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# Feedback Comments on Gravity

by Ron Kurtus

A total of **75** comments and questions have been sent in on Gravity. They are listed according to date.

## List of most recent 10 letters

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

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 |

Displacement Equations for Falling Objects |
Acceleration does not seem right | USA |

Equivalence Principle of Gravity |
Why aren't tides formed in lakes? | Nepal |

Equivalence Principle of Gravity |
Is the weight at low altitudes less than at high? | Nepal |

## 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.

## Acceleration does not seem right

Topic: **Displacement Equations for Falling Objects**

### Question

November 11, 2015

I understand y = gt^/2... and g is the same as (a) for acceleration in F = Ma... thus we can have y = at^/2. And with metric (a) or (g) is 9.8m/s^. ...What if... gravity acceleration were 1 m/s^... and a body of 1kg is dropped from zero to one second elapsed time, and it is displaced 0.5 meter... I assume it reached a velocity of 1 m/s in the one second... then this, describes the newton (N)...as one newton. I don't see how a body of 1kg body mass can be accelerated to an end of acceleration increment (from zero) to 1m/s velocity and be displaced one meter! A v = 1.414, and 1.414 s^,for 2 N... seems like what it would take to get 1kg displacement? (I have looked up and at what they call the newton meter of Pa for a cube of pressure for the continued force and work of 1 joule... but something still seems out of wack? Can you explain it better.

Ron Davis

Ronald A. - **USA**

26523

### Answer

Let's look at simple acceleration and not concerning gravity. Acceleration is the change in velocity over an increment in time. Thus, if the change in velocity is from 0 m/s to 1 m/s in one second, the acceleration is 1 m/s^2.

The distance an accelerating object travels is the average velocity times the time. In this case, the average velocity is 0.5 m/s and thus the distance traveled in one second is 0.5 m.

The force required to accelerate 1 kg, 1 m/s^2 is one newton. F = ma.

I know it seems confusing that the distance is only 0.5 m, but the average velocity is only 0.5 m/s.

## Why aren't tides formed in lakes?

Topic: **Equivalence Principle of Gravity**

### Question

June 20, 2015

tides are formed in seas not in lakes.why?

jyoti - **Nepal**

26101

### Answer

Tides are formed in lakes, but unless it is a very large lake the height of the tide is difficult to measure.

## Is the weight at low altitudes less than at high?

Topic: **Equivalence Principle of Gravity**

### Question

June 20, 2015

weight at low altitude is less than that of higher altitude?

jyoti - **Nepal**

26099

### Answer

Note that the Equivalence Principle of Gravity refers to heights relatively close to the Earth. At higher altitudes, the effect of gravity is less and the acceleration due to gravity is slightly smaller. This means that an object would weight less at higher altitudes. At very high altitudes, gravity can be so small that an object floats.

## Summary

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