Centripetal force is always a net force. Why?

Answer 1

First of all, this is nonsense, and you can tell your teacher I said so.

Centripetal force is the force directed toward the center. This may or may not be a net force.

Examples where centripetal force is NOT a net force:

1. A bug sits on the edge of a turntable, and the turntable starts from rest and then accelerates gradually to an angular speed of 33 revs/minute. While the turntable is gaining speed, the bug experiences both centripetal force and tangential force. The centripetal force is not the net force.

2. A spaceship in orbit around the earth fires its engines in a "horizontal" orientation. It experiences a centripetal force (due to gravity) plus a non-centripetal force due to the rocket engines. The centripetal force is not the net force.

In the special case where an object is traveling in a circle at a constant speed, THEN the centripetal force is definitely the net force. This is because you can determine (by analyzing its speed and direction) that the net acceleration is toward the center (in this special case). Newton said that the net force is always in the same direction as the net acceleration, so in this case the net force is also centripetal.