- #1

arhzz

- 241

- 46

- Homework Statement:
- A child m = 35 kg wants to buy balloons so that he can take off. How many balloons with a radius of 0.25 m full of helium,does he need to buy so that he can float away.The thickness of helium is 0,178 kg/m^3 and the thickness of the air is 1,29 kg/m^3 The ballons are shaped as a sphere

- Relevant Equations:
- Lift equation

Hello! So my thought process is in order to achive lift the lift force has to equal the force that is pulling the child to the ground.So the force pulling the child down we can calculate like this

$$ F = mg $$ S

F should be 343,35 N

Now to calculate the lift I've used this formula $$ p = \frac{F}{A} $$ Now we can write F like this ## F = \rho * g * V ##

Now here is where I am kind of having a dilema.I'm prett sure I need to calculate the Volume and Area of the balon (probably why the r was given) and I've done that;

$$ V = 0,065 m^3$$

$$ A = 0,785 m^2$$

Now if i put these back into my equation for p I get p = 0,144 N (assuming I've done everything correctly in the calculation). Now obviously this isn't really what I was expecting to happen.Where its the flaw in my logic? I've also noticed I haven't used the thickness of air.By logic it should also play a role into the lift force but I'm not really able to wrap my head around all of this. Any help would be great.

Thank you!

$$ F = mg $$ S

F should be 343,35 N

Now to calculate the lift I've used this formula $$ p = \frac{F}{A} $$ Now we can write F like this ## F = \rho * g * V ##

Now here is where I am kind of having a dilema.I'm prett sure I need to calculate the Volume and Area of the balon (probably why the r was given) and I've done that;

$$ V = 0,065 m^3$$

$$ A = 0,785 m^2$$

Now if i put these back into my equation for p I get p = 0,144 N (assuming I've done everything correctly in the calculation). Now obviously this isn't really what I was expecting to happen.Where its the flaw in my logic? I've also noticed I haven't used the thickness of air.By logic it should also play a role into the lift force but I'm not really able to wrap my head around all of this. Any help would be great.

Thank you!