Polly is pushing a box to the right across the floor with a force of 30 cap n The force of gravity is 8 cap n , and the normal force is 8 cap n Which value could describe the force of friction if Polly could not move the box?

If the box does not move, the horizontal forces must balance. Polly pushes with 30 N to the right, so friction must exert 30 N to the left. The correct choice is left 30 N; the 8 N forces act vertically and do not balance the push.

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