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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 |

Gravity |
Did Einstein discover gravity? | UK |

Gravity |
I believe gravity is caused by centrifugal force | USA |

Gravity |
Mistake in numbers given | USA |

Gravity |
Velcoity-Time equations needed | Sri Lanka |

Gravity |
Object is projected horizontally | USA |

Gravity |
Why is weight in kg and force in newtons? | Serbia |

Gravity |
Creating a feeling of weightlessness | USA |

Gravity |
What is center of gravity? | Ghana |

Gravity |
Dropping a 10 stone item 10 meters | UK |

Gravity |
Shooting gun and dropping hammer | USA |

## Did Einstein discover gravity?

Topic: **Gravity**

### Question

January 1, 2012

Albert Einstein discovered the effects that gravity had on the:

planets, stars and time its self. But if I'm right,he still

realised there was more to be learned about Gravity.

My question is, is this still true today?

I thank you in anticipation for you reply.

Wilfred - **UK**

22300

### Answer

Actually, Galileo did much of the early studies on gravity in the late 1500s. Then in the 1600s, British scientist Isaac Newton formulated the effects of gravity and gravitation on the planets and stars. Around 1900, Albert Einstein updated many of the laws or gravitation with his Theory of Relativity.

See Gravity and Gravitation for more information on the subject.

## I believe gravity is caused by centrifugal force

Topic: **Gravity**

### Question

December 11, 2011

The reason why gravity is not better understood is that Gravity is not a FORCE, but rather the EFFECT of the centrifugal force of Earth’s rotation pushing all the dark matter contained in the voids of space above our atmosphere being pushed away from the planet creating a space between the earth‘s crust and the outlying dark matter of outer space. If we had been born before this void was created, then we would have been flung beyond the atmosphere with the rest of the unattached matter. Thus the density of the earth’s crust keeps our atoms from being drawn to planet’s core. If we try to walk on water, we will be drawn further towards the earth’s core depending on our buoyancy. It is the presence of this SPACE devoid of dark matter that allows the earth’s core to impose the magnetic pull on the atoms of all other matter . It is the layer of dark matter that creates the envelope around our less denser atmospheric gasses. The matter that was created AFTER the centrifugal force of the planet’s rotation was established will continue to be drawn to the earth’s magnetic core as long as it’s rotation is strong enough to maintain the space between the plant’s crust and the dark matter that is waiting to be drawn to the earth’s core by it’s magnetic field. Why science insists on classifying Gravity as a FORCE I don’t understand. There seems to be enough evidence that it is NOT a FORCE, but the results of an effect of this type. If you want to indulge in an ANTI - GRAVITY environment, you’re going to have find the absolute North pole of the earth’s core and put together a device that will create enough of the same polar energy to push it away from the earth’s magnetic field. THAT is an ANTI-GRAVITY mechanism.

Dominick - **USA**

22265

### Answer

Gravity is gravitation near the Earth. You have to look into space and the attraction among planets, stars and galaxies in the study of gravitational forces.

Einstein states that gravitation is not a force between masses but instead is an effect of the curvature of space near objects of mass.

Also see Effect of Dark Matter and Dark Energy on Gravitation for more information on that subject.

## Mistake in numbers given

Topic: **Gravity**

### Question

November 5, 2011

Dear Ron,

Sometime ago I asked you the following question about gravity.

How fast will an object be falling if dropped from an elevation of 29,000 feet by the time the object reaches earth?

You gave me the following answer:

The equation for the velocity of a falling object from a given height is v = SQRT(2xg) or velocity equals the square root of 2 times the distances times the acceleration of gravity. v = SQRT(2 * 1900 * 32) = SQRT(121600) = 348.7 ft/sec = 238 mph.

In understand most of this but in your (2 * 1900 * 32), what is the 1900 and how is this figure derived?

Thanks for your assistance.

Harry - **USA**

22143

### Answer

The 1900 looks like a careless typo.

The equation for the velocity after falling 29,000 feet should be: v = SQRT(2gy) = SQRT(2 * 32 * 29000)= SQRT(1856000) = 1362.4 ft/s = 928.9 mph.

However, air resistance would greatly reduce the speed. The terminal velocity of a falling round object is about 200 mph. Also, 928 mph is greater than the speed of sound, so even if the object was propelled, it probably would not reach that speed.

Also see Gravity Velocity Equations for Falling Objects.

