# Homework Help: Does placing something in water affect the water's weight?

1. Jan 9, 2010

### Dmitri10

"A fire truck carrying a load of fire fighters and a large tank of water is about to cross a bridge that may not support the load. The chief suggests that some of the fire fighters get into the tank so the load will be less. Is this a good idea? Explain."

I have no idea where to begin. I know that the fire fighters will displace a volume of water equal to their own, but I don't know if that would change the weight of the whole system. Any help or leads would be appreciated.

2. Jan 9, 2010

### JaWiB

The weight of the system depends on the total mass of the system. By placing firefighters in the tank, does its mass change?

3. Jan 9, 2010

### ideasrule

Does the mass of something change when you rearrange its components? If you move a book from the side pocket of your backpack to the main pocket, is the backpack going to get lighter/heavier?

4. Jan 9, 2010

### Dmitri10

No... the mass of the system doesn't change. That's what I initially thought, but then I proceeded to confuse myself by trying to reason that if something were suspended in a liquid, the liquid exerts an upward buoyant force that balances out the downward force of gravity. If the downward force of gravity (the weight) of one of the objects is being balanced out, won't the weight of the system change?

5. Jan 9, 2010

### ideasrule

When the firefighter was sitting on his seat, his weight was balanced out by the normal force of the seat. If he goes into the tank, it'll be balanced out by buoyancy.

6. Jan 9, 2010

### Dmitri10

Your logic is sound. Thanks for the help!