Difference between revisions of "Forest UCM EnergyIntPart"

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:<math>= - \vec{\nabla}_1 U(\vec{r}_1 - \vec{r}_2) </math>
 
:<math>= - \vec{\nabla}_1 U(\vec{r}_1 - \vec{r}_2) </math>
  
just take appropriate derivative
+
Newton's 3rd law requries that
 +
 
 +
:<math>\vec{F}_{21} = - \vec{F}_{12} = - \left (- \vec{\nabla}_1 U(\vec{r}_1)  \right ) = \vec{\nabla}_1 U(\vec{r}_1 - \vec{r}_2) </math>
 +
:: <math>- \left (- \vec{\nabla}_2 U(\vec{r}_2)  \right ) = -\vec{\nabla}_2 U(\vec{r}_1 - \vec{r}_2)</math>
 +
 
 +
or
 +
 
 +
:<math> \vec{\nabla}_1 U(\vec{r}_1 - \vec{r}_2)  = -\vec{\nabla}_2 U(\vec{r}_1 - \vec{r}_2)</math>
  
 
==Total work given by one potential==
 
==Total work given by one potential==

Revision as of 14:57, 28 September 2014

Energy of Interacting particles


Translational invariance

Consider two particles that interact via a conservative force [math]\vec{F}[/math]


Let [math]\vec{r}_1[/math] identify the location of object 1 from an arbitrary reference point and [math]\vec{r}_2[/math] locate the second object.

The vector that points from object 2 to object 1 may be written as

[math]\vec r = \vec{r}_1 - \vec{r}_2[/math]


The distance between the two object is given as the length of the above vector

If the force is a central force

[math]\vec F = \frac{k}{r^3} \vec r = \frac{k}{\left | r_1 - r_2 \right |^3} \left ( \vec{r}_1 - \vec{r}_2 \right )[/math]


Notice
The interparticle force is independent of the coordinate system's position, only the difference betweenthe positions matters


If object 2 was fixed so it is not accelerating and we place the origin of the coordinate system on object 2

Then the force is that of a single object

One potential for Both Particles

Both forces from same potential

If the above force is conservative then a potential exists such that

[math]\vec{F}_{12} = - \vec{\nabla}_1 U(\vec{r}_1) [/math]
[math] = - \vec{\nabla}_1 U(\vec{r}_1) = - \left( \hat i \frac{\partial}{\partial x_1} + \hat j \frac{\partial}{\partial y_1} + \hat k \frac{\partial}{\partial z_1} \right ) U(\vec{r}_1)[/math]
[math]= - \vec{\nabla}_1 U(\vec{r}_1 - \vec{r}_2) [/math]

Newton's 3rd law requries that

[math]\vec{F}_{21} = - \vec{F}_{12} = - \left (- \vec{\nabla}_1 U(\vec{r}_1) \right ) = \vec{\nabla}_1 U(\vec{r}_1 - \vec{r}_2) [/math]
[math]- \left (- \vec{\nabla}_2 U(\vec{r}_2) \right ) = -\vec{\nabla}_2 U(\vec{r}_1 - \vec{r}_2)[/math]

or

[math] \vec{\nabla}_1 U(\vec{r}_1 - \vec{r}_2) = -\vec{\nabla}_2 U(\vec{r}_1 - \vec{r}_2)[/math]

Total work given by one potential

Elastic Collisions

Definition

BOTH Momentum and Energy are conserved in an elastic collision

Example


Consider two object that collide elastically

Conservation of Momentum
[math]\left ( p_1 + p_2 \right ) _{\mbox{initial}} = \left ( p_1 + p_2 \right ) _{\mbox{final}}[/math]
Conservation of Energy
[math]\left ( T + U \right ) _{\mbox{initial}} = \left ( T + U \right ) _{\mbox{final}}[/math]

When the initial and final states are far away fromthe collision point

[math]U_{\mbox{initial}} = U_{\mbox{final}} = 0 =[/math] arbitrary constant


Example

Consider an elastic collision between two equal mass objecs one of which is at rest.

Conservation of momentum
[math] m \vec{v}_1 = m \left (\vec{v}_1^{\;\prime} + \vec{v}_2^{\;\prime} \right )[/math]
Conservation of Energy
[math] \frac{1}{2} m v_1^2 = \frac{1}{2} m \left (v_1^{\;\prime} \right )^2 + \frac{1}{2} m\left ( v_2^{\;\prime} \right )^2[/math]


Square the conservation of momentum equation
[math] \vec{v}_1 \cdot \vec{v}_1 = \left (\vec{v}_1^{\;\prime} + \vec{v}_2^{\;\prime} \right ) \cdot \left (\vec{v}_1^{\;\prime} + \vec{v}_2^{\;\prime} \right )[/math]
[math] v_1^2 = \left (v_1^{\;\prime} \right )^2 + \left ( v_2^{\;\prime} \right )^2 + 2 \vec{v}_1^{\;\prime} \cdot \vec{v}_2^{\;\prime} [/math]

compare the above conservation of momentum equation with the conservation of energy equation

[math] v_1^2 = \left (v_1^{\;\prime} \right )^2 + \left ( v_2^{\;\prime} \right )^2[/math]

and you conclude that


[math]2 \vec{v}_1^{\;\prime} \cdot \vec{v}_2^{\;\prime} = 0 \;\;\;\; \Rightarrow \vec{v}_1^{\;\prime} \perp \vec{v}_2^{\;\prime} [/math]

Forest_UCM_Energy#Energy_of_Interacting_Particles