Chapter 4 Floatation

Thrust and Pressure

Force acting on an object perpendicular to the surface is called thrust.

Effect of thrust depends on the area of contact.

The pressure is thrust per unit area.

SI unit is the pascal (Pa).

Force acting on a smaller area applies more pressure than the same force acting on a larger area.

• Pressure depends on two factors

(i) Force applied

(ii) Area of surface over which force acts

Examples of Pressure

→ The base of high buildings is made wider so that weight of walls act over a large surface are

pressure is less.

→ School bags are having broad strap so that the weight of school bags fall over a larger area so that student feel less pressure on shoulders.

--> liquids and gases are fluids and they exert pressure in all directions.

Archimedes’ Principle

→ It states, when a body is immersed fully or partially in a fluid, it experiences a upward force

equal to the weight of the fluid displaced by it.

Applications of Archimedes’ Principle

i) It is used in determining relative density of substances.

(ii) It is used in designing ships and submarines.

(iii) Hydrometers and lactometers are made on this principle.

→ It is because of this ship made of iron and steel floats in water whereas a small piece of iron

in it.

Buoyancy

→ The upward force experienced by an object when it is immersed into a fluid is called force of

buoyancy.

→ It acts in upward direction and it depends on the density of the fluid.

→ When force of gravitational attraction of the earth on the surface of the object < buoyant force

exerted by fluid on the surface of the object, object floats in the fluid.

• When force of gravitational attraction of the earth on the surface of the object > buoyant force

exerted by fluid on the surface of the object, the object sinks in the fluid.

→ This is the reason, why allpin sinks and boat/ship floats on the surface of water. (Archimedes

principle)

Relative density

→ The ratio of the density of a substance to that of the density of water is called relative density.

• Relative density = Density of a substance/Density of water

→ It has no unit as it is a ratio.

Kepler’s laws of planetary motion

Kepler observed that different planets take different time to complete one revolution around the sun. From s this observation of planetary motion he deduced that square of time period is proportional to the cube ob its distance from the sun.

Kepler’s First Law

Kepler’s first law of planetary motion states that planets move around the sun in such a way that sun always remains at one of its focus.

Kepler’s second law

According to second law the line joining planet to the sun sweeps out equal area in equal time interval, as the planet revolve around the sun in a elliptical orbit.

 Kepler's Third Law

It states that square of time period is proportional to the cube of its semi major axis.  

Introduction

Floatation depends upon the density. If an object has density less than the density of water, it floats. Principle of floatation is stated by the Archimedes. This chapter deals with what is floatation, laws of floatation, its applications and examples.

Thrust and Pressure

Force acting on an object perpendicular to the surface is called thrust.

Effect of thrust depends on the area of contact.

The pressure is thrust per unit area.

SI unit is the pascal (Pa).

Force acting on a smaller area applies more pressure than the same force acting on a larger area.

• Pressure depends on two factors

(i) Force applied

(ii) Area of surface over which force acts

Examples of Pressure

→ The base of high buildings is made wider so that weight of walls act over a large surface are

pressure is less.

→ School bags are having broad strap so that the weight of school bags fall over a larger area so that student feel less pressure on shoulders.

--> liquids and gases are fluids and they exert pressure in all directions.

Buoyancy

→ The upward force experienced by an object when it is immersed into a fluid is called force of

buoyancy.

→ It acts in upward direction and it depends on the density of the fluid.

→ When force of gravitational attraction of the earth on the surface of the object < buoyant force

exerted by fluid on the surface of the object, object floats in the fluid.

• When force of gravitational attraction of the earth on the surface of the object > buoyant force

exerted by fluid on the surface of the object, the object sinks in the fluid.

→ This is the reason, why allpin sinks and boat/ship floats on the surface of water. (Archimedes

principle)

Density

→ The mass per unit volume is called density of an object.

→ If M is the mass and V is the volume, then

• Density (d) = Mass(M)/Volume(V)

• SI unit = kg/m³

Archimedes’ Principle

→ It states, when a body is immersed fully or partially in a fluid, it experiences a upward force

equal to the weight of the fluid displaced by it.

Applications of Archimedes’ Principle

i) It is used in determining relative density of substances.

(ii) It is used in designing ships and submarines.

(iii) Hydrometers and lactometers are made on this principle.

→ It is because of this ship made of iron and steel floats in water whereas a small piece of iron

in it.

Relative density

→ The ratio of the density of a substance to that of the density of water is called relative density.

• Relative density = Density of a substance/Density of water

→ It has no unit as it is a ratio.

Kepler’s laws of planetary motion

 Kepler observed that different planets take different time to complete one revolution around the sun. From s this observation of planetary motion he deduced that square of time period is proportional to the cube ob its distance from the sun.

Kepler’s First Law

Kepler’s first law of planetary motion states that planets move around the sun in such a way that sun always remains at one of its focus.

Kepler’s second law

According to second law the line joining planet to the sun sweeps out equal area in equal time interval, as the planet revolve around the sun in a elliptical orbit.

 Kepler's Third Law

It states that square of time period is proportional to the cube of its semi major axis.