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I have been asking and learning about angular momentum. I know that an object traveling straight has ang. momentum about a point of rotation.

Just curious, this example shows the torque acting on a meteor traveling in a straight line. Therefore, wouldn't there be work done by torque? Normally work would be $W=\int \vec{\tau} \cdot \overrightarrow{d \theta}$, but here that doesn't work. https://courses.lumenlearning.com/suny-osuniversityphysics/chapter/11-2-angular-momentum/

I know that it is the same as $W=\vec{F}\cdot\vec{d}$, and can calculate as such, but was trying to see how they are the same.

What angle would you use since it doesn't change like it would due in true circular motion?

1 Answers1

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Work is defined as $$W=\int\vec{F}\cdot\mathrm{d}\vec{s}.\hspace{1in}(1)$$ The relationship for torque and work which states $$W=\int\vec{\tau}\cdot\mathrm{d}\vec{\theta}\hspace{1in}(2)$$ is only true if the particle experiencing the torque is at a constant distance from the axial point, i.e., rigid body rotation or circular orbits. Derivations which develop equation (2) are assuming circular motion.

That is not the case here. The meteor is getting closer to the origin (the point chosen for torque calculations). To account for the change in distance one must add a term to equation (2), $$W_r=\int\vec{F}\cdot\hat{r}\mathrm{d}r.\hspace{1in}(3)$$

Edit

For this problem the $x$ cooridinate is fixed at $X$, but $y$ is changing. So we can write $$r=\frac{X}{\cos\theta}.\hspace{1in}(4)$$ $\theta$ is the angle with respect to the horizontal ($x$-axis). We also note that $\vec{F}$ makes an angle $\pi/2 - \theta$ with the $\hat{r}$ so $$\vec{F}\cdot\hat{r}=F\sin\theta.\hspace{1in}(5)$$ From (4) we can determine d$r$: $$\mathrm{d}r=\frac{X\sin\theta}{\cos^2\theta}\mathrm{d}\theta.$$

Now we can put all the pieces together for (3) and integrate from $\theta_1$ to $\theta_2$ and see that (2)+(3) gives the same result as (1). I'll leave the details of the integrals to the curious person, but it does work out properly.

End Edit

If one considers elliptical planetary orbits, this makes sense: the kinetic energy of the satellite or planet or moon is increasing and decreasing which means work is being done, but the torque is zero which means equation (2) gives zero. The work being done is done by the radial component of the attractive force as the object moves toward or away from the attractive center.

Bill N
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