Final answer:
The value of M • r, where M is the torque obtained from the cross product of the position vector r and force F, is zero because a vector is always perpendicular to its cross product with another vector. Therefore, the correct answer is option A) 0
Step-by-step explanation:
If the equation M = r x F represents the torque (M) being the cross product of the position vector r and the force F, and we want to find the value of M • r (the dot product of torque and position vector), then the answer is A) 0. This is because the cross product of two vectors is perpendicular to the plane containing the original vectors.
Thus, the dot product of the torque and the original position vector r will be zero, since the angle between a vector and a perpendicular vector is 90 degrees, and the cosine of 90 degrees is zero, making the product of their magnitudes times the cosine of the angle between them zero.
The value of M . r can be found by substituting the given value of M=r x F into the expression. We have M . r = (r x F) . r = r^2 x F. Therefore, the value of M . r is r^2 x F which corresponds to option C.
Therefore, the correct answer is option A) 0