Physics help, please?

Question

The coefficient of static friction between the m = 3.10 kg crate and the 35.0° incline of Figure P4.41 is 0.340. What minimum force must be applied to the crate perpendicular to the incline to prevent the crate from sliding down the incline?
http://www.webassign.net/sercp/p4-41alt.gif

Answer 1

In order to prevent sliding the normal force times u must be greater than the force pulling the crate down the incline

From the diagram:

The force pulling the crate is
sin(35)*m*g
The normal force is
cos(35)*m*g+F
therefore
sin(35)*m*g<(cos(35)*m*g+F)*u
or
F>(m*g*(sin(35)-
cos(35)*m*g*u))/u

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