Conceptual Physics Homework Questions

1. Mar 16, 2013

kevinnn

1. The problem statement, all variables and given/known data
A large barge filled with scrap iron is in a canal lock. If the iron is thrown overboard, does the water level at the side of the lock, rise, fall, or remain unchanged?

2. Relevant equations

None. I like math a lot but this class i'm taking is a prerequisite to college physics. I enjoy the class though because I do believe that the fundamental conceptual understanding this class provides for physics is great.

3. The attempt at a solution

I think the water level will remain unchanged. I draw this conclusion because as the iron is thrown in the water it displaces an amount of water equal to the mass of the iron. But as this happens the boats weight is reduced so it displaces less water because of the decrease in weight. So with all said and done there should be no change.

Thanks for the help.

2. Mar 16, 2013

phinds

Think about this. If you have a 10,000 ton volume of incredible dense material only 1 foot cube and it is in a big boat that is able to support it in the water, how much weight of water does it displace? If the same 1 cubit foot of material is put in the water, how much volume of water does it displace? How much does that volume weigh?

Would be results be different if the material was the same volume but 10 times the weight?

3. Mar 16, 2013

kevinnn

Ohhh I was on the wrong track. So to answer your questions the dense material on the boat will displace a volume of water equal to its weight. But if the object is put in the water then it will only displace an amount of water equal to its volume. It will displace one cubic foot of water.

If the material was the same volume but ten times the weight then it would displace a larger volume of water on the boat, a volume equal to its weight, but the same volume of water if it was submerged as the less heavy object, one cubic foot.

So the answer to my question is that if the iron was to be thrown over board the height of the water would decrease correct? Because the iron out of the water displaces a volume of water equal to its weight, but in water it displaces a volume of water equal to its volume.

4. Mar 16, 2013

phinds

Yes, now you've got it. The lesson here is to really think about ALL the variables when solving physical problems.

5. Mar 17, 2013

kevinnn

Great thanks for the help!