Engineering Statics

E-11


The 20 lb. block, A, hangs from a cable as shown. Pulley C is connected by a short link to block E, which rests on a horizontal rail. Knowing that the coefficient of static fric;tion between block E and the rail is .35 and neglecting the weight of block E and the friction of the pulleys, determine the maximum allowable value of q if the system is to remain in equilibrium.

The FBD of the block and pulleys and cable is drawn.
Since two cables are attached to the pulley connected to 
the 20 lb. weight, the tension in the cable must be 10 lb.
If we look at the sliding block the force equation in 
the x-direction can be written.  Note that although the 
block E is considered weightless and normal force appears 
because the tension has a vertical component.
The force equation can be written in the y-direction.
If we solve for N in the second equation and substitute 
into the second equation we obtain: