# 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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