# Vertical Circular Motion

Hamish Cruickshank
I am having some difficulty in understanding vertical circular motion.

As I understand it, the only time the object (lets say an aeroplane flying in a vertical circle) is in uniform circular motion is at the top and the bottom of the circular path.

So if you want to find the net force on an object at the top of the circle it will be the centripetal force, because in UCM the net force is the centripetal force.

Is this right?

Homework Helper
Generally uniform circular motion means a constant speed throughout the circle...

In the case of a ball attaches to a string going through a vertical circle... you don't have uniform circular motion... however at the top and bottom we do have dv/dt = 0 where v is the speed of the ball (careful to note, acceleration is not 0, ie: $$\frac{d\vec{v}}{dt}$$ is not 0 because direction is changing)...

This happens because the net force is centripetal at the top and bottom. The tension and gravity both act along the radius of the circle... There is no tangential component to the net force, so speed is constant at this moment... tangential force changes speed... centripetal force doesn't (centripetal force only changes direction).

However, in between the top and bottom we have a centripetal force and a tangential force. The tension acts along the radius... but gravity can be divided into 2 perpendicular components... one along the radius and one tangent to the circle... this results in a changing speed...

Centripetal vs. tangential are "components" of the net force... (don't think of them as independent forces in and of themselves... they are components of the net force which in this case is the vector sum of tension and gravity).