# Is there any frame of reference in which we can neglect the gravitational force?

There is a laboratory in which there is a string tied to a bob on the earth.Consider the earth is perfect sphere ,rotates on its own axis in a circular path with uniform angular velocity. Main Q of this concept is to find apparent weight of the object but I have some more Q related to this.

So , I tried to draw every possible frame of reference I can. Pls check if it is correct or not.

In earth frame of reference , I excluded the centripetal force and added a pseudo force. My confusion in this part is that if as an NIF , I look at the box from earth. Does it mean I am inside the earth on the ground or can I say I am a person running on the earth surface (I.e as if I am 1million feet in height )who has same acc as that of the box on the ground of earth or are both the situations same in meaning ?.

My Q 1:is that is it possible if we can take a non inertial frame due to which when I consider the FBD of the box , the mg or N force is not present.

It is more like I also wish to know when do cos die the mg , N forces. I am not getting when to exclude them in NIF.

• The gravitational force varies with distance to the earth's core, and always pointing to the core. You need to edit your question with more details or examples so people can understand what is the reference frame about.
– r13
Commented May 19, 2021 at 16:08

The classic example of a frame of reference in which gravitational forces disappear is called a freely-falling reference frame.

This is what you get when in deep space, or in orbit around the earth, or when falling off a tall building or down a mine shaft on earth.

In a freely-falling reference frame, pendulums do not swing because all the (mg) terms vanish.

• Ok . Right examples. Thanks. What if I take NON inertial reference as inside the box . What about the FBD I made sir? Is it correct Commented May 19, 2021 at 18:08
• Suggest you redraw the FBD with forces pointing in their actual directions, and the surface of the earth shown below the FBD. Commented May 20, 2021 at 4:49
• Ok. I’ll redraw FBD Commented May 20, 2021 at 6:04

In the so called center of mass frame the gravitational forces of the constituents sums to zero per definition. That's the reason why we describe two body problems as (1) motion of the center of mass, and (2) relative motion with respect to the cantor of mass.

it is called free float. And Einstein used it as an intuition leading to his theory of general relativity. In a small local spacetime no physical ebservation or experiment can tell the difference between weightlessness and free float.

the picture is from A Jurney into Gravity.. by John A Wheeler.

It is said that he considered this was one of his the happist thought.