Flashcards · 7357 · Topic 20

S: Moments flashcards

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Light (modelling word): No weight.
Uniform (modelling word): Weight at the midpoint.
Non-uniform (modelling word): Weight at an unknown point; find it with moments.
Rigid (modelling word): Does not bend.
Moment of a force about a point: Force × perpendicular distance from the point to the line of action of the force, in N m, with a sense (clockwise or anticlockwise).
Conditions for a rigid body to be in equilibrium: The resultant force is zero, and the total moment about any point is zero (clockwise moments = anticlockwise moments).
A beam on two supports is about to tilt about support D. What do you use?: The reaction at the other support is zero. Take moments about D.
A uniform rod AB, 2 m long and weight 40 N, hangs horizontally from vertical strings at A and B. A 20 N load is 0.5 m from A. Find the tensions.: Moments about A: 2T_B = 40 × 1 + 20 × 0.5 = 50, so T_B = 25 N. Vertically: T_A = 60 − 25 = 35 N.
Choosing the point to take moments about: Take moments about the point where most unknown forces act: their moments are zero there, so they drop out of the equation.