Recent content by hhjjy

Ok, thanks.

figure 11.12 I need someone to explain why the angular momentum of the ball is ## L_{f} = -rm_{d}V_{df} + I\omega## rather than ## L_{f} = rm_{d}V_{df} + I\omega ##. How to distinguish the sign of the angular momentum?p.s. ##\Delta\vec{L}_{total} = \vec{L}_{f} - \vec{L}_{i} = (-rm_{d}v_{df} +...

I appreciate your help. Thank you everyone .

Oh , I got it . I miss a important idea.The point P is at (-R,0), so ## \vec{r} = (R + R\cos{\theta}) \vec{i} + R \sin{\theta} \vec{j} ## . That makes sense if we take it back to the equation. ## \vec{L} = mvR(\sin^2{\theta}+\cos^2{\theta} + \cos{\theta}) = mvR(1 + \cos{\theta})\vec{k} ## By...

I How to use geometric sight to solve this problem? I have no idea when I first thought, so I use the algebraic approach to solve, but the result looks weird. According to Charles, I made some mistakes. If I correct the calculation mistake, It looks weird. ## \vec{L}(\theta) =...

the figure is this one.

I don't understand why my solution is wrong. Here is my solution. $$ r_{\theta} = R\cos{\theta} \vec{i} + R\sin{\theta} \vec{j} $$ $$ v_{\theta} = v\cos(\theta + \frac{\pi}{2}) \vec{i} + v\sin(\theta + \frac{\pi}{2}) \vec{j} $$ $$ p_{\theta} = mvR(-\sin{\theta}) \vec{i} +mvR(\cos{\theta}...

I got it . It's ## W = F x =(30)( \frac{1}{2} 2 \pi r) ##. I forgot there is a 2 . Thank you .

Yes , I can accept this concept .Potential energy is transferred to Kinetic energy. Because of this equation ## \Delta Kinetic = Work +(-\Delta U) ## , we can know that both work and potential energy are increasing the kinetic energy . Right ?

Thank you . I have corrected it . 4. It works along half of the circle . 5. I am not sure , but I think the pitcher's arm is the only force on the ball. I am confuse that whether gravity has worked on the motion.

So If we take another examples ,such as uniform circular motion or a pen fell from sky, $$ \Delta KE = 0 - \Delta U $$ ,which means there is no force working on it ,so we call it conservative of energy . Non-uniform circular motion is $$ \Delta KE = W - \Delta U $$, which means there is other...

Can you explain that what 's the different between $$ E_{system} = \Delta K + \Delta U = W_{circular} $$ and $$ E_{system} = \Delta K + \Delta U = -W_{circular} $$ .And thank you for your reply .

This is my solution ,and I just use the definition .But I still feel unclear about the concept of non-conservative force.$$ W = F x = 30N (\frac{1}{2}\pi r ) = 56.2 J $$ $$ E_{system} = \Delta K + \Delta U = W $$ $$ (K_{f}- K(i))+(U(f)-U(i)) = W $$ $$ (\frac{1}{2} *m{V_{f}}^2...