- 15,738
- 8,937
This got me thinking. In our part of the world we teach that the force exerted on the stone by the string is "centripetal", i.e. directed towards the center of rotation. Centripetal defines a direction, not a special kind of force and has an orthogonal counterpart that is given the name "tangential." We also teach, as shown in the quote above, that "centrifugal" is a force that appears in a non-inertial frame when the right hand side of Newton's second law is moved to the left hand side with a change in sign. In either frame, when the system is the stone, the tension acts towards the center of the circle. We can write unambiguous equations using radial unit vectors for the two casesharuspex said:You have made responding a bit awkward by embedding your responses in your quotes of my posts. It means I cannot use the quote button on them.
"My teacher told me so, sir. He said they both counterbalances and cancel themselves."
Oh dear. Your teacher is incompetent. You have my sympathy.
"Never both, why sir? we do have centripetal and centrifugal force, right?"
As I wrote, in an inertial frame there is no centrifugal force; in the noninertial frame locked to the body, the body is not moving, so there is no centripetal acceleration nor force. In some other noninertial frame there can be both, but it would be a very odd choice of frame.
$$\begin{align} & T(-\mathbf{\hat r})=m\omega^2 r(-\mathbf{\hat r}) \\
& T(-\mathbf{\hat r})+m\omega^2 r(\mathbf{\hat r})=0. \end{align}$$Equation (1) is in the inertial frame and says the the centripetal force, which is the net force, is the same as mass times the centripetal acceleration. Equation (2) is in the non-inertial frame and says that there are two forces (let's not give them names) that add to give zero. That's what we teach in our part of the world. It would be much simpler if everybody agreed to drop "centripetal" and "centrifugal" altogether and use "radially in" and "radially out" instead.
So here is my question. What if in OP's part of the world the terms "centrifugal" and "centripetal" are used simply to designate direction, respectively ##(\mathbf{+\hat r})## and ##(-\mathbf{\hat r})##, with no reference to inertial or non-inertial frames? Then, in view of equation (2) the teacher's claim that "they both counterbalance
I should add that it is here, at PF, that I learned that when someone from India says "I have a doubt about your solution", he does not mean that he questions the validity of my solution. He is simply informing me that he has a question about my solution that needs further clarification. That kept my feathers from being ruffled.