Ahorizontal force of 90.0 n is required to push a 75.0 kg along a horizontal surface at a constant speed. what is the magnitude of the force of friction (90.0n)

== You're pushing the load straight along the horizontal surface at a constant speed.

== The speed and direction of the load are not changing, so there is no acceleration.

== From Newton's 2nd law of motion, we know that  F = m A .  Since A=0, F must also be zero.  That means NO NET FORCE is acting on the load.

== But how can that be ? You're pushing it with 90 N !

== The NET force is zero.  So something else must be pushing the box, also with 90 N of force, exactly opposite to YOUR force, and they add up to zero.

== THERE's your force of friction !

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