Calculating Buoyant Force of Ice in Water: Homework Practice

[mg1977]

1. a)Homework Statement What is the buyant force on 0.90kg of a block of ice floating on liquid water? B) What is the buyant force on 0.90kg of ice held completely submerged under water?

Homework Equations

Fbuy+mg=0
Fb=mg

The Attempt at a Solution

Fb=.90kg * 9.18m/s^2=8.8
so i figured out part a, but part b isn't so easy

 

[berkeman]

1. a)Homework Statement What is the buyant force on 0.90kg of a block of ice floating on liquid water? B) What is the buyant force on 0.90kg of ice held completely submerged under water?

Homework Equations

Fbuy+mg=0
Fb=mg

The Attempt at a Solution

Fb=.90kg * 9.18m/s^2=8.8
so i figured out part a, but part b isn't so easy

For part b, you need to look up the density of ice. Since it is less dense than water, you need the density to figure out the volume of water that is displaced by the block when it is held underwater.

 

[mg1977]

So what is the density of ice and how do I find the volume of water displaced by the block?

 

[berkeman]

So what is the density of ice and how do I find the volume of water displaced by the block?

There are lots of places you can look up the density of ice. There's a popular search engine that should be of help...

And if you have the weight and the density, I hope you can figure out the volume...

 

[mg1977]

I ended up with V=m*d is 917 kg/m3 *.90kg= 825.3 so now that i got the volume do i find the rest in formula, please forgive me i need to see symbols to figure this out.

 

[berkeman]

I ended up with V=m*d is 917 kg/m3 *.90kg= 825.3 so now that i got the volume do i find the rest in formula, please forgive me i need to see symbols to figure this out.

Your equation is not correct. Volume has units of m^3, mass has units of kg, and density has units of kg/m^3. Use the units to help check whether the equation you wrote is correct.

It's a good idea to carry units along in all of your calculations, to keep checking that you haven't made an error along the way (dropped a quantity, for example). You multiply and divide and cancel the units as you go, and the units of the LHS of the equation have to match those on the RHS.

 

[mg1977]

Could you help me figure this problem out I am confused...Please show me how to do this.

 

[berkeman]

Could you help me figure this problem out I am confused...Please show me how to do this.

We can't do your homework for you, but I'll try to help a bit more. On your equation, do you see why it's not correct to write V = m * d ? That would give you units of m^3 on the LHS, and units of kg * kg/m^3 = kg^2/m^3 on the RHS. How can you re-arrange the equation to get the same units on both sides? Besides, your book must have the correct form of this equation, no?

 

[mg1977]

all it says is that d=m/V so what i did was solve for V. There isn't a clear cut example unless I'm missing something which is possible.

 

[berkeman]

all it says is that d=m/V so what i did was solve for V. There isn't a clear cut example unless I'm missing something which is possible.

You started with d = m/V

and solved for V and got V = m*d

Do you see the problem? No biggie, but you need to get the right form of the equation for V = ____ before you can use it to solve this problem.

Once you figure out the volume that the ice displaces, you figure out the weight of that amount of water, and that's the answer for the last part of this problem. Remember to carry units along in your calculation to be sure your equations are consistent

 

[iRaid]

917 kg/m^3 is the density of ice.
Use the summation of forces to get Fb = Fg -- You need to substitute:
pvg = mg
pv = m
(p = density)