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

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
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 Blog Entries: 27 Recognitions: Gold Member Homework Help Science Advisor 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!
 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?

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

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

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hi martijn-arts!
 Quote by martijn-arts By the way, this should be the perfect question for you, looking at your profile picture :)
yes, i used to have one of those in my little bowl …
it's one of the reasons i eventually studied fishics!
 Quote by martijn-arts 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?
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?
 That's where I got stuck and my knowledge was too old (it's been 10 years)
 Blog Entries: 27 Recognitions: Gold Member Homework Help Science Advisor 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?
 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

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