Acceleration is the rate of the velocity change, i.e. \vec{a}\equiv\frac{d\vec{v}}{dt}.
Because velocity is a vector, there are many ways to change velocity:
1. change the magnitude of the velocity (change speed) without change it's direction.
2. change direction of the velocity without change it's magnitude: this is the circular motion.
3. change both magnitude and directtion of the velocity.

If the acceleration is in the direction of velocity, the acceleration will change the magnitude the of velocity(change speed).
However, if the acceleration is always perpendicular to the velocity, the velocity will change direction without change it's magnitude.  This is the circular motion.
So, for circular motion, the force(acceleration) always perpendicular to it's velocity.

You can drag the velocity to change its direction and magnitude, the magnitude of the force is changed by slider bar.

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