# Kinetic friction

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.

## Answers and Replies

Doc Al
Mentor
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.