The following is a pendulum.
The blue arrow indicate the weight mg of the pendulum,
and the square shows two component of weight (m*g*sin?,m*g*cos?).
The red arrow is the velocity vector (v).
The cyan arrow shows the tension of the string (T= m*g*cos?+m*v[sup]2[/sup]/L, where L is the length of the pendulum). The second term is the centripetal force required for the circular motion.)
You can click the stop button to pause the simulation and drag the black dot to change length and angle of the pendulum (the velocity will be set to zero when you drag the black dot).
You can drag the end of the velocity arrow to change it's velocity and watch how tension force changed,too!