **Kenya**

26626

### Answer

When the number is behind the variable (vi) and is above it, that means the variable is squared. Another way of writing vi squared is vi^2.

## Projection at a 45 degree angle

Topic: **Velocity Equations for Objects Projected Upward**

### Question

November 17, 2015

What if you were throwing at a 45 degree angle ? What's equation would be.

Fuad - **USA**

26542

### Answer

See Effect of Gravity on an Artillery Projectile to see the equation at various angles.

## Why doesn't book crash through table?

Topic: **General**

### Question

November 16, 2015

why does a book sitting on the table not crashing through the table, and the table not crashing through the earth because of the earth's force of gravity, do two objects that are in contact exert a force of gravity on each other

sidanga - **South Africa**

26534

### Answer

A book has a certain weight and exerts that force on the table. However, the material of the table has enough strength that it can withstand the force or weight. However, if the table was made of paper, the book would crumple the table and fall to the floor.

The force of attraction between the book and the table is really too small to measure. It takes a large mass to create a measurable force with most instruments.

## What is a point mass?

Topic: **Center of Gravity**

### Question

November 16, 2015

The book is sitting on the table, and the table is sitting on group, is there a force of gravity between the book and the table, the earth and the book, plz also tell me what is meant by point mass

sidanga - **South Africa**

26533

### Answer

Gravitation equations for large (and even small) objects can be complex. A good approximation is to assume all the mass is concentrated at a point, called the center of mass. See Center of Mass Definitions for more information.

For smaller objects, like a book, you can refer to its Center of Gravity, as if the mass was concentrated at a point.

## Speed on Earth and on the Moon

Topic: **Mass and Weight**

### Question

November 13, 2015

if some known force is applied on some known mass.will it move with the same speed on earth as well as on moon?

Mahrooz - **Pakistan**

26525

### Answer

If the force is applied parallel to the surface of the Earth, as well as the Moon, the speed will be the same, since it is independent of the gravity force.

See Horizontal Motion Unaffected by Gravity for information.

## Summary

Hopefully, this reader feedback has helped provide information about Gravity issues.

Appreciate what you have

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