by Ron Kurtus

A total of 81 comments and questions have been sent in on Gravity. They are listed according to date.

## List of next 10 letters

### Country

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

Vectors in Gravity Equations Validating the acceleration due to gravity USA

Center of Gravity Making a Center of Gravity balance USA

Vectors in Gravity Equations Shooting an arrow at a target USA

Artificial Gravity How are gravity, velocity, and time related? India

Next 10

First 10 letters

## 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

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

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

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

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

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

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.

## Validating the acceleration due to gravity

Topic: Vectors in Gravity Equations

### Question

May 21, 2015

I am still struggling with gravity.
Has the 9.8 m/s ever been validated with real data?
When you leap out of an aircraft you go from nothing to something as Newton observed with the apple.
But after a very brief period of acceleration you slow the rate of acceleration as you get closer to the Earth.
Drag is real. How did scientists ever validate the 9.8 meter thing?
Bear with me. My last physics class was in 1959 and I only got a C.

Allen - USA

26019

The acceleration due to gravity on the Earth is set at about 9.8 m/s^2. This number is an average value, since the force varies in different locations. It has been measured by numerous experiments and compared with the theoretical value.

When an object is dropped, starting at zero velocity, it increases its speed at the rate of 9.8 m/s^2. However, air resistance can slow down the object as it falls. The resistance is a function of the speed of the object.

See Work Done by Gravity Against Inertia and Air Resistance for more information.

## Making a Center of Gravity balance

Topic: Center of Gravity

### Question

April 24, 2015

Hi

Thanks for your explanation of Center of Gravity. All the examples are very good. I particularly like your illustrations..

I want to be able to build and balance something like a U-shaped bar:

---------------
l
l
l
---------------

How do I go about it?

I've seen it done on this YouTube Video. Please advance to the 5:23 minute point and onwards in the video. You see what I have in mind.

Question:

Do you first find the Center of Gravity of the connecting piece (the middle point?) and then the Center of Gravity somewhere in space between the two projecting pieces?

I need to build this for my grandson for a school project. I plan to use wood initially and if it works, I will do it in metal just as shown in the Youtube video.

Many thanks for your guidance. Much appreciated.

Cheers!

Anil - USA

25947

The balanced u-shaped metal tubing piece in the YouTube video looked pretty cool. Note that he added some weight to the ends of the arms, so that the center of gravity (CoG) was not too close to the sideways beam. If the total weight at the end of the arms equaled the weight of the cross bar, then the CoG would be in about the middle of the U.

Here is my idea of how to do it:

First assemble the three parts of the U. Then tape a sheet of paper or thin cardboard over the U, in order to mark the CoG.

Attach a string to a point on the U and hang it from that string. The arms of the U will face somewhat downward. From that same point hand another piece of string with a small weight on its end. That string should pass through the CoG. You can mark a line on the cardboard around where the CoG would be.

Repeat that by hanging the U from another point and marking where the lines cross. That is the CoG of the U. You might have to do it a few times.

Then attach your wire hook to reach the CoG.

I hope this helps. Best wishes for a successful demonstration. Let me know how things turn out.

## Shooting an arrow at a target

Topic: Vectors in Gravity Equations

### Question

April 23, 2015

I do not understand gravity calculations in which only the horizontal component of an arrow’s flight is used.
Specifically, I am trying to understand how much I should adjust my bow sight when shooting an arrow at a target 40 yards from me if the target is down from where I am in a tree stand at an angle of 45 degrees.
The web literature suggests that when shooting relatively level horizontal if I have zeroed for 40 yards I can leave the sight pin there even if the target is up or down 15 degrees or less from my position relative to it.
Here is my problem: The sources I checked said the effect of gravity only acts during the horizontal travel component of the arrow’s flight. The arrow’s diagonal flight is 40 yards but its horizontal distance covered is less at 45 degrees.
If gravity effects are proportional only to the horizontal surface of the earth covered it does not make sense to me. I do not understand something here. What if I was shooting straight up?
The exact scenario is this: I am zeroed at 40 yards on my bow sight. If I am in a tree stand and am shooting down at an angle of 45 degrees to an arrow flight distance of 40 yards shouldn’t I adjust my aim higher for the arrow hitting higher than my point of aim? If I ignore wind resistance and gravity is accelerating the arrow’s flight it would be going faster than the force given to it initially by the bow. Faster means it will hit higher on the target.

Allen - USA

25945

Since the arrow is traveling at a constant horizontal speed, the only force on it to change its direction comes from gravity, which is toward the ground.

When trying to calculate the angle at which to shoot an arrow in order to hit a target at some distance, you really need to know the initial velocity of the arrow. See Effect of Gravity on an Artillery Projectile for some explanations, equations, and examples.

I hope that helps.

## How are gravity, velocity, and time related?

Topic: Artificial Gravity

### Question

April 18, 2015

how the gravity velocity time are related?
how the time on space and earth are different?

ATHULYA - India

25931

See Derivation of Velocity-Time Gravity Equations for information.

The measurement of time is dependent on the velocity of objects. It only becomes different at extremely high velocities--close to the speed of light. In general, large objects in space do not travel that fast. However, some small particles do travel close to the speed of light, and time to them is slowed down.

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