Chapter 27: SURFACE
tension explains why Bubbles are spherical.
doesn’t spread on some surfaces but forms droplets.
and water don’t mix; water wets us but not ducks.
of brush stick together when removed out of water.
are spherical. oil rises in wick; sap and water rise
of soil remain separated under water but stick
together when taken out.
glass pieces are melted the edges take rounded shape
and become smooth, this is called fire polishing.
Properties of Surface Tension
tension decreases with temperature.
of contact determines if liquid shall spread or form
droplets. It is reduced by wetting agents so they
can penetrate well; whereas it is increased by
waterproofing agents so water doesn’t stick.
Detergents molecules attract water on one side and
grease-dirt on other so they help in washing by
reducing surface tension of water-oil.
Fig 1: Angle of Contact
is due to the friction between different
liquid layers as they slip past each other when the
Properties of Viscosity
viscosity means slower is the flow. Increasing the
temperature decreases the viscosity.
Glass is an extremely viscous liquid; it’s so
viscous that it resembles solids. The windowpanes of
old buildings are thicker at the bottom as glass
flows down due to gravity.
friction [rolling a body on ground] is less
than sliding friction [pushing a body
already in motion] is less than static friction
[pushing a static object].
Use of ball bearings in hubs and axels of ceiling
fans and bikes to reduce contact surface between two
solid parts. Thin cushion of air is maintained
between solids in relative motion for same purpose.
Lubricants too reduce friction.
occur when continental plates brush past each
other or collide. Earthquakes are measured in
Richter’s scale. An earthquake of intensity 6 is
1000 times more destructible than earthquake of
intensity 2, so Richter scale is not linear.
escape velocity on moon is lower so gas
molecules if formed escape as they can easily
overcome the gravitational pull of the moon. Thus
the moon has no atmosphere.
satellites appear to be appearing fixed from
any point on the earth. To do this the satellite
must be at a height of 35800km from the earth’s
surface and in the equatorial plane. Since EM waves
beyond a certain frequency can’t be reflected by the
ionosphere [TV], for reflecting such waves we use
satellites go around earth in north south
direction, their time period is 100 mins approx and
their altitude is 500-800km. The satellites at a
higher height have a lower speed of revolution to
stay in orbit as gravitational pull is lesser but
this is independent of their weights. They don’t
fall as earth’s gravitation provides necessary
acceleration to it.
2: Types of Satellite
are launched preferably from equator and eastwards
as it aids the speed due to lower acceleration due
to gravity ‘g’. Speed of rotation of satellites is
independent of its size and mass.
centrifugal force is more at equator than poles.
Gravity is higher at poles than equator.
waves in decreasing order of wavelength
Radio wave AM radio > TV and FM waves > short
radio > microwave > infrared > visible >
UV > X-ray > gamma
satellites, the acceleration towards the center of
the earth is equal to earth’s acceleration due to
gravity at that point. Thus the objects inside the
satellite experience weightlessness. A body in free
fall experiences no upward force on him and so feels
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