###
Determination Of Gravitational Constant L-3
###
Determination of Gravitational Constant
###
Determination Of Gravitational Constant L-3
###
Determination of Gravitational Constant
- 1.Newton's Laws of Motion $2^{\text {nd }}$ and $3^{\text {rd }}$ Laws - 2.Centripetal Force - Example: The center of the orbit from the sun to that from the earth to that of the sun. - $F_c=\frac{m v^2}{r}$
###
Determination Of Gravitational Constant L-3
###
Galilean Law
- 3.The Galilean Law of Freely falling bodies. - 4.The second law describes how the acceleration takes place. - 5.The Newton's third law describes the symmetry. - ${F_G}=+\frac{G M m}{r^2}$
###
Determination Of Gravitational Constant L-3
###
Galilean Law
- $F_G=\frac{Gm_1 m_2}{r^2}$
###
Determination Of Gravitational Constant L-3
###
Galilean Law
- $A: M_A: \vec{R}_A$ - $B: M_B : \vec{R}_B$ - $\vec{R}_{A. B}=\vec{R}_A-\vec{R}_B$ - $\vec{F_{A \rightarrow B}}=-\frac{G M_A M_B}{R_{A B}^3} \vec{R_{A B}}$
###
Determination Of Gravitational Constant L-3
###
Galilean Law
* Acceleration independent of mass. * Acceleration independent of the hight.
###
Determination Of Gravitational Constant L-3
###
Galilean Law
- Two bodies start with a separation d. - The distance between them should keep on decreasing as they fall towards because both of them fall towards the center. - Acceleration should increase.
###
Determination Of Gravitational Constant L-3
###
Acceleration Due to Gravity at Height
- $m {{a}}=\frac{G m M_E}{\left(R_E+{{h}}\right)^2}$ - $\frac{h}{R_E} \ll 1$ - $a=\frac{G M_E}{h^2}\left[\frac{1}{\left(1+\frac{h}{R_E}\right)^2}\right]$
###
Determination Of Gravitational Constant L-3
###
Acceleration Due to Gravity at Height
- $x=\frac{h}{R_E} \ll 1$ - So, $\frac{1}{\left(R_E+h\right)^2}$ - $=\frac{1}{R_E^2\left(1+\left(\frac{h}{R_E}\right)\right)^2}=\frac{1}{R_E^2(1+x)^2}$
###
Determination Of Gravitational Constant L-3
###
Acceleration Due to Gravity at Height
- $\frac{1}{(1+x)^2}=\frac{1}{\left(1+2 x+x^2\right)}$ - $x^2 \ll x \ll 1$ - $\simeq \frac{1}{1+2 x}=(1-2 x)$ - $m a=\frac{F_{G}}{m}=\frac{G M_E m}{R_E^2}\left(1-2 \frac{h}{R_E}\right)$
###
Determination Of Gravitational Constant L-3
###
Acceleration Due to Gravity at Height
- $h=100 \mathrm{~m}$ - $R_E=6.4 \times 10^6$ - $x$ is really small - Reconciled all aspects of law of freely falling body with the Netonian law
###
Determination Of Gravitational Constant L-3
###
Acceleration Due to Gravity at Height
- $R \sim 6400 \mathrm{Km} ; \quad h \sim 100 \mathrm{~m}$ - $\frac{2 h}{R} \sim 3 \times 10^{-4}$
###
Determination Of Gravitational Constant L-3
###
Cavendish Experiment
* Distances are known * Masses not known * G Value not determined. * Galilean law determines $M_EG$. - The Cavendish Experiment (1797-98):Directly determine G.
###
Determination Of Gravitational Constant L-3
###
Cavendish Experiment
- **A Null-Torque Experiment**
###
Determination Of Gravitational Constant L-3
###
How To Determine Forces
* Measure the bodies acceleration * Arrest the motion by known force.
