Density Calculation.

[fishslayer]

I got this question sent to me:

 

An 2 kg mass object is held by a spring balance in a liquid of density 1200kg/m3. What reading does the spring balance show (in N)?

 

So i wanted to cheak my that i was doing it properly can somebody cheak what i did and say. This is what i did.

 

Weight = mg

Weight = 2*10

Weight = 20

 

Upthrust = Density of liquid/ Density of Solid (mg)

 

So: Density of solid = Density of liquid (mg)

Denity of solid = 1200*20

Density of Solid = 24000

 

Apparent wieght = mg (1- density of liquid/density of solid)

= 20 (1- 1200/24000)

= 20 (1- 0.05)

= 20*0.95

= 19N

 

Is that right?

[John Cuthber]

There isn't enough information to answer the question.

 

Imagine 2 Kg of polystyrene foam hanging from a balance. If you put it in a container of water it would float and aparently weigh nothing.

[DJBruce]

You are correct in that:

 

[math]m_{b}=m_{o}\left(1-\frac{p_{f}}{p_{o}}\right)[/math]

 

However your calculation of density is incorrect. According to your calculations your density has the units of [math] \frac{kg^{2}}{m^{2}s^{2}}[/math] which is not right. Unless you have a volume for the immersed solid there is not enough information to do this question.

[fishslayer]

Ok thank you for your help (i kind have knew i couldnt be right )