Physics 11

A 140 kg stationary crate is pulled by a force of 390 N along a horizontal surface.
a) The coefficient of static friction between crate and surface is s = 0.25. Is the applied
force large enough to begin moving the crate? Use physics calculations to prove that it is.
b) The coefficient of sliding friction between crate and surface is k = 0.17. Find the net
force and the acceleration of the crate once it does move.

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  1. (a) Normal force, N= 140g N
    μs=0.25
    frictional force to move crate
    = μN
    = 0.25*140g
    = 343.35
    Assuming force of 340N is acting horizontally, crate will move since 390N > 343.35 N

    (b) μk=0.17
    kinetic frictional force
    = μkN
    = 0.17*340*9.81
    = 233.48 N
    Net force causing acceleration
    =390-233.48
    =156.52 N

    Newton's second law: F=ma =>
    a=F/m
    =156.52/140
    =1.12 m/s²

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