## 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?

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 Recognitions: Homework Help Looks right to me. Can you show your calculations?
 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?

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Homework Help

## Helium Balloon Question!!!

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

 Recognitions: Gold Member Science Advisor 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.)
 ok thanks for the help, i will try this
 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..

Recognitions:
Homework Help
 Quote by 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?
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.

 Recognitions: Homework Help 18L = 0.018m^3, not 18000m^3.
 lol ur rite