###
Determination Of Gravitational Constant L-3
###
Cavendish Experiment
###
Determination Of Gravitational Constant L-3
###
Cavendish Experiment
- The whole Apparatus was placed in a thick wooden box. - The wooden box was enclosed in a shed. - Observations were made through two small holes fitted with telescopes. - The experiment was not null. A Large period of $20 \mathrm{~min}$. - The vernier scale had a least count of $0.1 \mathrm{~mm}$.
###
Determination Of Gravitational Constant L-3
###
Cavendish Experiment
| Column 1 | Column 2 | Column 3 | Column 4 | | -------- | -------- | -------- | -------- | | Mass of large ball | $m_W$ | 2439000 grains | 158.04 kg | | Mass of small ball | $m_b$ | 11262 grains | 0.73 kg | | Mass of supporting rod - insertial (equivalent small ball mass) | $m_I.rod$ | 398 grains | .03 kg | | Mass of supporting rod - gravitational (equivalent small ball mass) | $m_G.rod$ | 157 grains | .01 kg | | Mass of copper rods (equivalent small ball mass) | $m_copper$ | 18800 grains | 1.22 kg | | Distance between large ball | L | 73.30 in | 1.860 kg |
###
Determination Of Gravitational Constant L-3
###
Experimental Details
- Large Lead balls: ${158.04 \mathrm{~kg}}$ - Small Lead balls: $0.73 \mathrm{~kg}$ - Mass of the rod: $0.03 \mathrm{~kg}$
###
Determination Of Gravitational Constant L-3
###
Experimental Value
- The distance between small rod and the big rod = 0.225 millimeter. - We want a very large gravitational force. Therefore, the distance must be as small as possible. - Measure the angle. - Make an analysis and find out how determine the value of G.
###
Determination Of Gravitational Constant L-3
###
Analysis
- $\mathscr{T}=K \theta=L F=L \frac{G M m}{d^2}$ - $\therefore G=\frac{K \theta d^2}{L M m}$ -
How to determine K?
- Natural Oscillations. - $T=2 \pi \sqrt{\frac{I}{K}}=2 \pi \sqrt{\frac{m L^2}{2 K}}$
###
Determination Of Gravitational Constant L-3
###
Analysis
- $T=L F=L\frac{G Mm}{d^2}$ - $ ={k \theta}$ - $\theta$= angle of deflection - $G =\frac{K \theta d^2}{M m L}$
###
Determination Of Gravitational Constant L-3
###
Determination of K
-
How To Determine K?
- Look at the Natural Oscillation of the system.
###
Determination Of Gravitational Constant L-3
###
Time Period
- $T=2 \pi \sqrt{\frac{I}{K}}$ - $I$: moment of Inertia - $I = 2 \times m\left(\frac{L}{2}\right)^2$
###
Determination Of Gravitational Constant L-3
###
Experimental Value
- $G=\frac{2 \pi^2 L d^2}{M T^2}$ - $G($ Cavendish $)=6.74 \times 10^{-11}$ - $G($ Modern $)=6.67408(31) \times 10^{-11}$ - Relative error is about $1\\%$. - Absolute error $7\\%$.
###
Determination Of Gravitational Constant L-3
###
Calculated Value of Mass of Earth
- $m g=G \frac{m M_E}{R_E^2} \Longrightarrow$ - $M_E=\frac{g R_e^2}{G}=\frac{4 \pi}{R_e^3} \rho_E$ - $\frac{\rho_E}{\rho_w}=5.448 \pm 0.033$
###
Determination Of Gravitational Constant L-3
###
Calculated Value of Mass of Earth
- $mg=\frac{G M_E m }{R_E^2}$ - $M_E=\frac{g \bar{R}_E^2}{G}$ - $=\frac{4 \pi}{3} \bar{R}_E^3 \bar{p}$
###
Determination Of Gravitational Constant L-3
###
Calculated Value of Mass of Earth
- $\frac{\bar{\rho}_E}{\rho_W}=5.448 \pm 0.033$
###
Determination Of Gravitational Constant L-3
###
Thank You