Wednesday, April 13, 2016

homework and exercises - Motion of a rod struck at one end


Imagine a strong metal rod of uniform density and thickness floating in a weightless environment. Imagine it lies on an X-Y plane, with one end (A) lying at 0,0, and the other end (B) at 0,1. Then it is struck at A in the direction of increasing X. Please describe the trajectory of the bar. In particular, will the point B move in the opposite direction (i.e. decreasing X) momentarily? Is there a website or article that goes through the physics of this problem?


EDIT: some specific questions: What would be the trajectory of the centre of gravity? Would it just travel horizontally along the line y = 0.5? What would be the ratio of the velocity of the centre of gravity to the rotational velocity of the ends of the bar?


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Answer



You need Euler's laws of motion for Rigid Bodies.






  1. Sum of vector force impulse acting on a rigid body equal to the change of linear momentum through the center of gravity.




  2. Sum of vector torque impulse acting on the center of gravity of a rigid body equal to the change of angular momentum.





If the hammer impulse is $\vec{J}$ along the $\hat{x}$-axis, the location of $A$ is $\vec{r}_A = (0,0,0)$ and the center fr gravity $C$ is $\vec{r}_C = (0,\frac{L}{2},0)$ where $L$ is the length of the rod the above equations are



$$ \vec{J} = m \Delta \vec{v}_C $$ $$ (\vec{r}_A-\vec{r}_C)\times\vec{J} = I_C\,\Delta\vec\omega $$


The change in motion of point $B$ is $$\Delta\vec{v}_B = \Delta\vec{v}_C + \Delta\vec{\omega}\times (\vec{r}_B-\vec{r}_C ) $$ and point $A$ is $$\Delta\vec{v}_A = \Delta\vec{v}_C + \Delta\vec{\omega}\times (\vec{r}_A-\vec{r}_C ) $$


If you put it all together then you will find the motions of different parts of the rod (at least in direction and sign).


Note that the mass moment of inertia $I_C$ for a thin slender uniform rod of mass $m$ and length $L$ is $I_C = \frac{m}{12} L^2 $.


Here is a url of a website that describes what you need to solve this kind of problems.


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