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Feedback Comments on Gravity

by Ron Kurtus

A total of 79 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




Mass and Weight Confusion Clarifying confusion USA
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

Next 10


Clarifying confusion

Topic: Mass and Weight Confusion


February 28, 2018

I found this page in Google search and I would like to state that I believe you are incorrect in your assertion that pounds measure weight (or more accurately: force). You state confidently that "as a student of science, you need to be careful in what you call things." I do not believe you are being very careful when you state that pounds refers to weight. Pounds has always referred to mass (even if the unaware conflate that with weight, as you have done here).

Because weight is defined in relation to the gravitational constant, the side effect of this is that most of the time it works out and that is in fact why this mistake is so pervasive.

The correct terminology to use is pounds (or pounds-mass if you are being overly verbose) when referring to mass and pounds-force when referring to weight. This is codified by law and numerous standards. See NIST and the US Code for more information. The pound is defined as a direct ratio to kilograms. The transitive property applies here in an almost comically textbook way. Kilograms are a measure mass, pounds are defined in direct relation as a ratio to kilograms; thus pounds are a measurement of mass. There can be *no* other interpretation here.

Richard - USA



Thanks for the comments. I updated Confusion about Mass and Weight Units to clarify the material.

Although some standards refer to pounds as mass, I found the definition of pounds as weight in five college Physics textbooks.

Also, weight refers to the force of gravity and not the gravitational constant. Although the mass is the same, the weight of an object on the Moon is only 1/6 the weight on Earth.

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Problem with spinning spaceship

Topic: Artificial Gravity


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



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.

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Center of gravity when standing on a ball

Topic: Center of Gravity


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



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.

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Would the Gravity book be helpful?

Topic: Work Against Gravity


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



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.

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Finding the horizontal velocity

Topic: Horizontal Motion Unaffected by Gravity


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




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.

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Idea to use Lenz"s Law for artificial gravity

Topic: Artificial Gravity


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



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.

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Is the number in the equation squared?

Topic: Velocity Equations for Objects Projected Downward


December 15, 2015

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

patrick - Kenya



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.

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Projection at a 45 degree angle

Topic: Velocity Equations for Objects Projected Upward


November 17, 2015

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

Fuad - USA



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

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Why doesn't book crash through table?

Topic: General


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



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.

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What is a point mass?

Topic: Center of Gravity


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



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.

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