Calculating Buoyant Force: Ice in Water

[Jtappan]

Homework Statement

(a) What is the buoyant force on 0.70 kg of ice floating freely in liquid water?
_____ N upward

(b) What is the buoyant force on 0.70 kg of ice that is held completely submerged under water?
______N upward

Homework Equations

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The Attempt at a Solution

How do you calculate the buoyant force of an object in another object as it states here?

 

[Dick]

The buoyant force is equal to the weight of the water displaced. Is that enough to get started?

 

[BlackWyvern]

The buoyant force is equal to the weight of the water that is displaced.
If we look at the first problem, we can see that it's in equilibrium, which means all the forces are equal. Should be easy to solve the first one now.

The second one, I think you have to assume that the ice's density is the same as the waters. Because just from the mass, that doesn't help us. A 0.70kg piece of gold will have far less buoyant forces than a 0.70kg piece of ice. Since there's no other information here, we'll have to assume the ice's density.

 

[Galileo's Ghost]

Objects will sink until the bouyant force is equal to the object's weight. The size of the bouyant force is equal to the weight of the displaced water.

 

[Jtappan]

ok i got the first one, now i am still confused on how to find it if it is held under the water.

 

[BlackWyvern]

Density = mass/volume
You should know the density of water.

The density of ice is more, but using the density of water should be alright here. Unless they've given you it's density.

 

[Jtappan]

so it is the density of water = mass of ice cube/volume of ?? what

 

[BlackWyvern]

Volume of the ice cube, if it's density were equal to that of water, which it almost is.
Then refer back to the question with your new information, and you can solve it.

 

[Jtappan]

i am still not getting the right answer..I put it into the equation (rho)(gravity)(volume)

 

[Jtappan]

got it.. thanks everyone for the help!