How Does Throwing a Sack Affect Scale Readings?

AI Thread Summary
When the man throws the 20kg sack upwards, the scale initially reads 120kg, reflecting both his weight and the sack's weight. As he releases the sack, the forces acting on the scale include his weight and the force exerted by the sack. According to Newton's Third Law, the force of the sack on the man is equal and opposite to the force of the man on the sack, leading to a momentary decrease in scale reading. The scale will read 100kg at the instant before the sack leaves his hand, as the upward force exerted by him on the sack does not contribute to the scale's reading. Understanding the forces involved clarifies the dynamics of the situation.
Opteron
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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
 
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20kg is not negligible compared with 100kg. Think about Newton's Third Law and forces.
 
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.
 
Opteron said:
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?
 

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