# Homework Help: Iron and plumbum (lead) on a balance in air and water

1. Dec 28, 2012

### martijn-arts

On the flat bottom of an aquarium is an old fashioned balance. On one scale of the balance is a piece of plumbum, on the other side is a piece of iron. The scales are exactly in balance.
Now we let the aquarium fill slowly with water, until the entire balance is under water. What happens now:
A The scale with plumbum drops
B The scale with iron drops
C The scales remain in balance

2. Dec 28, 2012

### tiny-tim

welcome to pf!

hi martijn-arts! welcome to pf!

(btw, the ancient romans called it "plumbum", but we call it "lead" )

tell us what you think (and why), and then we'll comment!

3. Dec 28, 2012

### martijn-arts

Re: Iron and plumbum on a balance in air and water

Thanks!

I think lead because of the difference in kg/dm^3 of lead and iron and that of water.
Lead => 11.3 kg/dm^3
Iron => 7.86 kg/dm^3

This is both in air. Water is 0.998 kg/dm^3

Am I on the right track?

4. Dec 28, 2012

### martijn-arts

Re: Iron and plumbum on a balance in air and water

By the way, this should be the perfect question for you, looking at your profile picture :)

5. Dec 28, 2012

### tiny-tim

hi martijn-arts!
yes, i used to have one of those in my little bowl …

it's one of the reasons i eventually studied fishics!
yes, it's certainly the density that makes the difference

maybe you're right and maybe you're wrong …

what is your laws-of-physics reason for saying that the greater density should push the scale down more?

6. Dec 28, 2012

### martijn-arts

Re: Iron and plumbum on a balance in air and water

That's where I got stuck and my knowledge was too old (it's been 10 years)

7. Dec 28, 2012

### tiny-tim

aha!

ok, apply the principles of buoyancy

what are the forces on the lead (if it isn't moving), and so what is the force the lead exerts on the scale?

8. Dec 28, 2012

### martijn-arts

Re: Iron and plumbum on a balance in air and water

To be honest: I do not really understand buoyancy because of my knowledge of English.

Would you please be so kind to give the right answer with the reason

9. Dec 28, 2012

### tiny-tim

sorry, martijn-arts, on this forum we don't just give the answer

look up "bouyancy" (or "bouyant force") in wikipedia or the pf library

the important thing is that there is an extra force on the lead (the bouyant force), which acts through the centre of gravity of the displaced water