# A man on an air mattress in a swimming pool

1. Sep 25, 2007

### Smessy

1. The problem statement, all variables and given/known data
A man is lazily floating on an air mattress in a swimming pool. (a) Draw a free body
diagram for the system of the man and mattress. (b) Apply Newton’s second law to the
man and mattress system. (c) If the weight of the man and air mattress together is 806 N,
what is the upward force of the water acting on the system of man and mattress? (d)
Identify the Newton’s third law forces pairs for the forces you have in part a. (e) What
force (give both magnitude and direction) does the man and mattress exert on the water?
Explain.

2. Relevant equations
F=ma

3. The attempt at a solution

c)=806N

2. Sep 25, 2007

### learningphysics

Where are you getting stuck?

3. Sep 25, 2007

### Smessy

Well, I understand how to do the Free Body Diagram, but then after that I have no idea what to do.

4. Sep 26, 2007

### learningphysics

For part b) The acceleration of the system is 0. You have 2 forces... the net weight of the system acting downward, and the upward force of water acting on the man and the mattress...

c) yes, it is 806N

d) What is the third law pair of the weight of the system... what is the 3rd law pair of the upward force that water exerts on the system?

e) This is directly from part c) and d) and applying newton's third law.