What is Buoyancy Force?

Question

And how does it apply to a boat on the water.

Answer 1

Archimedes sez:

Buoyant force is the weight of the fluid displaced by the object (boat).

So to float, a boat has to displace water with a weight equal to the total weight of the boat (and everything in it).

Answer 2

Buouancy force is an upward force (or push) that is exerted on any object that is immersed in a gas or a liquid. This has to happen some place where there's gravity--for example, if you immerse a rock into a tank of gas in a weightless environment, there is no buoyant force on the rock.

The reason the buoyant force exists is this: Whenever you have some gas or liquid where there's gravity, the pressure that the gas or liquid produces, increases as you go down. For example, the air pressure on the 1st floor of your house is slightly greater than the air pressure on the 2nd floor. And the water pressure at the bottom of a swimming pool is greater than the water pressure at the top. The increased pressure is caused by the weight of the additional gas or liquid on top, being pushed down by gravity.

But how does that cause buoyancy? Well, let's say you have a beach ball sitting on a table. There is air pressure pushing on all sides of the beach ball. But the pressure on the bottom of the ball (pushing up), is just slightly greater than the pressure on the top of the ball (pushing down). When you combine the upward and downward pressures, the upward wins by a slight amount. So there is a very slight excess upward push by the air pressure. That's the buoyant force.

In the case of the beach ball, that upward push is not enough to make the ball rise in the air, because, the buoyant force is small compared to the ball's weight. But if it were a very light object (like a balloon filled with helium), the buoyant force would be enough to make it rise.

Or, if you took the beach ball and submerged it in water, in that case the pressure differences are much greater than the pressure differences in air, so in that case the buoyant force is easily enough to counteract the balls weight, and the ball rises up through the water.

It turns out that the amount of buoyant force is exactly equal to the weight of the fluid (gas or liquid) that is "displaced" (pushed out of the way) by the immsersed object. So for example if you have a balloon that takes up 500 cubic feet of space, all you need to do is figure out how much 500 cubic feet of _air_ weighs--and that is the amount of buoyant force that pushes up on the balloon. (Whether or not the balloon floats will depend on whether the buoyant force is more than or less than the balloon's weight.)

When a boat is floating on the water, the buoyant force is exactly equal to the boat's weight (think about it: if the buoyant force were greater than the weight, the boat would rise up; and if the weight were more than the buoyant force, the boat would sink. It's doing neither, so the two opposing forces (weight and buoyancy) must be equal). This means that the weight of the water displaced by the boat, is exactly equal to the boat's weight.

Answer 3

If you immerse an inflated baloon in the bath tub, the balloon will try to push itself up and out of the water. The force by which the a fully immersed balloon pushes itself up is exactly equal to the amount of weight of water that balloon could hold but acts in the reverse direction (upwards).
If the balloon is half immersed in water, then the force by which it tries to push itself up is equal to the weight of 1/2 the water it could hold but again in the upward direction.

Now consider that you are immersing the balloon in let us say a bathtub full of mercury (liquid metal, which is very heavy). If you try to push in the same balloon you will notice that the force by which it resists being immersed will be much much higher. This is because the force required to push the balloon down is now equal to the weight of mercury that balloon could hold.

In case of a boat, let us first measure the weight of the boat, which is say 250 Kg. Now when the boat is placed in water, it will continue to sink in till the amount of water it has displaced equals 250 Kg. At that point, it will not submerge any more and begin floating. If you were to place another 100 Kg in the boat, it will sink in deeper till the amount of water displaced is now 250+100 = 350 Kg.

I hope you can get the idea.

Answer 4

Buoyancy force or upthrust is the upward force exerted on a fully or partially submerged object in a fluid as a result of the difference in pressure between the layers of fluid.

It is numerically equal to the weight of water the object displaces:

U = pVg

U = Upthrust/buoyancy
p = density of fluid
V = volume of object displacing the fluid
g = gravitational field strength

For a boat to float on water, the amount of water it displaces is equivalent to that of the upthrust force, hence it floats.

A concrete block sinks as its weight is greater than the upthrust force the water can provide.

Answer 5

It is the weight of what ever you are displacing. My body in air displaces a volume of air. I would actually weigh more in a vacuum.

A boat sitting on the surface of the water will sink until such time as the weight of the water displaced by the boat is equal to the weight of the boat. If the density of the boat, including the air space, is less than the density of the water, it should float.

Answer 6

The force on a body due to buoyancy is given by Archimedes' principle,
fb=mg



where m is the mass of liquid displaced and g is the gravitational acceleration. Therefore, a more useful form of Archimedes' principle is
fb+pliquid gVsolid
Hope this helps