Question

Could someone give me some pointers on this one please?

A body is positioned at point A (5,-6, 9) and is subjected to a force F = -4i +7j +14k. Calculate the magnitude of and the direction cosines of the moment of force about the a fixed point P (3,5.-2)

Answer 1

do you have the answer?

Answer 2

no i'm afraid I don't

Answer 3

um i think you hafta find the distance betwen the two points.
multiply it with the given force vector. as moment=force applied * perpendicular dist. from the given point.
and find the magnitude and direction cosines of the vector.

Answer 4

ok that's great thankyou

Answer 5

Toby: What course is this for? Sounds like Multivariable and Vector Calculus to me, or Mechanics at the university level.

random 231 is on the right track in suggesting that you find the distance between the center about which the body would rotate if it could rotate. This would be your moment arm. Now that force is applied to the body. If we're looking for moment, then we need to find the component of the force F that is perpendicular to the moment arm at the body's location (wouldn't that be a tangent vector to the curve along which the body would move?). It occurred to me that you could do this by taking the dot product of that force with that tangent vector.