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

[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

 

[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! ​

 

[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?

 

[martijn-arts]

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

 

[tiny-tim]

hi 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! ​

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? ​

 

[martijn-arts]

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

 

[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? ​

 

[martijn-arts]

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

 

[tiny-tim]

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

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