# Homework Help: Density 1

1. Jul 3, 2009

### Miri

1. The problem statement, all variables and given/known data
The mass of an unfilled balloon is 7.5g. How much helium has to be filled in the balloon to make it float in air?
density of helium: 0.18kg/m^3
density of air: 1.29kg/m^3

2. Relevant equations
When I calculate the volume like this: (7.5*10^-3kg)/(0.18kg/m^3)=0.041m^3 and than I don't know what to do with the density and the volume of water...Solution would be 1.2g

2. Jul 3, 2009

### Staff: Mentor

When will the balloon float?

3. Jul 3, 2009

### Miri

When it has 1.2g helium in it. But I don't know how I can calculate this...

4. Jul 3, 2009

### RoyalCat

What are the conditions required for the balloon to float? What should the net force on it be in order for it to float?

5. Jul 3, 2009

### Miri

I don't know what the conditions are. I just know that it has to float. And why do we need a net force? And how would we calculate it? Thx for your patience

6. Jul 3, 2009

### Staff: Mentor

What forces act on balloon?

7. Jul 3, 2009

### LowlyPion

When your balloon is filled with helium, how much less mass is in it per unit volume than if it was filled with air?

Think buoyancy. Displaced air ...

When you know what volume you need to lift the mass of the balloon shell itself, then you can figure how much helium is in that volume can't you?

8. Jul 4, 2009

### Miri

So the volume needed to lift the mass of the balloon shell itself is calculated like this: 7.5*10^-3kg / 1.29kg/m^3=0.006m^3. But what's the point now? Should I calculate 7.5*10^-3kg / 0.18kg/m^3=0.04m^3 and then??

9. Jul 4, 2009

### LowlyPion

No. Not like that.

Think of the difference in density needed.

Air is 1.29 kg/m3. If you replace it with helium then that means that you have a buoyancy of (1.29 - .18) or 1.11 kg/m3.

The volume needed to lift the .0075 kg of the balloon itself is what you want. TRhen figure the mass of helium in that volume.