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# Feedback Comments on Gravitation

by Ron Kurtus

A total of **64** comments and questions have been sent in on Gravitation. 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 |

Force Between Two Objects |
Objects being pulled together | USA |

Force Between Two Objects |
Relationship of force between two objects | Nepal |

Force Between Two Objects |
Why aren't we pulled toward a building? | India |

Gravitational Escape Velocity Derivation |
Velocity at infinity | India |

Force Between Two Objects |
Finding force of attraction | Nigeria |

Cavendish Experiment |
How was G calcualted | Nepal |

Gravitational Escape Velocity |
Throwing a ball upward | USA |

Gravitational Escape Velocity |
Escape velocity equation | Malaysia |

Force Between Two Objects |
Why doesn't the Moon collide with the Earth? | India |

Law of Universal Gravitation |
Why don't lakes have tides? | Nepal |

## Objects being pulled together

Topic: **Force Between Two Objects**

### Question

July 28, 2014

When body A and body B are being pulled together by F =Gmm/d^2,

If body A's mass is 100 times greater than body B's mass, what are their relative accelerations, when they being pulled together by gravity?

Is this like a lead-filled ping pong ball and a hollow ping pong ball being pulled together by one rubber band, which is wrapped around both ping pong balls. Obviously, the hollow ball will travel farther than the lead-filled ball. But how much farther? What is the formula for their relative accelerations?

Dick - **USA**

25182

### Answer

The force on each is determined by F = GMm/d^2. Thus F(1) = Ma(1) and F(2) = ma(2), where a(1) and a(2) are the accelerations.

However, the objects will move together toward their center of mass. See Center of Mass Location and Motion for more information.

## Relationship of force between two objects

Topic: **Force Between Two Objects**

### Question

May 16, 2014

what is the relationsip between the fore of gravitation and masses of two object and their central distance?

jitu rana - **Nepal**

24966

### Answer

The lesson Gravitational Force Between Two Objects gives the equation:

F = GMm/R^2

## Why aren't we pulled toward a building?

Topic: **Force Between Two Objects**

### Question

May 5, 2014

gravitational force is between a boy and building.Why we are not pull towards the building?

Prameet - **India**

24928

### Answer

You are pulled toward a building, but the force is so small that you do not notice it or it has no effect on you. The reason is that the mass of the building does not create enough gravitational force. Look at how large the Earth is and compare it with the force the Earth-s gravity has on your body.

## Velocity at infinity

Topic: **Gravitational Escape Velocity Derivation**

### Question

April 26, 2014

a body is projected from the surface of earth with a velocity v=2((gR)1/2) where g is acceleration due to gravity on earth surface and R is the radius of earth then what is the velocity of body at infinity ? plse sir give quick response to my question!!!!

PRIYANKA - **India**

24898

### Answer

At infinity, the kinetic energy and thus the velocity of the object = 0.

## Finding force of attraction

Topic: **Force Between Two Objects**

### Question

March 4, 2014

A body of mass 25gm was placed at a distance of 15cm from another body of mass 42gm, find the force of attraction

Kingsley - **Nigeria**

24571

### Answer

Apply the equation F = GMm/R^2 shown in Gravitational Force Between Two Objects, where G = 6.674*10^?11 N-m^2/kg^2, M = 25 gm, m = 42 gm and R = 15 cm.

You must convert gm to kg and cm to m before doing the calculations.

## How was G calcualted

Topic: **Cavendish Experiment**

### Question

February 5, 2014

How cavendish caculate the value of 'G' gravitational constant? Explain it details?

bikash - **Nepal**

24494

### Answer

The lesson Cavendish Experiment to Measure Gravitational Constant shows how masses turned a fine wire. The amount of torques was measured and used in the equations in the lesson to calculate G.

## Throwing a ball upward

Topic: **Gravitational Escape Velocity**

### Question

January 31, 2014

I have the strength to throw a basketball straight up into the air at 2meters per second and the ball returns and hits the ground lets say after 15 seconds , how high has the ball travelled ? thank you so much

K - **USA**

24474

### Answer

There is something wrong with your numbers. If it took 15 seconds for the ball to hit the ground, your initial velocity would have to be 284 m/s.

If you threw the ball upward at 2 m/s, the maximum height would be 0.2 m.

See Gravity Displacement Equations for Objects Projected Upward for the equations,

## Escape velocity equation

Topic: **Gravitational Escape Velocity**

### Question

January 19, 2014

This is the question that was given in my school.

'A quantity X is defined by the expression X = ?(2GM/R)

where G is a constant which has a units of Nm2kg-2

M represents the mass of a planet

R represents the radius of the planet.

What physical quantity does X represent? '

So what is X? Velocity or Speed ?

Thank You :)

Jia Jia - **Malaysia**

24446

### Answer

The escape velocity equation is v = -SQRT(2GM/R), where SQRT means square root.

If you would square both sides of the equation, you would get v^2 = 2GM/R.

Thus, in your problem, X is the escape velocity squared. Units would be m^2/s^2.

## Why doesn't the Moon collide with the Earth?

Topic: **Force Between Two Objects**

### Question

December 20, 2013

i have a dout sir ,that is why moon does not collied earth due to gravitational force and planets also why does collide sun due to gravity

jayanth - **India**

24361

### Answer

Although the gravitational force of the Earth attracts the Moon, the velocity of the Moon creates a centrifugal force that pulls the Moon away from the Earth. The gravitational force and centrifugal forces cancel out, allowing the Moon to rotate around the Earth.

However, if the Moon would slow down, then it could spiral into the Earth.

The same effect works with the Earth and planets rotating around the Sun. Their velocities form an equilibrium with the gravitation.

See Centripetal Force and Centrifugal Force for more information.

## Why don't lakes have tides?

Topic: **Law of Universal Gravitation**

### Question

October 9, 2013

Why does the tides occur in ocean but in lakes???

The mass of big body attract small body if so why doesn't satellite attract the astronaut towards it self

Asesh - **Nepal**

24086

### Answer

Actually, tides do occur in lakes. But since the amount of water and depth of the water in a lake is much less than in the ocean, the tides in lakes is much smaller and sometimes not noticeable.

See: Gravitation Causes Tides on Earth for more information on tides.

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