Question

Why is there no centripetal force at the highest point on the pendulum?

Answer 1

To have a centripetal force, you need a radius, as show by
\[F=mv^2/r\]
If radius equals zero at the focal point or anchor, then the force goes to zero.

Answer 2

I'm still a little confused. Sorry. How do you know that the radius is 0 at the highest point on the pendulum?

Answer 3

My apologies, you must mean "highest point" to be the highest point to which the pendulum may swing. In this case, ignore what I said ;) and read this:
Using the same equation,\[F=mv^2/r\]
we can see that it is dependent on velocity, mass and radius. While mass and radius is unchanging for the pendulum throughout its flight, the velocity does change. At the "lowest point" the velocity is the highest, because the energy of the system is all kinetic. At the "highest point", the pendulum has converted all the energy to potential energy and is about to turn around and continue back down to the lowest point. It is at this point of highest potential energy (highest point) that the velocity is zero because it is stopping and then turning around. Because velocity equals zero, so does the centripetal force.

Answer 4

Ah I see. Thanks!