# Homework Help: Barbell Underwater

1. Dec 8, 2004

### SnowOwl18

-----A bodybuilder is holding a 29.9kg steel barbell above her head. How much force would she have to exert if the barbell were lifted underwater?----

Ok...I'm really not sure of this one. I thought maybe the answer was the weight of the barbell (mg) plus the atmospheric pressure at sea level...does anyone know if I'm headed in the right direction? Thanks.

2. Dec 8, 2004

### Staff: Mentor

buoyant force

Consider the buoyant force that the water exerts on the barbell. This force acts upwards, making the barbell easier to lift under water.

3. Dec 8, 2004

### SnowOwl18

most equations say that the buoyant force= density of water x volume of object x gravity. but i do not know the volume...i do know that mass= density x volume...but if that is so, couldn't i theoretically just multiply the mass of the barbell by gravity to get the answer?

4. Dec 8, 2004

### Staff: Mentor

No. One equation uses the density of water; the other, the density of steel. (But you're getting closer.)

5. Dec 8, 2004

### SnowOwl18

Steel...I should've seen that. Well, what I tried to do was find the volume of steel using what I knew... 29.9kg= 7860kg/m^3 x V ...and for that value I got 0.0038m^3...and so I multiplied that by the density of water and then by gravity to get the buoyant force and got 37.28N, but the program says it's wrong. And I thought I was headed in the right direction...lol. Is there another huge mistake I'm making?

6. Dec 8, 2004

### Staff: Mentor

That's the correct value for the buoyant force. So, how does that relate to the force that the bodybuilder needs to exert on the barbell?

7. Dec 8, 2004

### SnowOwl18

Well...the buoyant force is the weight of the displaced water...aha! I have it. You subtract the buoyant force from the force that would normally be exerted above water (mg). Thank you for your help! :D