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DYNAMICS - EXAMPLE


Box Force
  Example

 

A box that weighs 100 lb is pushed along a surface by a variable force, F = 25 t0.5 lb. The coefficient of friction between the box and the floor is 0.25 for both static and kinetic friction. What is the velocity of the box after 2 seconds? Assume the box starts from rest.

 

   
    Solution


Impulse Action on Box

 

It is important first to note that t he block will not move until the force is large enough to overcome static friction. Thus, the exact time the box first moves needs to be determined.

The block will move when the force is equal to friction force to move. Summing the forces in the x-direction, gives,

     25 t0.5 - f = 0
     μN = 25 t0.5
      0.25 (100) = 25 t0.5

     t = 1.0 s

The motion of the box will depend on the linear impulse between 1 and 2 seconds.

     
     (1.8856 - 2) - (0.6667 - 1) = 0.1242 v2

v2 = 1.763 ft/s

     
   
 
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