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--- 183_notes:examples:the_moment_of_inertia_of_a_diatomic_molecule [2014/10/31 14:17] – created pwirving
+++ 183_notes:examples:the_moment_of_inertia_of_a_diatomic_molecule [2014/11/05 20:40] (current) – pwirving
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 ===== Example: The Moment of Inertia of a Diatomic Molecule  =====
-What is the moment of inertia of a diatomic nitrogen molecule $N_(2)$ around its center of mass. The mass of a nitrogen atom is 2.3 x 10^-26 kg and the average distance between nuclei is 1.5 x 10^-10 m. Use the definition of moment of inertia carefully.
+What is the moment of inertia of a diatomic nitrogen molecule $N_{2}$ around its center of mass. The mass of a nitrogen atom is $2.3$ x $10^{-26}$ kg and the average distance between nuclei is $1.5$ x $10^{-10}$ m. Use the definition of moment of inertia carefully.
 === Facts ===
-Metal block of mass 3 kg
+Mass of nitrogen atom is 2.3 x $10^{-26}$kg
-The stiffness of the spring is 2000 N/m.
+Average distance between nuclei is 1.5 x $10^{-10}$m
-The block has the following initial conditions for both parts:
+=== Assumptions and Approximations ===
-a:
+The distance between the atoms in the molecule does not change.
-Initial State: Block 0.8 m above floor, moving downward, $v_{i}$ = 2 m/s, spring relaxed
-Final State: Block 0.3 m above floor, spring compressed
-b:
-Initial State: Block 0.8 m above floor, moving downward, $v_{i}$ = 2 m/s, spring relaxed
-Final State: Block at highest point, $v_{f} = 0$, spring relaxed
+The model of the system you are using includes a spring between the atoms but these are not actual springs so the spring has no mass.
 === Lacking ===
-a: The speed of the block when it is 0.3 m above the floor
+The moment of inertia of a diatomic nitrogen molecule $N_{2}$ around its center of mass?
-b: The maximum height
-=== Approximations & Assumptions ===
-Air resistance and dissipation in the spring are negligible.
-$U_{g} \approx mgy$ near Earth's surface.
-${\Delta K_{Earth}}$ is negligible.
 === Representations ===
-System: Earth, block, spring
+$I = m_{1}r^{2}_{\perp1}$
-Surroundings: Nothing significant
+{{course_planning:course_notes:mi3e_09-018.jpg|?300}}
-Energy Principle: $E_{f} = E_{i} + W$
-$K =  \dfrac{1}{2}mv^2$
-$U_{spring} = \dfrac{1}{2}k_{s}s^2$
-$U_{gravitational} = mgy$
 === Solution ===
-a:
+For two masses, $I = m_{1}r^{2}_{\perp1}$ + $m_{2}r^{2}_{\perp2}$.
-From the Energy Principle:
-$E_{f} = E_{i} + W$
-The total final energy is the sum of the kinetic energy, spring potential energy and gravitational potential energy which is equal to the total initial energy which is the sum of the kinetic energy, spring potential energy and gravitational potential energy initially.
-$K_{f} + U_{s,f} + U_{g,f} = K_{i} + U_{s,i} + U_{g,i} + W$
-The work is equal to zero as no work is being done by the surroundings since the system is the Earth, block and spring.
-$U_{s,i}$ is also equal to zero as there is no initial spring potential energy as the spring is not compressed.
-We then substitute in the equations for kinetic and potential energies.
-$\dfrac{1}{2}mv^2_{f} + mgy_{f} + \dfrac{1}{2}k_{s}s^2_{f} = mgy_{i} + \dfrac{1}{2}mv^2_{i}$
-Multiply across by 2 and divide across by m in order to simplify the equation. Also group like terms.
-$v_{f}^2 = v_{i}^2 + 2g(y_{i} - y_{f}) - \dfrac{k_{s}}{m}{s^2_{f}}$
-Isolate $v_{f}$ to solve for it:
-$v_{f} = \sqrt {(2 m/s)^2 + 2(9.8 N/kg)(0.5 m) - \dfrac{2000 N/m}{3 kg}{(0.1 m)}^2}$
-$v_{f} = 2.7 m/s$
-b:
-We want to find the value for $y_{f}$ which is the maximum height reached by the bottom of the block above the floor.
-From the Energy Principle:
-$E_{f} = E_{i} + W$
-The total final energy is the sum of the kinetic energy, spring potential energy and gravitational potential energy which is equal to the total initial energy which is the sum of the kinetic energy, spring potential energy and gravitational potential energy initially.
-$K_{f} + U_{s,f} + U_{g,f} = K_{i} + U_{s,i} + U_{g,i} + W$
-The work is equal to zero as no work is being done by the surroundings since the system is the Earth, block and spring.
+The distance between the masses is d, so the distance of each object from the center of mass is $r_{\perp1} = r_{\perp2} = d/2$.
-$U_{s,i}$ and $U_{s,f}$ are both equal to zero as there is no initial spring potential energy or final spring potential energy as the spring is not compressed in either case.
+Therefore:
-$K_{f}$ is also equal to zero as the maximum height is reached when the block has a final kinetic energy of zero.
+$I = M(d/2)^{2} + M(d/2)^{2} = 2M(d/2)^{2}$
-Substitute in equations for kinetic and potential energy for remaining terms.
+Where you substitute in M for $m_{1}$ and $m_{2}$ as it is the same total mass we are talking about.
-$mgy_{f} = mgy_{i} + \dfrac{1}{2}mv^2_{i}$
+Substitute in given values for variables.
-Divide each term by mg.
+$I = 2 \cdot (2.3$ x $10^{-26}kg)(0.75$ x $10^{-10}m)^2$
-$y_{f} = y_{i} + \dfrac {v_{i}^2}{2g}$
+Compute moment of inertia of diatomic nitrogen molecule
-Solve for $y_{f}$
+$I = 2.6$ x $10^{-46} kg \cdot m^2$
-$(0.8 m) + \dfrac {(2 m/s)^2}{2(9.8 N/kg)} = 1.0 m$