Internal forces do not perform any work in the virtual displacement. This is not based on Newton's III law. In fact, the III law is a result of this rather than the cause. As you read the equation on slide 34 from left to right, you will find that we have not employed in that relation that action and reaction are equal and opposite. As the argument continues on slide 35, you will find that for the result on the first equation of slide 35 to hold for an *ARBITRARY* displacement (and this is the key issue), it follows that the sum of all the internal forces vanishes, which is the conclusion reached at the bottom of slide 35 - the conclusion being that the total momentum is independent of time. Newton's III law then follows from this as we find on slide 36.
Prof. P.C. Deshmukh Date: