Science CBSE Physics Flotation Term-II Class IX Force :
Pressure :Thrust : Atmospheric pressure: Buoyant force
Pressure in fluids – All liquids and gases are fluids.
- A solid exerts pressure on a surface due to its weight
- Similarly, fluids have weight, and they also exert pressure on the base and walls of the container in which they are enclosed.
- Pressure exerted in any confined mass of fluid is transmitted undiminished in all directions.
- The pressure in a liquid is the same at all points at the same horizontal level. As we go deeper in the liquid, the pressure increases.
- When an object is placed in a liquid, the liquid exerts an upward force on it e.g. When a piece of cork is held below the surface of water and then released the cork immediately rises to the surface.
- It is a common experience that a mug filled with water appears to be heavier when it is lifted above the surface of water in .
- In general, whenever an object is immersed in water, it appears to and feels lighter. The weight of the object in water is called apparent weight. It is less than its true weight.
- The objects appear to be less heavy when submerged in water because the water exerts an upward force on them.
- The upward force acting on an object immersed in a liquid is called buoyant force. The buoyant force is also known as upthrust. It is due to the buoyant force exerted by the liquid that the weight of an object appears to be less in the liquid than its actual weight in air.
- It is due to the buoyant force exerted by water that we are able to swim in water and ships float on water.
- The tendency of a liquid to exert an upward force on an object placed in it is called buoyancy.
- As more and more volume of the object is immersed in a liquid, the upward buoyant force acting on it increases. But once the object is completely immersed in a liquid, then lowering it further in the liquid does not increase the buoyant force. This means that maximum upward buoyant force acts on an object when it is completely immersed in the liquid.
1. The buoyant force exerted by a liquid depends on the volume of the solid object immersed in the liquid.
- As the volume of the solid object immersed inside the liquid increases, the upward buoyant force also increases. And when the object is completely immersed in the liquid, the buoyant force becomes maximum and remains constant.
- The magnitude of buoyant force acting on a solid object does not depend on the nature of the solid object, e.g. if two balls made of different metals having different weights but equal volumes are fully immersed in a liquid, they will experience an equal loss in weight and thus equal upward buoyant force. This is because both the balls displace equal weight of the liquid due to their equal volumes.
- The liquid having higher density exerts more upward buoyant force on an object than another liquid having lower density. Thus, as the density of liquid increases, the buoyant force exerted by it also increases,
- e.g. sea water has higher density than fresh water, therefore, sea-water will exert more buoyant force on an object immersed in it than the fresh water. It is easier to swim in sea water because it exerts a greater buoyant force on the swimmer.
- Similarly, mercury is a liquid having very high density. So, mercury will exert a very great buoyant force on an object immersed in it. Even a very heavy material like an iron block floats in mercury because mercury exerts a very high buoyant force on iron block due to its very high density.
2. Buoyant force (B) acting upwards.
An object will float or sink in a liquid will depend on the relative magnitude of these two forces acting on the object in opposite directions. Three cases arise:
1. If B exerted by the liquid < W of the object, the object will sink in the liquid.
2. If B = W, the object will float in the liquid.
3. If B > W, the object will rise in the liquid and then float.
Thus an object will float in a liquid if the upward buoyant force it receives from the liquid is great enough to overcome the downward force of its weight.
For an object to float, Weight of object = Buoyant force
But, Buoyant force = Weight of liquid displaced by the object\Weight of object = Weight of liquid displaced by the object.
Thus an object will float in a liquid if the weight of object is equal to the weight of liquid displaced by it.
The above relation holds true if the object has a lower density than the liquid.
- If the object has a higher density than the liquid, then the weight of liquid displaced will be less than the weight of object, and the object will sink.
- · An object will also float in a liquid if its density is equal to that of the liquid.
- When we put a piece of iron in water, it sinks immediately because iron is denser than water. But a ship made from iron and steel floats on water. This is because a ship is a hollow object having a lot of air in it. Air has low density due to which the average density of ship becomes less than the density of water and the ship floats in water.
- This can be explained in another way. A heavy ship floats in water as it displaces a large weight of water which provides a great buoyant force to keep it afloat.
Buoyant force acting = Weight of liquid displaced on an object by that object
Archimedes’ principle is applicable to objects in fluids, i.e. liquids as well as gases.
and Buoyant force = Loss in weight of body in water.\ Loss in weight of body in water = Weight of water displaced by body.
Applications of Archimedes’ principle –
1. It is used in designing ships and submarines.
2. It is used in determining the relative density of a substance.
3. The lactometers used for determining the purity of milk are based on Archimedes’ principle.
4. The hydrometers used for determining the density of liquids are based on Archimedes’ principle.
Density – The density of a substance is defined as mass of the substance per unit volume.
- Density = Mass of the substance/Volume of the substance
- The SI unit of density is kilograms per cubic meter (Kg/m3).
- The density of a substance, under specified conditions, is always the same. So, the density of a substance is one of its characteristic properties.
- The density of a given substance can help us to determine its purity.
- Different substances have different densities e.g. density of water is 1000 Kg/m3 which means that the mass of 1 cubic metre volume of water is 1000 kg.
- Relative density of a substance = Density of the substance/Density of water
- Since the relative density is a ratio, it has no units. It is a pure number.
- The relative density of a substance expresses the heaviness (or density) of the substance in comparison to water e.g. the relative density of iron is 7.8, which means iron is 7.8 times as heavy as an equal volume of water.
- The relative density of water is 1. If the relative density of a substance is more than 1, then it will be heavier than water and hence it will sink in water.
- On the other hand, if the relative density of a substance is less than 1, then it will be lighter than water and hence float in water. e.g. Ice has a density of about 900 kg/m3 and water has a density 1000kg/m3.
- Thus an ice cube has a relative density of 0.9 so it floats in water. The relative density of iron is7.8, so an iron nail sinks in water.