Engr Help

Forces and Moments - continued- (Vectors)

Key Concepts: The moment of a force about a designated point is just the magnitude

of the force times the perpendicular distance from the point to the line of action of the

force whereas the moment of a force about a line is just the dot product of the moment

of the force about any point on the line with a unit vector along the line.

In a Nut Shell:

Forces acting on structures can cause a “twisting” action on the structure. A good

example is the use of a wrench to tighten the head of a bolt. Note that the moment of

a force can be calculated about a designated point or about a designated line.

Example: Calculate the moment, M, of force, F, about point P. The line of action of

the force, F, is from O to Q. The perpendicular distance from point P to the line of

action of the force is PQ. So the moment of the force about point P is PQ x | F |.

M = ( r sin θ) F = | r x F | Note r sin θ is perpendicular distance from point P

to the line of action of the force F

M = r x F English units are lb ft and Metric units are Nm.