A weight lifter struggles but manages to keep a heavy barbell above his head. Occasionally he slips and the barbell starts to fall downward, but he always recovers. Compare the force exerted by the weight lifter on the barbell to that exerted by the barbell on the weight lifter. Is one force larger than the other or are they equal in magnitude to each other? Determine does this change during the times when the barbell slips downward? Explain.