Every layer of a fluid contributes to the buoyant force. The weights of each fluid that is displaced add together to give the total buoyant force. The buoyant force of the air is almost nothing compared to that of water, but it can be calculated. The 22% of the block that was originally in the air displaces some weight of air, so it gets a little buoyant force from that; most of the buoynat force is from the water. The combined weight of displaced water and air is the weight of the block. When the oil is added, the two fluids contributing a buoyant force are the water and the oil. Their combined displaced weight has to be the weight of the block. You need to figure out what fraction of the block is in water and what fraction is in oil.physgirl said:So there's a wooden block floating on water in such way that 78% of the block is submerged in water. Now oil (with density of 700 kg/m^3) is poured on top so that the wooden block is wholly covered by the oil. Now how much of the block is submerged in water? (Hint: do not ignore the buoyant force of air before the oil is submerged)
I'm not really sure where to go with this question :(
You know the density of the oil and you can look up the density of the water and air. You can find the density of the block from the information given. Everything is proportional. You can assume any volume you want for the block. The volume will divide out.physgirl said:but how can you find the buoyant force that a fluid is exerting if you don't have the volume of the fluid? the numbers i gave on the original post are the only numbers that are given in the problem...
The hint you were given about the air is telling you that you cannot use the 78% of the block submerged in water to conclude the density of the block is 78% of the density of water. Instead, you have to use 78% of the volume of the block displacing water plus 22% of the volume of the block displacing air and add together the weight of the water and the air to equal the weight of the block. It's not a big difference, but the concept is important for doing the rest of the problem. You will apply the same reasoning when the block is surrounded by water and oil instead of water and air.physgirl said:wait... I am confused. so let the volume of block be "x"... and the density of the block is 780 kg/m3 (0.78*1000)... and an object floats when the weight of fluid displaced equals the weight of the object... the weight of the object is "(780)(9.8)x"... so that's also the weight of the water displaced... and now what?
Buoyant force is the upward force exerted on an object immersed in a fluid, caused by the displacement of fluid by the object.
Buoyant force is calculated by multiplying the volume of the fluid displaced by the object and the density of the fluid.
This is due to the difference in density between the two fluids. Oil is less dense than water, so the buoyant force exerted on the wooden block is greater in oil than in water, causing it to float in oil but sink in water.
Yes, the buoyant force on an object can change if the density of the fluid or the volume of the object changes.
The shape of an object does not affect the buoyant force, as long as the object displaces the same volume of fluid. This is because the buoyant force is dependent on the volume of fluid displaced, not the shape of the object.