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# Physics *sigh* watch

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1. Guys could you help me with this answer, I think it's not that hard i just can't get my fingers on the answer, thanks a millon. Oh yeah could you explain how you got the answer too!

A piece of metal weighs 50 N in air, 36 N in water and 41 N in oil. Find the densities of the following:

a)the metal

b) the oil
2. (Original post by ace_justncase)
Guys could you help me with this answer, I think it's not that hard i just can't get my fingers on the answer, thanks a millon. Oh yeah could you explain how you got the answer too!

A piece of metal weighs 50 N in air, 36 N in water and 41 N in oil. Find the densities of the following:

a)the metal

b) the oil
To be honest I suck at Physics, but I'll try and help, maybe we can work it out together.

It weighs 50N in air, so just the metal on its own has a mass of 500kg.
We know the density of water (1g/cm^3 I think..). Using density = mass/volume, 1 = 140 000 [converted from kg to g] / volume, volume of water is therefore 140 000 cm^s [assuming that I've got the density of water right, might be wrong]. Therefore, in that volume of water, the force on the metal (its weight) is 360kg (from the question). Density = mass/volume, density of metal = 360 000 [converted from kg to g]/140 000 = 2.57 g/cm^3. <= That's the density of the metal.
It weighs 410kg in oil, so we know the mass of the metal and its density, can use this to calculate the volume of the oil. The mass of the oil we don't know... Ok now I'm stuck.

Please don't trust any of what I just wrote up there, it could be all wrong, but hopefully it'll give you some idea on how to approach the question... Maybe post it in the physics forum as well?
3. (Original post by blabber92)
To be honest I suck at Physics, but I'll try and help, maybe we can work it out together.

It weighs 50N in air, so just the metal on its own has a mass of 500kg.
We know the density of water (1g/cm^3 I think..). Using density = mass/volume, 1 = 140 000 [converted from kg to g] / volume, volume of water is therefore 140 000 cm^s [assuming that I've got the density of water right, might be wrong]. Therefore, in that volume of water, the force on the metal (its weight) is 360kg (from the question). Density = mass/volume, density of metal = 360 000 [converted from kg to g]/140 000 = 2.57 g/cm^3. <= That's the density of the metal.
It weighs 410kg in oil, so we know the mass of the metal and its density, can use this to calculate the volume of the oil. The mass of the oil we don't know... Ok now I'm stuck.

Please don't trust any of what I just wrote up there, it could be all wrong, but hopefully it'll give you some idea on how to approach the question... Maybe post it in the physics forum as well?

i think it would be 5kg as weight = mass * gravity

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