Homework Help: Physics

Posted by Sarah on Monday, September 20, 2010 at 7:50pm.

The drawing shows a large cube (mass = 27 kg) being accelerated across a horizontal frictionless surface by a horizontal force vector P . A small cube (mass = 4.0 kg) is in contact with the front surface of the large cube and will slide downward unless vector P is sufficiently large. The coefficient of static friction between the cubes is 0.71. What is the smallest magnitude that vector P can have in order to keep the small cube from sliding downward?

Physics - bobpursley, Monday, September 20, 2010 at 7:53pm
how does a small cube go "downward" if both are on a horizontal surfce? I dont understand the picture painted.
Physics - Sarah, Monday, September 20, 2010 at 7:56pm
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That is what the picture looks like. Hopefully the formatting won't mess up once it is posted. Its like one cube is temporarily glued to the other.
Physics - Sarah, Monday, September 20, 2010 at 7:57pm
Since the formatting messed up...

Imagine a regular, large cube. A smaller cube is placed on the right hand side of it but it appears as if it is glued to the side of the cube a couple inches above the horizontal surface.
Physics - bobpursley, Monday, September 20, 2010 at 8:10pm
ok, I get it. force on the surface keeps it up (actually friction does.)

The normal force on the front surface of the large cube to the small cube is

forcenormal=smallcubemass*a
the friction force then holding up the small cube is forcenormal*mu

and that has to equal smallcubemass*g

so smallcubemass*g=smallcubemass*a*mu
or mu=a/g

Now, what is a? Pushing force= (total mass)a or a=P/totalmass
so we have...
mu=a/g=P/(totalmass*g)
solve for P.
Physics - Dawn, Tuesday, September 21, 2010 at 12:20am
thanks...got the right answer!
Physics - lo, Wednesday, November 3, 2010 at 7:44am
chuck norris

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