24 October 2011

9th CBSE PHYSICS Gravitation -Flotation Term-II

 Science CBSE Physics Flotation Term-II Class IX Force : Pressure :Thrust : Atmospheric pressure: Buoyant force

Thrust – The force acting on a body perpendicular to its surface is called thrust.

The S.I. unit of thrust is Newton (N).
e.g. For fixing a poster on a bulletin board one has to press drawing pins with the thumb.

When pressing a drawing pin, force is applied on the surface area of its head.

The force is directed perpendicular to the surface of the board. This force is called thrust.

Pressure – The thrust per unit area is called pressure.

Pressure = Thrust/ Area

The S.I. unit of pressure is Newton per square metre (N/m2) which is also called pascal (Pa).

Many times a bigger unit of pressure called kilopascal (kPa) is used.

The pressure depends on two factors:
(a). Force applied              (b). Area over which force acts.

The same force can produce different pressures depending on the area over which it acts
e.g. when a force acts over a large area of an object, it produces a small pressure.

But if the same force acts over a small area of the object, it produces a large pressure.

Let we take  two similar bricks lying on the ground, one in the lying position and another in the standing position. The two bricks exert the same force on the ground because they have the same weight.

But the two bricks exert different pressures on the ground because their areas in contact with the ground are different.

The brick in the lying position has a large area in contact with the ground. So, the force of the weight of the brick falls on a large area of the ground and the ‘force per unit area ‘ or pressure on the ground is less.

The brick in the standing position has a small area in contact with the ground. So, the force of the weight of the brick falls on a smaller area of the ground and the pressure on the ground is more.

A school bag has wide straps made of thick cloth

=  The weight of bag may fall over a large area of the shoulder of the child producing less pressure on the shoulder. And due to less pressure, it is more comfortable to carry the heavy school bag.

=  On the other hand, if the school bag has a strap made of thin string, then the weight of school bag will fall over a small area of the shoulder. This will produce a large pressure on the shoulder of the child and it will become very painful to carry the heavy school bag.

A sharp knife cuts better than a blunt knife.

A sharp knife has a very thin edge to its blade. Due to its very thin edge, the force of our hand falls over a very small area of the object producing a large pressure. And this large pressure cuts the object easily.

On the other hand, a blunt knife does not cut an object easily because due to its thicker edge, the force of our hand falls over a larger area of the object and produces lesser pressure. This lesser pressure cuts the object with difficulty.

The tip of a sewing needle is sharp so that due to its sharp tip, the needle may put the force on a very small area of the cloth, producing a large pressure sufficient to pierce the cloth being stitched.

The pressure on ground is more when a man is walking than when he is standing.

When a man is walking, then at one time only his one foot is on the ground.
Due to this, the force of weight of man falls on a smaller area of the ground and produces more pressure on the ground.

On the other hand, when the man is standing, then both his feet are on the ground.
Due to this the force of weight of the man falls on a larger area of the ground and produces lesser pressure on the ground.

The depression is much more when a man stands on the cushion than when he lies down on it.
When a man stands on a cushion then only his two feet (having small area) are in contact with the cushion. Due to this the weight of man falls on a small area of the cushion producing a large pressure. This large pressure causes a big depression in the cushion.

On the other hand, when the same man is lying on the cushion, then his whole body (having large area) is in contact with the cushion. In this case the weight of man falls on a much larger area of the cushion producing much smaller pressure. And this smaller pressure produces a very little depression in the cushion.

The tractors have broad tyres so that there is less pressure on the ground and the tyres do not sink into comparatively soft ground in the fields.

A wide steel belt is provided over the wheels of army tanks so that they exert less pressure on the ground and do not sink into it.

Wooden sleepers (or concrete sleepers) are kept below the railway line so that there is less pressure of the train on the ground and railway line may not sink into the ground.

The snow shoes have large, flat soles so that there is less pressure on the soft snow and this stops the wearer from sinking into it.

It is easier to walk on soft sand if we have flat shoes rather than shoes with small heels (or pencil heels). This is because a flat shoe has a greater area in contact with the soft sand due to which there is less pressure on the soft ground. Due to this the flat shoes do not sink much in soft sand and it is easy to walk on it.

On the other hand, a small heel (or sharp heel) has a small area is contact with the soft sand and so exerts a greater pressure on the soft sand. Due to this greater pressure, the small heels tend to sink deep into soft sand making it difficult for the wearer to walk on soft sand.

The foundations of buildings and dams are laid on a larger area of ground so that the weight of the building or dam (to be constructed) produces less pressure on ground and the building or dam may not sink into the ground.

Atmospheric pressure

The pressure at any place due to the atmosphere is called atmospheric pressure. Its value varies from place to place and also with the time.

Atmospheric pressure at the earth’s surface near the sea level is around 1.01x105 Pa. This value is known as 1atmosphere of pressure (1atmosphere = 760mm of Hg).

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.
Buoyancy
• 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 a bucket.
• In general, whenever an object is immersed in water, it appears to lose some weight 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.
Factors affecting buoyant force

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.
1. The buoyant force exerted by a liquid depends on the density of the liquid in which the object is immersed.
• 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.
Why objects float or sink in a liquid –

A wooden block floats in water whereas a steel rod sinks in it. Thus some objects float and some sink in water.

When an object is put in a liquid, then two forces act on it:

1. Weight (W) of the object acting downwards,

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.
Archimedes’ principle

“When an object is wholly or partially immersed in a liquid, it experiences a buoyant force (or upthrust) which is equal to the weight of liquid displaced by the object”.

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.

Gases (like air) exert an upward force (or buoyant force) on the objects placed in them but in most cases it is so small that we usually ignore it. It is the buoyant force due to displaced air which makes a balloon rise in air.

Buoyant force = Weight of water displaced by body.
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 – The relative density of a substance is the ratio of its density to that of water.
• 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 is 7.8, so an iron nail sinks in water

CBSE Class 9 - Science - Chapter 10: Flotation:Thrust and Pressure, Pressure

 X Thrust and Pressure, Archimedes’ Principle, Relative Density CBSE Class 9 - Science - Chapter 10: Flotation: Notes and Quest MCQ: Flotation: Thrust, Pressure, Buoyancy and Density Thrust and Pressure, Archimedes’ Principle, Relative Density key point Notes : Flotation: Thrust, Pressure, Buoyancy and Density Physics Flotation Term-II Class IX  Buoyant force Detail Study 9th Physics Solved Numerical Floating bodies