# Conceptual Doubt regarding Simple Harmonic Motion

While studying about a simple pendulum, I came across the following line.

At the bottom of the swing the tension will actually be greater than the weight, causing the bob to move in a circle.

Now, before this, the author was trying to explain how the two vectors, tension, and gravity add up to give a force that is directed towards the equilibrium point. As the bob tends towards the equilibrium point, the tension begins to grow as a result the force vector decreases. Shouldn't this force vector become zero at the equilibrium point and if so, what propels the bob to move forward in a circle. Now the only way to prevent this conflict is by assigning the tension a greater magnitude, which will help in creating some force that will drive the bob in a circle. But, I don't see any explanation as to why this should be true. As far as I understand, the bob should stop. Moreover, the energy conservation also does not seem to work here as at the equilibrium the bob's kinetic energy is maximum, implying that energy is conserved, then why would the bob "like" to convert its energy into potential energy. I know that I am missing something or maybe a lot of things, but if you spot the error in my reasoning(s) please help.

Thank You.

## 1 Answer

But, I don't see any explanation as to why this should be true. As far as I understand, the bob should stop.

Look at it like this. When the bob is at its lowest point, no horizontal forces act on it.

By Newton's Law of motion that means that the horizontal speed vector is conserved: the bob MUST by Newton keep moving. We call it inertia.