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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 next 10 letters

Topic

Title

Country

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
 
Horizontal Motion Unaffected by Gravity re: Do objects hit the ground at the same time? Netherlands
 
Horizontal Motion Unaffected by Gravity Do objects hit the ground at the same time? Netherlands
 
Displacement Equations for Falling Objects Moving objects India
 

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First 10 letters





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.

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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.

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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.

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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

Answer

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.

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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

Answer

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.

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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?
I enjoyed reading your web page and you seem approachable. I thought I would ask.
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.
Thank you for your time.

Allen - USA

25945

Answer

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.

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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

Answer

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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re: Do objects hit the ground at the same time?

Topic: Horizontal Motion Unaffected by Gravity

Question

April 11, 2015

Appreciate for your response but i would like you to refer from this discussiom "http://physics.stackexchange.com/questions/10773/how-does-gravity-affect-bullets". Referring to a long range gun, accordingly if im not mistaken, it is possible that an object may orbit the earth with the right velocity and force applied to an object. So it is possible that they will not hit the ground....

Priori - Netherlands

25910

Answer

For relatively short horizontal distances, the objects will hit the ground at the same time. But if the bullet travels far enough, the curvature of the Earth becomes a factor. If the projectile velocity is great enough, it can orbit the Earth or even go into space. See: Newton's Cannon for an example of this with illustrations.

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Do objects hit the ground at the same time?

Topic: Horizontal Motion Unaffected by Gravity

Question

April 3, 2015

Sir, two object identical same mass in same altitude and position were release at same instance but other one horizontally applying certain force or greater, will it touch the ground at same time? Please answer. Aprreciate and thanks for effort in advance

- Netherlands

25884

Answer

Both objects will hit the ground at the same time. You can verify this by putting a coin near the edge of a table and holding another coin at the same height. Drop the one coin while flicking the coin on the table outward. Surprisingly, both coins will hit the ground at the same time.

They fall at the same rate, even though one is projected sideways,

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Moving objects

Topic: Displacement Equations for Falling Objects

Question

March 11, 2015

1. An object is projected vertically upward with a certain velocity. After sometime it reaches to a height of 100 m and come back again to the point of projection. The total time taken in this journey is 8 sec. Calculate the distance covered and displacement of the body during this time.
2. An object moves from a certain position with uniform velocity of 20 m/sec. After 2 sec. another body moves from the same position. Calculate the velocity of the second body with which it has to move to get at the first body in 4 sec.

- India

25826

Answer

1. The object travels upward for 100 m and back downward for a 100 m. So the total distance covers is 200 m. Displacement is the change in position. Since the object ends up at the starting point, its displacement is 0.

2. After 2 sec, the first object has traveled 40 m. At a total of 6 sec (2 + 4) it travels 120 m. That means the second object must travel 120 m in 4 sec or 30 m/s.

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