Question

Omar is standing still on the sidewalk. Aisha bumps into him as she is walking by. When are the forces on Omar balanced and when are they unbalanced in this situation?
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1 point
They are balanced when Aisha bumps into him and unbalanced when he is standing still.
They are balanced when he is standing still and unbalanced when Aisha bumps into him.
They are unbalanced when he is standing still and unbalanced when Aisha bumps into him.
They are balanced when he is standing still and balanced when Aisha bumps into him.

Answer 1

They are balanced when he is standing still and unbalanced when Aisha bumps into him.

Reason: while standing, the upward normal force equals his weight and horizontal forces sum to zero (net force = 0). When Aisha bumps him she applies a horizontal force (an unbalanced force/impulse) so the net force is not zero and his motion changes.