Posted by **Anonymous** on Sunday, May 13, 2012 at 12:15am.

A block of mass m lies on a rough plane which is inclined at an angle è to the horizontal. The coefficient of static friction between the block and the plane is ì. A force of magnitude P is now applied to the block in a horizontal direction, towards the plane.

Show that the minimum value of P which is necessary to ensure that the block remains at rest on the plane, is

P=[mg(sinè-ìcosè)]/(ìsinè+cosè)

Please help! Thanks!

- Physics -
**Elena**, Sunday, May 13, 2012 at 12:15pm
Take x-axis directed along the incline downwards, and y-axis directed normaly away from the incline.

Projections of the equation of motion on the x- and y- axes are:

m•g•sinα - P•cosα – F(fr) = 0,

-m•g•cos α + N -P•sinα = 0.

F(fr) = μ •N = μ •(m•g•cos α+P•sinα).

0 = m•g•sinα - P•cosα - μ •(m•g•cos α+P•sinα) =

= m•g•sinα - P•cosα - μ •m•g•cos α+μ•P•sinα.

P = m•g•( sinα - μ•cos α)/(cos α + μ•sinα).

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