Application of Newton's Laws of motion

[Aggie]

A block of mass 20kg is pushed against a vertical surface at an angle of 30 degrees as shown. The coefficient of friction between the surface and the block is 0.2. What is the minimum magitude of P to hold the block still?


I got 271.5 when I tried but the answer is 202.2

 

[Edgardo]

How did you come to the 271.5?

 

[H_man]

Have you taken into account the fact that the frictional force will depend also on P, ie how strongly it is being pressed against the wall?

 

[Aggie]

I came up withthe equation

mg+ Fr = mg cos theta

 

[H_man]

That equation does not look right. Where is P?

Also how do you determine Fr. You need another equation right?

 

[kritnu]

You guys should try Laws of Physics app on iPhone http://itunes.apple.com/in/app/laws-of-physics/id444709628?mt=8