1. The problem statement, all variables and given/known data Hi, I don't know if it's the appropriate forum to post this question... Basically I've been talking with a professor at my university (a professor that will evaluate me in less than 15 days) about a problem we had on a test and I must admit I got very stubborn (I hate me for that) because or I didn't understand her or I was right. I told her the problem we had on a test was badly made. I'll show you my argument : The problem was "a bar of mass m with 2 points of mass m at its extremity is rotating with an angular velocity of [tex]\vec \omega[/tex] around an axis passing by the center of mass of the system bar-points. At a moment a point drop out the bar. 1) Calculate the angular velocity of the system bar-point. (I assumed it was rotating on an axis passing by the center of mass of the new system but I was wrong and I got a result of [tex]\vec \omega _f[/tex] greater than [tex]\vec \omega[/tex] . While according to her the angular velocity doesn't change. I trust her on this.) Then the next question really surprised me : 2) Is the energy conserved? Calculate the energy of the system before and after the mass drop." I did calculate it and I found it to be superior after the drop than before. (Which was against my intuition that says the energy is conserved. I got this false result because of my mistake in the first question). But, by calculating out [tex]\vec \omega[/tex], don't we suppose that the energy is conserved? That's what I've been telling her again and again. As they don't say "the falling point is dropped with a velocity of [tex]\vec v[/tex]" which could have let us thought about an explosive drop or a drop that "consume" energy, I concluded we simply assume that the energy is conserved. She told me that no, we cannot say this because the axis of rotation will suffer a force [tex]\vec F[/tex] (I agree with her on that... the next question was to calculate it) that could cause some loss of energy and we don't know if the body would suffer deformations. I told her that if the body suffers deformations then it's not a rigid body. She didn't change her mind. I told her that as we aren't told about any friction in any part of the body (not even it's axis of rotation), then THERE'S NO dissipative force that could consume energy. According to her I was wrong and I have to calculate the energy before and after. It was asked so I simply did it. But what I meant by all this discussion is that we could have answered the second question very simply without any calculus... Was I right? I feel a bit sad to have been so stubborn but it seems I'm like that and despite she will judge me for the final exam I stood my ground.