Homework Help: Physics

Posted by Litany on Friday, November 13, 2009 at 6:02am.

The drawing shows a large cube (mass = 25 kg) being accelerated across a horizontal frictionless surface by a horizontal force . A small cube (mass = 4.0 kg) is in contact with the front surface of the large cube and will slide downward unless is sufficiently large. The coefficient of static friction between the cubes is 0.71. What is the smallest magnitude that can have in order to keep the small cube from sliding downward?

Physics - supersmart, Sunday, October 17, 2010 at 2:54pm
the answer is 400 N
Physics - physics_savier :P, Sunday, September 25, 2011 at 8:59am
first we find the normal force on the small cube:
F=ma (where f=force,a=acceleration,m=mass)
a=f/m
acceleration on small box is same as that on the large box. let force to find be P.
then:
a=p/(25kg+4kg)
a=p/(29kg)m/s^2
force acting on small box:
f=ma
f=4*(p/29)N(normal force)
friction force= Us*(normal force) Us is coeff. of static friction.

friction= 0.71*(4*p/29)
weight= mg= 4*9.8
for the object(small box) not to slide down the friction force b/w the two objects have to be exactly the same as the weight of the object.
0.71*(4*p/29)=4*9.8
solving..
p= 400.29N

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