"In Cavendish experiment we do not know the following parameters

1) Air Pressure

2) Direction of air

3) Masses and the densities of M and m

4) Distance between M and m

5) Height of the rope/string

6) Nature of the string

7) Polarity of M and m

Material (iron,steel,plastic wood etc) of M and m

9) Is it shows the same results for different materials as the gravity is constant for all the objects?

10) Is this experiment gives the same results all the time in all conditions?"

In the real world, rather than one where gravity doesn't exist and where this whole thread makes sense, we do know most of those things.

Cavendish was a good experimenter and will have logged most of them like the masses and distances.

There are some like #10 which no single experiment can show, but we have been using spring balances (and the more modern equivalents) for a long time. We know by direct experiment that gravity hasn't changed much since Cavendish's day.

The gravitational constant appears in Newton's law of universal gravitation, but it was not measured until 1798 — 71 years after Newton's death — by Henry Cavendish (Philosophical Transactions 1798). Cavendish measured G implicitly, using a torsion balance invented by the geologist Rev. John Michell. He used a horizontal torsion beam with lead balls whose inertia (in relation to the torsion constant) he could tell by timing the beam's oscillation. Their faint attraction to other balls placed alongside the beam was detectable by the deflection it caused. Cavendish's aim was not actually to measure the gravitational constant, but rather to measure the Earth's density relative to water, through the precise knowledge of the gravitational interaction. In retrospect, the density that Cavendish calculated implies a value for G of 6.754 × 10−11 m3/kg/s2.[5]

The accuracy of the measured value of G has increased only modestly since the original Cavendish experiment. G is quite difficult to measure, as gravity is much weaker than other fundamental forces, and an experimental apparatus cannot be separated from the gravitational influence of other bodies. Furthermore, gravity has no established relation to other fundamental forces, so it does not appear possible to calculate it indirectly from other constants that can be measured more accurately, as is done in some other areas of physics. Published values of G have varied rather broadly, and some recent measurements of high precision are, in fact, mutually exclusive.[3][6]

In the January 5, 2007 issue of Science (page 74), the report "Atom Interferometer Measurement of the Newtonian Constant of Gravity" (J. B. Fixler, G. T. Foster, J. M. McGuirk, and M. A. Kasevich) describes a new measurement of the gravitational constant. According to the abstract: "Here, we report a value of G = 6.693 × 10−11 cubic meters per kilogram second squared, with a standard error of the mean of ±0.027 × 10−11 and a systematic error of ±0.021 × 10−11 cubic meters per kilogram second squared."[7] (Wikipedia)

From the above history, it is quite clear that Cavendish wanted to calculate the density of the Earth rather to calculate the Gravitational Constant.

It is quite clear that G is dependent of density and acceleration due to gravity it dependent of G.

We have estimated value of G not the exact one and we are rotating the whole science on the basis of G which is dependent of density.

I am still confused on one side this force of gravity is so strong that is holding the whole universe and on the other hand it appear so weak in Cavendish experiment as described in Wikipedia.

What would be the results if we use plastic balls of same density instead of lead balls? Results should be the same if the gravity exists.