*64.  A block is pressed against a vertical wall by a force , as the drawing shows. This force can either push the block upward at a constant velocity or allow it to slide downward at a constant velocity. The magnitude of the force is different in the two cases, while the directional angle q  is the same. Kinetic friction exists between the block and the wall, and the coefficient of kinetic friction is 0.250. The weight of the block is 39.0 N, and the directional angle for the force   is q = 30.0°. Determine the magnitude of  when the block slides (a) up the wall and (b) down the wall.

Consider two free body diagrams, one for the block going up wall and one for the block sliding down the wall.

For block going up wall sum of force equations are

 

Since block is not accelerating horizontally

With the angle defined as it is in the picture, the horizontal or x component is opposite of the angle and therefore goes with Sin instead of the usual Cos

 

 

Since we are told the block is traveling up the wall with constant speed.

 

 

 

 

 

For block going down wall sum of force equations are

 

Since block is not accelerating horizontally No real change

 

 

Since we are told the block is traveling up the wall with constant speed.

 

 

 

 

 

  Going Up

  Going Down

 

 

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This page last updated on January 11, 2020