How Does Throwing a Sack Affect Scale Readings?

[Opteron]

Homework Statement

A man with a mass of 100kg stands on a scale with a sack of potatoes weighing 20kg. He throws the sack straight up. At the instant before he releases the sack, what does the scale read, in kg?

Homework Equations

n/a

The Attempt at a Solution

I assumed that because the sack had sufficient velocity to overcome gravity and continue moving once released, its mass is negligible with respect to the scale, and the scale should read 100kg

 

[Dick]

20kg is not negligible compared with 100kg. Think about Newton's Third Law and forces.

 

[Opteron]

Ok not negligible, i misspoke. I assumed that since that since F(of the sack on the person)=ma and the acceleration was positive rather than negavite, the sack did not contribute anything to the force of the person on the scale.

 

[Dick]

Ok not negligible, i misspoke. I assumed that since that since F(of the sack on the person)=ma and the acceleration was positive rather than negavite, the sack did not contribute anything to the force of the person on the scale.

What are all of the forces acting on the bodies on the scale? Which two cancel?