Q. What is Thrust?
Ans: Thrust is the force acting on a body perpendicular to its surface.
The SI unit of thrust is Newton.
Q. Define Pressure?
Ans: Pressure is defined as the force acting per unit area.
The pressure is derived as,
Pressure, P = Force
Area
2
The SI Unit of Pressure is Pascal or N/m.
Q. Why do the school bags have wide straps?
Ans: The school bags have wide straps as the weight gets distributed over a larger area. This creates less pressure and hence, no pain to shoulders.
Q. Why is it difficult to hold a school bag having a strap made of a
thin and strong string?
Ans: It is difficult to hold a school bag having a strap made of a
thin and strong string because the force of the bag acts on smaller surface
area.
In other words, the bag exerts more pressure on the shoulders.
Q. Describe Buoyancy? What are the factors that affect buoyancy?
Ans: The tendency of fluids to exert upward force on object immersed in it is called buoyancy.
It is also called as upthrust.
Buoyancy helps things to float on water. It is because of buoyancy that the stones lifted from bottom of river feel lighter in weight.
Factors affecting Buoyancy:
The buoyancy depends on volume of solid object immersed in it.
The buoyancy depends on density of liquid in which object is immersed.
Q. Why does an object float or sink when placed on the surface of water.
Ans: When the density of an object is less than the water, it
floats and when the density is more than water, it sinks.
Q. In what direction does the buoyant force on an object immersed
in a liquid act?
Ans: The buoyant force acts in vertically upward direction on an
object immersed in a liquid.
Q. Why does a block of plastic released under water come up to the
surface of water?
Ans: The plastic has lower density than the water. When the block
of plastic is released under the water, it experiences upthrust greater than
its weight and it comes up to the surface of water.
Ans: When we stand on weighing machine, our weight acts in downward direction, whereas upthrust due to air acts upwards.
So the mass is always more than shown on machine.
Q. Explain the Archimedes Principle? What are the applications of Archimedes principle?
Ans: Archimedes Principle states that when an object is wholly or partially immersed in a liquid, it experiences an upthrust and displaces some liquid. The weight of fluid displaced by object is equall to force applied on object.
Archimedes principle has many applications;
It is used for designing ships, submarines, lactometers, hydrometers etc.
Q. You have a bag of cotton and an iron bar, each indicating a mass
of 100 kg when measured on a weighing machine. In reality, one is heavier than
other. Can you say which one is heavier and why?
Ans: In reality, the bag of cotton will be heavier than the Iron
bar. This is because, a 100 kg cotton bag will take large volume, thus will
have large upthrust.
The upthrust will result in weighing machine showing less mass for
cotton bag.
Q. Explain what is density and Relative density?
Ans: Density: The density is defined as mass of substance per unit volume.
Density = Mass
Volume
3
The SI Unit of Denisty is Kg/m
Relative density: Relative density is the ratio of density of a substance to the density of water.
Relative Density = Density of substance
Density of water
The relative density has no Unit.
In other words, Relative density expresses heaviness of object with respect to water, e.g relative density of Iron is 7.8, i.e Iron is 7.8 times heavy than water.
Q. The volume of 50 gram of substance is
2 3
20 cm . If density is water is 1 g/cm. Will substance float or sink.
Ans: Volume of Substance = 20
Mass of Substance = 50
Density = Mass
Volume
3
= 50/20 = 2.5 gm/cm
The density of object is more than water. Hence it will sink.
Q. Volume of 500 gm sealed packet is
3
350 cm . Will packet float or sink? Find the mass of water displaced.
Ans:
Mass of packet = 500
Volume of packet= 350
Density = M/V
= 500/250
= 1.42
The density of packet is clearly more than water. Hence it will sink.
Mass of water displaced
= Volume of packet × Density of water
= 350 × 1
= 350 g
Q. Why do ships sail on water while iron nail sinks?
Ans: The density of iron is 7.8 times more than that of water, hence iron nail sinks.
Ships float on water because its average density is less than that of water. This is due to the presence of air spaces within the ship.
When ship is placed in water, it sinks to certain level and displaces large weight of water which provides buoyancy to it.
Q. The volume of 50 g of a substance is 20 cm3. If the
density of water is 1 g/cm3, will the substance float or sink?
Ans: Given;
Mass of substance = 50 g
Volume of substance = 20 cm3
Density of water = 1 g/cm3
We know,
Density of substance = Mass/Volume
Density of substance = 50/20
Density of substance = 2.5 g/cm3
Clearly, the density of substance is more than the water. Hence,
it will sink.
Q. The volume of a 500 g sealed packet is 350 cm^3 . Will the
packet float or sink in water if the density of water is 1 g/cm^3? What will be
the mass of the water displaced by this packet?
Ans:
Given;
Mass of Sealed packet = 500 g
Volume of Sealed pack= 350 cm^3
Density of water = 1 g/cm3
We know,
Density of Sealed pack= Mass/Volume
Density of Sealed pack= 500/350
Density of Sealed pack = 1.4 g/cm^3
Clearly, the density of sealed pack is more than the water, hence it
will sink.
Mass of Water displaced:
According to the Archimedes Principle, the Mass of Water displaced
by the sealed packet is equal to the volume of the pack i.e. 350 g.
