prediction vs explanation

The other day I was brushing up on some physics (it has been a long time since I did physics and I need to take a physics test at some point so that MIT will let me graduate).

I’ve been studying mechanics and I noticed something interesting.

”There’s no such thing as centripetal force”

Centripetal force is just the label that we give to a net force that results in an object following a circular motion pattern. That is, if you show me a ball swinging on a rope in a circular trajectory, then it is quite true that there is a net force acting on the ball, accelerating it towards the center of the circle.

But this is explanative. Not predictive.

If you give me a setup, you say “Here’s a rocket, it has velocity straight away from the center of earth and is distance from the center of earth, will this rocket end up orbiting the earth, and if so at what radius?” I can’t predict the answer based on “centripetal force”. I need to know about gravitational attraction.

But this reminded me of something important:

• It’s super easy to explain things with hindsight.
• But much harder to predict them in advance.

So, be wary of after-the-fact explanations.