Sorry about the mistake in numbers.

## Velcoity-Time equations needed

Topic: **Gravity**

### Question

October 20, 2011

Dear sir,

I'm from grade-10.And,i do want to ask some questions regarding acceleration due to gravity(velocity-time graphs)as follows,

Qst-01:

A pebble at rest was dropped and it took 6 seconds to reach the ground.Draw the velocity-time graph for this motion.Using the graph find the height that the pebble had fallen.

Qst-02:

Draw the velocity-time graph for the motion of an object projected vertically up with an initial velocity of 20ms-1 and returns back to the earth.

Hope you would give me the answers soon.

Fathima - **Sri Lanka**

22102

### Answer

1. Use the equation y = gt^2/2 from Gravity Displacement Equations for Falling Objects to get the answer: y = 9.8*6*6/2 = 176,4 m.

You can get the velocity-time equation in Gravity Time Equations for Falling Objects, which is v = t/g. However, using that equation seems to be a round-about way of finding the solution.

2. See Gravity Velocity Equations for Objects Projected Upward to get the equation.

## Object is projected horizontally

Topic: **Gravity**

### Question

June 1, 2011

Object is projected horizontally 'h' height above da ground at 100 meters per second after 10s. Find the horizontal range & 'h' if a=10 meters per square seconds

- **USA**

21660

### Answer

See Effect of Gravity on Sideways Motion for an explanation and equations.

Also see Effect of Gravity on an Artillery Projectile and set the angle to zero.

## Why is weight in kg and force in newtons?

Topic: **Gravity**

### Question

May 18, 2011

I am confused if W=mg and F=mg why is W in kilograms (Kg) and F in newtons (N), shouldn't they be in newtons and only m in kilograms

Despot - **Serbia**

21598

### Answer

Although a kilogram (kg) is supposed to be a unit of mass, it is sometimes carelessly considered as a unit of weight. To avoid confusion, sometimes they are separated by calling them kg-mass and kg-weight.

A kg-weight = g * kg-mass. Since the units of g are m/s^2, the units of kg-weight are (kg-mass)*m/s^2, which is the definition of a newton (kg-m/s^2).

## Creating a feeling of weightlessness

Topic: **Gravity**

### Question

April 26, 2011

Hi, I have a question. If you were flying in a jet around the equator at, lets say, 10000 feet above sea level; at what speed would the centrifugal force of flying in that BIG circle overcome the effects of gravity therefore giving a feeling of weightlessness? I understand there are mountains in the way, etc. I am considering no changes in speed or altitude. What equation would be used to calculate that?

Thanks!

Larry - **USA**

21516

### Answer

The equation to use is v = SQRT(gr), where v is the velocity in feet/sec, SQRT is the square root, g is 32 ft/s^2 and r is the radius of the Earth plus 10000 ft.

The radius of the earth is about 4000 miles, so the extra 10000 ft would only add 0.02%.

The resulting velocity would be about 26,000 ft/s or about 18,000 mph.

However, usually aircraft creating weightlessness fly in a big vertical arc that has a reasonable radius and allows for a lover speed.

They are usually called microgravity aircraft. NASA has a problem on such aircraft to train astronauts.

## What is center of gravity?

Topic: **Gravity**

### Question

February 18, 2011

WHAT IS A CENTRE OF GRAVITY?

GHANA - **Ghana**

21224

### Answer

See Center of Gravity.

## Dropping a 10 stone item 10 meters

Topic: **Gravity**

### Question

February 14, 2011

hello questios are good. i was wondering if u could tell me how heavy a 10 stone item would weighe after falling 10 meters when it hit the ground at the moment of impact with only gravity involved?

many thanks

dave

dave - **UK**

21199

### Answer

The item always weighs the same. You could find the momentum of the object after it fell 10 m: momentum equals mass times velocity. The velocity is found from the equation: v = (2gy)

However, you cannot determine the force the object creates on the ground, because it would be different depending on how hard the ground was

## Shooting gun and dropping hammer

Topic: **Gravity**

### Question

January 26, 2011

If you shot a gun that was horizontal and dropped a hammer from the same height all at the same time which would hit the ground first?

Thomas - **USA**

21088

### Answer

They would hit the ground at the same time. See "Effect of Gravity on Sideways Motion" at:

http://www.school-for-champions.com/science/gravity_sideways_motion.htm

## Summary

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