Class 8, Science, Chapter-8, Lecture-2, Pressure (Notes)
THRUST:
The total force acting perpendicular to the surface of an object is known as thrust.
- The thrust on a body does not depend on shape or position of the body.
- SI unit of thrust is newton (N)
PRESSURE:
The force exerted by a body on a unit surface area and perpendicular to the surface is termed as the pressure.
The pressure exerted by the body depends on shape or position of the body.
Mathematically: ${\rm{P = }}{{\rm{F}} \over {\rm{A}}}$
where,
P = Pressure
F = Force
A = Surface area of application of force
SI unit of pressure is pascal (Pa)
One pascal:
The pressure exerted by an object that exerts a force of 1 N on a surface area of 1 m2 is termed as one pascal.
School Bags have Wide Straps.
Reason:
Pressure is inversely proportional to area.
The large area of the wide straps reduces the pressure on the shoulder and thus makes it comfortable to carry a loaded bag.
Railway tracks are laid on large sized wooden or iron sleepers.
Reason:
Pressure is inversely proportional to area.
The large area of the wooden or iron sleepers reduces the pressure on the ground and thus prevents the railway track from sinking into the ground under the heavy weight of the train.
A camel walk easily on a sandy surface than a man.
Reason:
Pressure is inversely proportional to area.
The area of camel’s feet is large as compared to the area of man’s feet.
This reduces the pressure exerted by a camel on the sand and makes it possible for camel to walk.
A sharp knife is more effective in cutting objects than a blunt knife.
Reason:
Pressure is inversely proportional to area.
The area under the sharp knife is less than the area under the blunt knife.
This increases the pressure exerted by a sharp knife and thus penetrates easily into the object.
The Tip of a Needle is made Pointed.
Reason:
Pressure is inversely proportional to area.
The area under the pointed tip of needle is very less.
This increases the pressure exerted by the needle and thus penetrates easily into the cloth.
ATMOSPHERIC PRESSURE:
The force exerted per unit area by a column of air from the top of the atmosphere to a point is termed as the atmospheric pressure at that point.
Atmospheric pressure at sea level = 103360 Pa ≈ 103 kPa
Atmospheric pressure is measured by using Barometer.
In high altitudes, nose bleeds.
Reason:
At high altitudes, the atmospheric pressure is less than the pressure of blood flowing in the vessels.
This leads to rupture of the thin-walled blood capillaries present near the skin of the interior of the nose.
Devices used in everyday life based on atmospheric pressure
- Sipping Straw
- Vacuum Cleaner
- Suction Cup
- Syringe
HYDROSTATIC PARADOX:
“The pressure at a certain horizontal level in the fluid is proportional to the vertical distance to the surface of the fluid.”
OR
The pressure at any point in a fluid does not depend upon its shape and the base area.
The pressure depends upon the height of the fluid column above the point