Minimum Force to Make an Object Move

[uchicago2012]

Homework Statement

A 6 kg object sits on a horizontal surface with coefficient of friction = .8. What is the minimum force (not necessarily horizontal) necessary to make the object slide?

Homework Equations

F_smax = u * F_n
where F_smax is the force that must be overcome to make an object move, u is the coefficient of friction, and F_n is the normal force.

The Attempt at a Solution

I was just wondering if this seemed correct:
F_g = -58.8 N (mass * g) so
F_n = 58.8 N
I'm not clear on when exactly one can make the assumption that F_g = F_n
but then using that value of F_n I calculated F_smax, so the minimum force necessary to make the block move is any force greater than F_smax.

 

[stevenbhester]

You need to overcome the force of friction.

The force of friction is
Ff=(coefficent of friction)(weight of object in Newtons)
Ff=(.8)(6*9.81)
Ff=(.8)(58.86)
Ff=47.088N

I think :D

 

[cepheid]

I'm not clear on when exactly one can make the assumption that F_g = F_n

One can always make the assumption that, if the acceleration in some coordinate direction is zero, then by Newton's second law, the sum of all forces acting along that direction must be zero.

This condition for static equilibrium is the foundation for all introductory statics problems.