enter image description here

enter image description here

  1. is the linear momentum of the ball conserved in the $e_3'$ direction?

I figure since there is no force on this direction, but the direction is changing, the linear momentum will not be conserved.

  1. is the angular momentum around B of the ball conserved in the $e_3''$ direction?

Here I can say that since $\bar M=\bar x\times\bar F=-mgLsin(\theta)e_3''$ the angular momentum in this direction is not conserved.

  1. around the $e_1''$ direction?

Since in this direction the moment-force is 0 and it is fixed in space, the angular momentum is conserved. If $\bar H=x_{A/B}\times \bar G$ is the angular momentum, I find that $$\bar H \cdot e_1''=-m(Lsin(\theta))^2\dot \phi$$ conservation of $\bar H$ in this direction along with $Lsin(\theta_0)\dot \phi_0=v_0$ yields,$$\dot \phi=\frac{v_0sin(\theta)}{Lsin^2(\theta)}$$ note: In the formal answer to this question there is no square in the denominator, and I cannot see why.

  1. Does the rope do work on the ball?

None, since it acts normal to it's direction.

  1. Determine the kinetic energy of the ball.

Since no work is done by the rope and no other external forces, the following conservation of energy holds: $$E_0=E$$ $$E_0^{kinetic}+E_0^{potential}=E^{kinetic}+E^{potential}$$ $$mgL(1-cos(\theta))+\frac{1}{2}mv^2=const.$$ using this,$\dot \phi$ from before and $v^2=(Lsin(\theta)\dot \phi)^2+(L\dot \theta)^2$, $\dot \theta$ should be explicitly available.

  1. Determine $\dot\theta$ and $\dot\phi$ in terms of {$L,\theta,\theta_0,v_0,g$}

already found.

  1. Determine the absolute acceleration of the ball.

$$\bar a=\frac{\delta \bar V}{\delta t}+\bar \omega\times\bar V$$ $$\bar V=\frac{\delta \bar X}{\delta t}+\bar \omega\times\bar X$$ where $\frac{\delta}{\delta t}$ is the frame-derivative and $\bar \omega=e_1''(-\dot \phi)+e_3'(\dot \theta)$ is the angular velocity of the ball's frame.

  1. Determine the tension in the rope.

Using Newton's 2nd and 3rd law, and the acceleration above,$$T=-\bar F\cdot e_1'=-m\bar a\cdot e_1'$$

as mentioned before, the "formal" solution to this has quite different results, but I think the logic, I got correctly. I can't be sure so I'd love some feedback.

Your Answer


By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Browse other questions tagged or ask your own question.