Question

Find a real physical example of a mechanical structure or machine that has a moment acting on it.

(a) Make a clear, labeled drawing of the situation.

(b) Estimate the dimensions, and the magnitudes and directions of the forces that act on it. Show these on your drawing. Briefl y explain why you estimate the dimensions and forces to have the numerical values that you assigned.

(c) Select a moment pivot point, explain why you chose that point (perhaps it is important from the standpoint of the structure or machine not breaking), and estimate the moment produced about that point.

Answer 1

Example:

 A bicycle is suspended from two bungee cords (think of them as springs), as shown below. Find the position of the center of mass of the bicycle in terms of the length d and the two angles and , expressing your answer as a position vector relative to the attachment point A.

 

(a)

Diagram representation of street line pole:

 

(b)

Consider a rod AB supports the bulb, which weighs 50 N, self weight of rod AB is 10 N distributed uniformly along the length. So, weight of the rod acts at the center of AB. Due to rod and bulb weight, moment is created at pivot A, where rod is fixed to pole.

 

Weight of bulb, is 50N, weight of bulb, 10N and length of rod AB, 0.7m.

 

(c)

Weights of both rod and bulb is acting downwards and creates moment at A in clockwise direction.

 

Calculate the moment about A,

MA = WB × lAB + WAB × lAB/2

 

Here, weight of the bub is WB, length of the rod is AB and the weigh t of od AB is WAB

MA = 50 × 0.7 + 10 × 0.7/2

      = 38.5 N

 

Therefore, moment about A is 38.5 N.

Therefore, moment about A is 38.5 N.