Archimedes' Principle

by Jared Rovny

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    00:01 Now that we've discussed some basic properties of fluids including the density and specific gravity and surface tension and seen some examples of how to apply hydrostatic pressure, both the gauge pressure and the total pressure, we're now ready to move on to a few key principles that have been derived about fluids and the way they behave, Archimedes’ principle and Pascal’s law.

    00:19 Let’s start with Archimedes’ principle. This principle basically says that when you put an object under water, it will feel a buoyant force upwards. You might have noticed this. If you've ever been in water, you feel much lighter. This is because there's a force acting upwards on you.

    00:33 We call this force the buoyant force. The reason you have a buoyant force on you, the reason there’s something pushing up on you is that you have a pressure from the water on top of you in one direction. You also have a pressure on the bottom at the lower part of this mass as you can see it here. But as we just saw, the pressure at one height and the pressure at a different height are different pressures. So, the pressure at the bottom is actually more than the pressure at the top of the object, meaning it has a net force acting on it upwards.

    01:02 This object actually has displaced some water and we can use this idea of displaced water to figure out how much pressure and how much buoyant force is lifting the object upwards. The displaced water is always the exact same volume as an object completely under water. So, we could sort of imagine this object having replaced that water. We took this amount of water out and then we stuck this object in.

    01:24 So, we consider this displaced water which has some mass to it. The force upwards on you, the buoyant force will be equal to the weight of this displaced water. This is Archimedes’ principle.

    01:37 This buoyant force we can always find by considering how much water the object in the water displaced by being put there. The force, the buoyant force acting upwards on the object will be exactly equal to the force in newtons, the gravitational force that would be pulling the water downwards.

    01:54 So, you can always find this buoyant force using this principle of Archimedes just by finding the mass of the displaced water which you can find from the volume of the object that’s in the water and multiplying it by g since you want the force that would normally be pulling this water down by the force of gravity. So, the buoyant force in summary by Archimedes principle is equal to the mass of the displaced water by your object times the gravitational acceleration, g.

    02:20 We can also determine when an object will float and if it will float using this principle.

    02:27 What we need for an object in the water like this one which has a gravitational force pulling it downwards and a buoyant force trying to lift it upwards, for it to float, we need the buoyant force to be greater than the gravitational force. If it’s exactly equal to the gravitational force, the object would sort of stay still and not go up or down. What this means if we need the buoyant force to be greater than the gravitational force is that we need the buoyant force which we found as the mass of the displaced water times g to be greater than the mass of the object times g since that’s the force of gravity pulling it downwards. We can divide both sides by the volume since these volumes will be the same. We can see that this means that the mass of the object, the mass times gravity of the water that’s displaced has to be greater than the mass of the object itself.

    03:18 We rewrite mass as the density times the volume and then we divide it by the volume as I said.

    03:23 So, the conclusion is that for an object to float, we simply need the density of water to be greater than the density of that object. We can also determine if an object is floating, how much of it will emerge from the top of the water. Again, this object will have a buoyant force equal to the gravitational force pulling the object down. This is the condition for it to be floating in equilibrium. This means that the mass of the water displaced also has to be equal to the mass of the object just like we just saw. Therefore, once again rewriting the masses in terms of their densities times their volumes, we can see that for a floating object, the volume that is submerged of your object divided by the entire volume of the object has to be equal to the density of the object divided by the density of the water. In other words, if you have an object that’s more dense, it will have to submerge itself more and displace a little bit more water if it wants to float.

    04:20 The ratio of the amount of the object that’s submerged, V submerged over V object has got to be equal to the ratio of the density of your object to that of water.

    About the Lecture

    The lecture Archimedes' Principle by Jared Rovny is from the course Fluids.

    Included Quiz Questions

    1. 49 g
    2. 40 g
    3. 51 g
    4. 60 g
    5. Cannot be determined without the density of the material
    1. A and B only
    2. A only
    3. A, B, and C only
    4. All of these would float
    5. None of these would float
    1. 0.8 cm
    2. 0.5 cm
    3. 0.2 cm
    4. 1 cm
    5. 0 cm

    Author of lecture Archimedes' Principle

     Jared Rovny

    Jared Rovny

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