Related Rates (with circle/position)

**PhizKid** · November 5th 2012, 01:10 PM

A runner sprints around a circular track of radius 100 m at a constant speed of 7 m/s. The runner's friend is standing at a distance 200 m from the center of the track. How fast is the distance between the friends changing when the distance between them is 200 m?

The problem I am having here is expressing the runner's position on the circle as a function. I know the coordinates, but how can I turn that into a function? Can I use x^2 + y^2 = 100^2?

Even so, we cannot assume any angles here so I am guessing that this problem doesn't need any angles to solve.

But is my diagram correct? Since the friend is 200 m away from the center, does this form an isoceles triangle because the question is also asking if he is 200 m. away from the runner which is another point on the circle? Or is it the same exact point (a straight line)?

**skeeter** · November 5th 2012, 01:25 PM

$s = r \theta = 100\theta$

$\frac{ds}{dt} = 7 = 100 \frac{d\theta}{dt}$

$\frac{d\theta}{dt} = \frac{7}{100} \, rad/sec$

law of cosines ...

$D^2 = 200^2 + 100^2 - 2(200)(100)\cos{\theta}$

**PhizKid** · November 5th 2012, 01:30 PM

What is s? I don't see what the radius * angle is supposed to be. It looks like it's a function of the runner's position as the angle, but I don't see how that makes sense.