http://www.ck12.org Chapter 7. Centripetal Forces Version 2
In other words,it is much easier for a force to change the speed of an object when it points along its velocity vector.
Forces are capable of changing an object’s velocity — speed and direction: the more parallel to motion the force, the
more it changes speed; the more perpendicular, direction (if we had analyzed the effect on the angle of the velocity
vector above instead of speed, the results would have been reversed).
Circular Motion
Knowing this, we can understand why, when force isalwaysperpendicular to the velocity vector, an object’s speed
neverchanges, while its direction changes continuously. If the force is constant and magnitude, the direction of the
object’s velocity must change at a constant rate — otherwise the situation would be asymmetrical. In other words,
the object will travel in a circle, with instantaneous velocity tangent to it and instantaneous force pointing toward the
center. At any given time, the relationship between force, acceleration, and velocity is illustrated here:
Circular motion is kind of a limiting case of the ’first second’ scenario above — if the force had always been
perpendicular to the ball’s velocity, it wouldn’t have accelerated downward for an entire second. No matter how
weak a centripetal force, it will in principle always cause a moving object to travel in a circle. This may seem
counterintuitive, but is actually a direct result of the arguments above.