When an object is in circular motion, it need centripetal force.
1. Click the black ball and drag it up and down to change the radius r. 2. The torque acting on the red ball is zero since F is parallel to r. 3. Click the right mouse button to pause, click it again to resume. When the animation is suspended, Click at the red ball and drag the mouse button to change its tangential vector, and the mass of black ball will also change to keep in the same radius. If you click the right mouse button and drag it up/down/left/right, you can rotate the coordinate system. 4. Click trace check box to trace the trajectory, Press Clean Button to clear the trace. 5. Parameters : A short white arrow represents the velocity vector v = r w. where w : angular frequency , r : radius Mg/m: Mg gravitation force for black ball, m is the mass of red ball.