Engineering Statics

E-14


A force of 500 N is applied to the end of the cord that is attached to block A. Block A weighs 2000 N and block B weighs 4000 N. The coefficient of friction between the blocks is 0.3 while the coefficient of friction between B and the floor is 0.2. (a) Determine if the system would be in equilibrium for P=2500N, (b) Determine the maximum P for which the system is in equilibrium.

The FBDs for the upper and lower blocks are drawn.  Notice 
again how forces on touching members are equal and opposite 
in direction.
The force equation in the y-direction for the upper block 
yields the normal force between the blocks.
The force equation in the y-direction for the lower block 
yields the normal force between the block and the floor.
The force equation in the x-direction for the lower block 
is written.
The applied force P is found to be 875 N.  This means that 
for any force greater than that number the system will become 
dynamic.  This is the greatest force which may be applied.