Helium Balloon Question

1. Sep 22, 2007

physicsperson

Helium Balloon Question!!!

Hey I have a buoyancy question here, anyone know how to solve it? A 15 gram rubber balloon is filled with 18 L of Helium (density 0.179kg/m^3) and tied with a string to a child's wrist. Find the tension in the cord. The answer is 0.050N but what is the solution?
Im not sure if yuo need it but the denisty of air is 1.29kg/m^3. Thanks.

2. Relevant equations
Equations i used include F buoyancy = F tension + m helium * gravity + m balloon * gravity

and F buoyancy = density * gravity * volume displaced

3. The attempt at a solution

I tried to solve but got completely different answers, what is the volume displaced? is that the right thing to use?

2. Sep 22, 2007

learningphysics

Looks right to me. Can you show your calculations?

3. Sep 22, 2007

physicsperson

F tension = (18000 m^3 * .179) (9.8) + (15/1000kg * 9.8) + Force of Bouyancy

But im not sure how to get the force of buoyancy, can anyone help?

4. Sep 22, 2007

learningphysics

I think the buoyancy force is the volume of the balloon, times the density of air * g.

5. Sep 22, 2007

HallsofIvy

Staff Emeritus
The bouyancy is the difference between the weight of 18 L of air and the weight of the balloon.

(You are TOLD that the volume is 18L.)

6. Sep 22, 2007

physicsperson

ok thanks for the help, i will try this

7. Sep 22, 2007

physicsperson

Still doesnt help. My equations must be wrong. Lets see: the downwards forces are the tension, weight of helium, and weight of balloon, and the upwards force is force of buoyancy. the question is asking for tension force. the answer is apparently 0.050 N. did anyone get that answer, cause i sure cant.thanks..

8. Sep 22, 2007

learningphysics

It should be: F tension = -(0.018 m^3 * .179) (9.8) - (15/1000kg * 9.8) + Force of Bouyancy.

fbuoyancy = 1.29*0.018*9.8

Working out the above, gives me 0.05N.

Last edited: Sep 22, 2007
9. Sep 22, 2007

learningphysics

18L = 0.018m^3, not 18000m^3.

10. Sep 22, 2007

lol ur rite