Kinetic friction

  • Thread starter devnull
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5. The drawing shows a large cube (mass = 23 kg) being accelerated across a horizontal frictionless surface by a horizontal force P. A small cube (mass = 3.2 kg) is in contact with the front surface of the large cube and will slide downward unless P is sufficiently large. The coefficient of static friction between the cubes is 0.71. What is the smallest magnitude that P can have in order to keep the small cube from sliding downward?


I'm having trouble writing a force diagram. I know the formulas to use but I don't know which direction the forces are in. Any help would be appreciated.
 

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Doc Al
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Static friction, not kinetic

Originally posted by devnull
I'm having trouble writing a force diagram. I know the formulas to use but I don't know which direction the forces are in.
Give it a shot. Take each cube and write down all the forces acting on it. (Don't forget Newton's 3rd law.)

Hint: Besides the applied force P, I see three "kinds" of forces: weight, friction, and normal forces.
 

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