1. Apr 11, 2010

### lenarass

1. The problem statement, all variables and given/known data

A hollow but water-tight cubic box of wood, 1.2m on each side, weighs 961 N in air. It is placed in fresh water lake tied to an anchor made of solid iron (1m x 1m x 10 cm) with a weightless cord of such length that exactly half of the wooden box is submerged. What is the tension in the cord?

2. Relevant equations

i dont know how to start.if u can help me a little bit and then try together i will appreciate it!:)

3. The attempt at a solution

sorry:( i just need a little bit help

2. Apr 11, 2010

### kuruman

Start by drawing a free body diagram of the box showing all the forces acting on it. How many are there and in what direction does each force act? What is the net force on the box?

3. Apr 12, 2010

### lenarass

i did that but then? i dont know ..thx for helping me kuruman
why it says bout volume of anchor. why it gives me the dimension of each side of the box? (1.2)
how do u use them:/

4. Apr 12, 2010

### Rajini

when you tie you object/wood in air it weighs 961 N. That means the tension on the rope is 961 N. Now you find the same when the wood is tied to solid iron, and also when it is immersed in water...that is all you need...just apply buoyancy + Archimedes rule..
good luck

5. Apr 12, 2010

### kuruman

What is the buoyant force according to Archimedes' principle?

6. Apr 13, 2010

### lenarass

Volume of cube = 1.2^3 = 1.728 m^3

then divide by 2 because the cub is half in water so 1.728/2= 0.864 m^3

water displaced => m (mass) = V(volume) * p (density of water)
so

0.864 x 1000 = 864 kg

864 x 9.8 = 8467.2 N

so apparent weight of cube is 961 - 8467.2 = -7506.2 N

so tension in cord is -7506.2 N ?? ?

7. Apr 13, 2010

### Rajini

negative value means compression!
What happened to the iron solid?? include that also

8. Apr 13, 2010

### lenarass

what do u mean? should i include volume of anchor also? thats why he gave me its dimensios?

9. Apr 13, 2010