[Math] Line integral of the unit circle

calculusintegrationmultivariable-calculus

Let $C$ be the bottom half of the unit circle $x^2+y^2 = 1$ traversed
counter clockwise. Calculate $\int_{C} \vec{F}\cdot\, d\vec{r}$.

What is the question asking and why is a parameterization of $C$, $r(t) = (\cos{t}, \sin{t})$ where $\pi \le t \le 2\pi$. Is it because if we start from $(1,0)$ and move around the circle counter clockwise, the point $(-1, 0)$ corresponds to $\pi$ (half a circle) and arriving back $(1,0)$ corresponds to $2\pi$ (full circle)?

I'm asking: what do they mean by traversed counter clockwise? Where does $\pi \le t \le 2\pi$ come from?

Best Answer

"Traversed counter clockwise" means we go in the opposite direction to the direction followed by the hands of an analogue clock. That is, we go from 9 o'clock to 8 o'clock...to 4 o'clock to 3 o'clock.

Suppose we want walk along such a circle according to the rule that our position must at all times $t$ be $(\cos t,\sin t)$. If we start when $t=\pi$, we start at the position with coordinates $(\cos \pi,\sin \pi)=(-1,0)$. When $t$ has changed to $t=3\pi/2=\pi+\pi/2$, we have changed our position to $(\cos 3\pi/2,\sin 3\pi/2)=(0,-1)$. When $t=2\pi$, we are at $(\cos 2\pi,\sin 2\pi)=(1,0)$, etc. Thus as $t$ changes from $\pi$ to $2\pi$, our position on the circle changes accordingly.

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