You are Here: Home >< Physics

# calculating upthrust? (really basic physics question) watch

1. What is the upthrust on a 1cm diameter steel ball in water?
Density of steel = 8100kgm^-3
Density of water = 1000kgm^-3

if i were given the coefficient of viscosity id probably be able to work this out. any ideas?

2. Upthrust = Weight of Fluid Displaced
Hence , mass of water displaced = density * volume = (1000) * (4/3 pi (1/100)^3)
Hence weight displaced = mass * g = (1000) * (4/3 pi (1/100)^3) * 9.81 = Upthrust

Coefficient of visc needed only for drag force....

Upthrust + Drag = Weight of ball
[at terminal velocity]
3. (Original post by antonfigo)
Upthrust = Weight of Fluid Displaced
Hence , mass of water displaced = density * volume = (1000) * (4/3 pi (1/100)^3)
Hence weight displaced = mass * g = (1000) * (4/3 pi (1/100)^3) * 9.81 = Upthrust

Coefficient of visc needed only for drag force....

Upthrust + Drag = Weight of ball
[at terminal velocity]
hmm the answer should be 5.1 x 10^-3.

is anything supposed to be done with the steel density value?
4. 5.1x10^-3 or 4.1x10^-3
Remember to convert 1 cm to 1/100 m

No...Steel density value is not needed for upthrust...but we need steel ball's volume which is equal to volume of water displaced...
5. antonfigo has used 1cm in the calculation, but this is the diameter of the sphere. The formula requires the radius; half of this.
6. (Original post by Stonebridge)
antonfigo has used 1cm in the calculation, but this is the diameter of the sphere. The formula requires the radius; half of this.
Oh yea!
I overlooked that :P ...now theres the real answer
7. 1. Calculate the volume of the displaced liquid (volume of the block or object)
2. Calculate the mass of the displaced liquid using the formula=density x volume
3. Calculate the weight of the displaced liquid using the formula weight=massx10
Always write it down in Newton or N in kilograms so if the answer is 1.0x20=20grams its 0.020 kg then you use the formula weight = massx10 so the answer is: 0.20N
8. (Original post by Ripiramon)
1. Calculate the volume of the displaced liquid (volume of the block or object)
2. Calculate the mass of the displaced liquid using the formula=density x volume
3. Calculate the weight of the displaced liquid using the formula weight=massx10
Always write it down in Newton or N in kilograms so if the answer is 1.0x20=20grams its 0.020 kg then you use the formula weight = massx10 so the answer is: 0.20N
Welcome to TSR physics.

Thanks for this.

This thread was started in 2010 and is now closed.

### Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: January 21, 2015
The home of Results and Clearing

### 4,396

people online now

### 1,567,000

students helped last year
Today on TSR

### IT'S TODAY!

A-level results chat here

### University open days

1. Bournemouth University
Fri, 17 Aug '18
2. University of Bolton
Fri, 17 Aug '18
3. Bishop Grosseteste University