9th standard syllabus notes and 30 questions on the content attached

1. Gravitation 2

2. Thrust
▪ Thrust is the amount of force exerted by the body
on a surface.
▪ It is the net force in a particular direction.
▪ The force acting on an object perpendicular to the
surface is called thrust.

3. Pressure
▪ Definition:The thrust on unit area is called
pressure.
▪ SI unit: N/m2 or Pascal (Pa)

4. Dependence of pressure on area
Smaller
area
Larger
pressure
Larger
area
Smaller
pressure

5. Pressure in fluids
▪ Fluids exert pressure on the base and walls of the
container in which they are enclosed.

6. Upthrust or Buoyant force
▪ The upward force exerted by the water on an
object is known as upthrust or buoyant force.

7. Buoyancy in objects
Upthrust of
water.
Force due to the
gravitational
attraction of the
earth.
MORE
Iron nail sinks
MORE
Cork floats

8. When objects sink or float
Objects of density greater
than that of a liquid sink
in the liquid
Objects of density less
than that of a liquid
float on the liquid

9. Archimedes’s Principle
▪ When a body is immersed fully or partially in a
fluid, it experiences an upward force that is equal
to the weight of the fluid displaced by it.

10. Application of Archimedes's Principle
Designing
ships
Designing
submarines
Lactometer Hydrometer

11. Relative Density
▪ The relative density of a substance is the ratio of
its density to that of water.
▪ Relative density = Density of a substance
Density of water
 Unit: No unit.

12. 1. How can camels walk easily on sand?
A. Camels can easily walk on sand as they have wide foot. Since
they have wide foot the weight of their body acts on a larger
area and produce less pressure on sand. .[We know that pressure
is inversely proportional to area .]
2.When you stand in sand you feel your legs going inside but this
does not happen when you lie down why?
A. When you stand on loose sand, the force, that is, the weight of
your body is acting on an area equal to area of your feet.When
you lie down, the same force acts on an area equal to the contact
area of your whole body, which is larger than the area of your
feet.Thus, the effects of forces of the same magnitude on
different areas are different.The effect of thrust on sand is larger
while standing than while lying.

13. 3.Why does a knife have a sharp edge?
A. The same force acting on a smaller area exerts a larger pressure,
and a smaller pressure on a larger area.This is the reason why a
nail has a pointed tip, knives have sharp edges.
4. Calculate the pressure in a wooden block of dimensions
20cmsx10cms with a mass of 10kg.
A. F= 10x9.8=98N
Area= 20x10=200cm2= 0.02m2
pressure=Force/area= 98/0.02= 4900 Nm-2
5. Calculate the pressure exerted on a area of 20m2 by a force of
10N
A. Pressure = Force/Area
= 10/20= 0.5 Nm-2

14. 6. Explain pressure in fluids.
A. A solid exerts pressure on a surface due to its weight.
Similarly, fluids have weight, and they also exert pressure on
the base and walls of the container in which they are
enclosed. Pressure exerted in any confined mass of fluid is
transmitted undiminished in all directions.
7.What are two forces which act on a bottle in water?
A. The force due to the gravitational attraction of the earth
acts on the bottle in the downward direction. So the bottle is
pulled downwards. But the water exerts an upward force on
the bottle.Thus, the bottle is pushed upwards.
8.Why does a bottle float in water?
A. When the bottle is immersed, the upward force exerted by
the water on the bottle is greater than its weight.Therefore
it rises up when released.

15. 9. How can the bottle be kept immersed in the water?
A. To keep the bottle completely immersed, the upward force on
the bottle due to water must be balanced.This can be achieved
by an externally applied force acting downwards.This force must
at least be equal to the difference between the upward force and
the weight of the bottle.
10.What is buoyant force?
A. The upward force exerted by the water on the bottle is known as
upthrust or buoyant force.
11.Why does an iron nail sink in water?
A. The force due to the gravitational attraction of the earth on the
iron nail pulls it downwards.There is an upthrust of water on the
nail, which pushes it upwards. But the downward force acting on
the nail is greater than the upthrust of water on the nail. So it
sinks.

16. 12. Explain why and iron nail sinks in water on the basis of density?
A. The density of an iron nail is more than the density of water.This
means that the upthrust of water on the iron nail is less than the
weight of the nail. So it sinks.
13. Explain why a cork floats in water on the basis of density.
A. The density of cork is less than the density of water.This means
that the upthrust of water on the cork is greater than the weight
of the cork. So it floats.
14. On the basis of density explain when an object floats or sinks.
A. Objects of density less than that of a liquid float on the liquid.The
objects of density greater than that of a liquid sink in the liquid.
15. Explain Archimedes's principle.
A. When a body is immersed fully or partially in a fluid, it
experiences an upward force that is equal to the weight of the
fluid displaced by it.

17. 16. Give the applications of Archimedes’s principle
A. It is used in designing ships and submarines. Lactometers and
hydrometers also make use of Archimedes's principle.
17.What is density?
A. The density of a substance is defined as mass of a unit volume.
The unit of density is kilogram per metre cube (kg m–3).
18. Density depends on what characteristics.
A. The density of a given substance, under specified conditions,
remains the same.Therefore the density of a substance is one of
its characteristic properties. It is different for different
substances.
19.What does density help us to determine?
A. The density of a given sample of a substance can help us to
determine its purity.

18. 20.What is relative density?
A. The relative density of a substance is the ratio of its density to that
of water.
21.What is the unit of relative density?
A. Since the relative density is a ratio of similar quantities, it has no
unit.
22.What is the relative density of gold if density of gold is 19300 kg m-3
while that of water is 1000 kg m-3?
A. Relative density = Density of Gold
Density of water
= 19300/1000 = 19.3
23. Relative density when density of Hg= 13,546 kg/m^3 of water= 103
A. Relative density = 13546/1000= 13.546

19. 24.The relative density of steel is 7.7 and so find the density of it if the
density of water is 1000.
A. RD= Density of steel
1000
7.7x1000= Density of steel= 7700kgm^-3
25. Relative density of silver is 10.8.The density of water is 103 kg m-3.
What is the density of silver in SI unit?
A. Density of silver = Relative density of silver × density of water
= 10.8 × 103 kg m–3.
26. An iron nail sinks but an boat made of iron floats explain why?
A. A needle sinks in water because the weight of water displaced by it
is less than the weight of needle and an iron ship is shaped that it
can displace a large volume of water.The weight of the water
displaced by the immersed portion o the ship is equal to the weight
of the ship hence it can float.

20. 27.What happens to the string when you immerse the string with stone
into water?
A. You will find that the elongation of the string or the reading of the
balance decreases as the stone is gradually lowered in the water.
28.What do you infer from the decrease in the extension of the string?
A. We know that the elongation produced in the string or the spring
balance is due to the weight of the stone. Since the extension
decreases once the stone is lowered in water, it means that some
force acts on the stone in upward direction. As a result, the net
force on the string decreases and hence the elongation also
decreases.
29.What is the purpose of lactometer and hydrometer?
A. Lactometers, which are used to determine the purity of a sample of
milk and hydrometers used for determining density of liquids.

21. 30.What is the other unit of Pressure?Why was it given that unit?
A. In honour of scientist Blaise Pascal, the SI unit of pressure is
called pascal, denoted as Pa.