# Mass Relative to a scale.

1. May 4, 2012

### Opteron

1. The problem statement, all variables and given/known data
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?

2. Relevant equations

n/a

3. 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

Last edited: May 4, 2012
2. May 4, 2012

### Dick

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

3. May 4, 2012

### 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.

4. May 4, 2012

### Dick

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