please help

[lilei]

I am completely lost with these problems and would appreiciate help.

1. a 2-kg balloon is filled with 5-m3 helium tied to the ground by a cord. if the density of the helium in balloon is pHE=.2kg/m3 and density of air outisde the balloon is pAIR=1.3 kg/m3. Determine the tension of the cord?

2. A 5-kg book is resting on a horizontal tabletop. A force is then applied parallel to the top of the book. if the coefficient of static friction between the book and the tabletop is .6, determine how much the top of the book will shear before the book starts to slide along the tabletop. assume thickness of the book is 3cm, face of the book is 20 cm by 25 cm, and the shear modulus is S=4410 Pa.

[Crosson]

The ballon of certain density is submerged in a fluid (air), use archimedes principle:

The force of bouyuncy on an object submerged in a fluid is equal to the weight of the fluid that is displaced by the object.

[lilei]

thanks !!!
i never thought of air being fluid.
Greatly helps. :blushing:

[Doc Al]

Problem 2 requires understanding the definition of shear modulus (S):
S = (shear stress)/(shear strain) = (F/A)/(\Delta x/h)

Check your text to find out what these terms mean (your book probably uses different notation). \Delta x is what you are trying to find.