Unit Circle and Sine Curve
Instructions
Push the slider for theta back to 0. Then press the Play button. Thereafter, you will see the curve y = sin x being traced out as the angle theta increases.
How this applet works
The circle is a unit circle i.e., it has radius of 1, and it is centred at the origin O. A right-angled triangle is produced (as theta changes) such that the hypotenuse is always the radius of the circle, which is 1 unit long, and y is the opposite side. Thus. sin theta = y/1 = y, so the y value of triangle will always be equal to the corresponding y-value on the sine curve.