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Thrust and Pressure

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Thrust and Pressure - Lesson Summary


Thrust

The upward force exerted on a body by the fluid in which the body is submerged is called the upthrust or buoyant force. The property of liquid to exert an upward force on a body immersed in it is called buoyancy. Being a force upthrust is measured in newton or kgf in the system of international units.

Examples

  • Pushing an empty can into water experiences the buoyant force.
  • Pushing a cork into water into water experiences the buoyant force.

Effeect of Upthrust

The effect of upthrust is that the weight of body immersed in a liquid appears to be less than its actual weight.

Example
Lifting a bucketful of water appears lighter than its actual weightwhen it iswhen it is immersed in water.

Characteristic Properties of Upthrust

  • For the same volume inside the fluid more the density of fluid, greater is the upthrust.
  • Larger the volume of the bodysubmerged in fluid, greater is the upthust.
  • The upthrust acts on the body in upward direction at the centre of buoyancy i.e., the centre of gravity of the displaced liquid.

Factors Affecting the Upthrust
  • The magnitude of upthrust on a body due to a fluid (liquid or gas) depends on the volume of the body submerged in the fluid (liquid or gas) and
  • The magnitude of upthrust on a body due to a fluid (liquid or gas) depends on thedensity of the fluid (liquid or gas) in which the body is submerged.

Magnitude of the Upthrust
Up thrust = Weight of liquid displaced by the body i.e.,
Up thrust = Volume of the body submerged in the fluid × Density of the fluid × Acceleration due to gravity

F B = vρg, Where
F B = Up thrust
v =  Volume of the body submerged in the fluid
ρ = Density of the fluid in which the body is submerged
g = Acceleration due to gravity

Pressure

The suitcases or the handles of heavy luggage items have broad straps. Also the nails that are to be fixed into walls have pointed tips.  

The force that acts on an object perpendicular to its surface is the thrust, measured in newton in the SI system or dyne in the CGS system of units. Pressure is the thrust per unit surface, that plays an important role in these situations. Even though the luggage is heavy and it weighs more, the force acting per unit area decreases with broad straps of the handles. Thus lesser pressure facilitates a person holding these items.

Similarly, lesser surface area of the nails (hence the pointed tips) transfer a larger force exerted on them and facilitate easy fixing of them in walls. As pressure is force per unit area, its units are newton per metre square. One metre per metre square is termed Pascal (Pa) after the name of the scientist Blaise Pascal. The pressure exerted by solids depends on their weight and surface area through which the weight acts.

However, the pressure (P) exerted by liquids depends on their density (r), acceleration due to gravity (g) and the height (h) of the liquid column. Mathematically it is given by P = hrg. The pressure exerted at a point in a liquid is equal in magnitude in all directions, hence it is scalar.

Pressure =     Thrust (F) Area (A)     

    •  Pressure is measured in N m-2 or Pascal (Pa) in the SI system.
 
Pascal's Law
The increase in pressure of a liquid at a point is transmitted to all other parts of the liquid without any change. This is Pascal’s law and it is widely used in various applications like hydraulic brakes of vehicles, vehicle lift platforms in garages etc.

    •  The thrust exerted by a body remains constant placed in any position.
    •  The pressure exerted by the body changes if the surface area of contact of the body with another surface changes.
    •  Thrust and pressure are also applicable to fluids i.e., liquids and gases.
    •  Buoyancy is the upward force that a fluid exerts on an object when the object is immersed in that fluid.
    •  Floating and sinking of objects depends on how the density of the object compares with the density of water.
    •  The magnitude of the buoyant force depends on the density of the fluid.
    •  Objects with a density less than that of a given liquid float when placed in that liquid.
    •  Objects with a density greater than a given liquid sink when placed in that liquid.

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