9th Class Science Gravitation and Floatation Thrust and Pressure

          Thrust and Pressure

                Thrust

It is the force acting on the body perpendicular to the surface. Its unit is same as that of force. It is dyne in CGS system and Newton in SI system.

1 kgwt = 9.8 newton and 1 gm wt = 980 dyne

           Pressure

It is the force applied normal to the surface per unit area. It is given by

Pressure \[(P)=\frac{Force}{Area}=\frac{F}{A}\]

The unit of pressure is \[N/{{m}^{2}}\] or pascal (Pa). 1 Pa = 1 \[N/{{m}^{2}}=10\,\,dyn/c{{m}^{2}}\].

             Fluid Pressure

The pressure exerted by the fluid is called the fluid pressure. A fluid exerts pressure in all directions. Normally it exerts three types of pressure downward pressure, upward pressure, and lateral pressure.

              Atmospheric Pressure

The pressure exerted by air on the surface of earth is called the atmospheric pressure. It can be measured with the help of an instrument called barometer. 1 atmospheric pressure is equal to 76 mm of Hg.

               Pascal Law

It states that if a pressure is increased at any point inside the liquid at rest then it is equally transmitted in all directions of the liquid. This principle is used in designing the instrument like hydraulic press, Brahma press etc.

               Buoyant Force

It is the force exerted in upward direction by the liquid on the object which is fully or partially immersed into the liquid. The property of liquid to exert buoyant force on the object is called buoyancy.

            Archimedes Principle

It states that when a body is fully or partially immersed in a liquid at rest, it exerts an up thrust on the body which is equivalent to the volume of liquid displaced by the object. All the ships are designed on the principle of Archimedes principle.

             Relative Density

We can compare the density of the substance relative to the density of the water at 4°C . The ratio of density of the substance to the density of the water is called the relative density.

Relative Density \[=\frac{\text{Density of substance}}{\text{Density of water}}\]

        Find the depth in an ocean at which a bubble of air will have one fourth the volume as it have on the surface of ocean?

(a) 4.1560m

(b) 2.1002m

(c) 1.8908m

(d) 3.1008m

(e) None of these

Answer: (d)

        A compounder applies a force of 3.8 N on the piston of a syringe of radius 0.9 cm. Find the pressure on the piston of the syringe.

(a) 14.93 kPa

(b) 12.10kPa

(c) 10.89 kPa

(d) 13.10kPa

(e) None of these

Answer: (a)

        A water tank whose dimension is  \[6m\times 2m\times 2m\]. Find the force acting at the bottom of the tank if it is filled with water.

(a) \[20.02\times {{10}^{4}}N\]

(b) \[23.52\times {{10}^{4}}N\]

(c) \[22.22\times {{10}^{4}}N\]

(d) \[24.02\times {{10}^{4}}N\]

(e) None of these

Answer: (b)

        Which of the following statements is correct?

Statement 1: Acceleration due to gravity decreases with height.

Statement 2: The value of G changes from place to place.

(a) Statement 1 and 2 both are correct

(b) Statement 1 is correct and 2 is incorrect

(c) Statement 1 is incorrect and 2 is correct

(d) Statement 1 and 2 both are incorrect

(e) None of these

Answer: (b)

        If we mix equal mass of water with a liquid having relative density 2, then the density of the mixture is:

(a) \[2.11\,\,g/c{{m}^{3}}\]

(b) \[3.26\,\,g/c{{m}^{3}}\]

(c) \[1.33\,\,g/c{{m}^{3}}\]

(d) 4.01 g/cm3

(e) None of these

Answer: (c)

• The force of attraction exists between any two objects of this universe.
• Acceleration due to gravity is the acceleration with which an object falls towards the earth.
• The total quantity of matter present in the object is called mass.
• The force with which an object is attracted towards the earth is called weight.
• Acceleration due to gravity decreases with depth or height of an object.