Physics "wooden blocks" question?

A wooden block of mass m = 9 kg is at rest on an inclined plane sloped at an angle q from the horizontal. The block is located 5m from the bottom of the plane





(a) Find the magnitude of the frictional force for q = 20°.





(b) If the angle is increased slowly the block starts sliding at an angle of q = 30°. What is the coefficient of static friction?





(c) If the block slides to the bottom in t = 2 s, (q = 30°), what is the kinetic frictional force on the block?

Physics "wooden blocks" question?
(a) If the block doesn't move then mgsinq = Friction


Friction = 9*9.81*sin20 = 30.20 Newtons





(b) At 30 degrees, Friction = mgsin30 = 44.15 Newtons


This = µR


R, the reaction force = mgCos30 = 76.46





µ = 44.15 / 76.46 = 0.58





(c) Use s=ut+1/2at^2


u=0, t=2, s=5, giving a=2.5ms^-2





Now use F=ma


F-friction = ma


mgsin30-friction = ma


9*9.81sin30 - Friction = 3*2.5


Friction = 36.65 Newtons





Friction=µR


µ = friction / R = 36.65/mgcos30 = 0.48

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