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help in physics

A man of 75 kg is pushing a heavy box along a flat floor. The coefficient sliding friction between the floor and the box is 0.20, and the coefficient of static friction
between the man’s shoes and the floor is 0.80.
(a) If the man pushes downward on the box at an angle of 30◦ what is the maximum mass of the box he can move? (3 marks)
(b) If the man pushes upward on the box at an angle of 30◦
, what is the maximum mass of the box he can move?

Reply 1

Physics Enemy

Original post by studentzz123

...

30° to the horiz or vert? In a), man's normal reaction decreases by Fsin30 (if 30° is to the horiz) and box's increases by an equal amount. For max mass, box is about to shift: friction uR for both, each has net horiz force = 0. Man feels reaction Fcos30 by the box, he's about to move back. b) is basically role reversal of a).