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Q 1. What is meant by free fall? Two bodies, one
of mass 1g and other of mass 1 kg are dropped from the same height in vacuum.
Compare the two time intervals in which the two bodies will hit the ground.
Solution
Whenever
objects fall towards the earth under the influence of gravitational force
alone, it is called free fall. Both bodies will hit the ground at the same time
as acceleration due to gravity is independent of mass of the falling object.
Q 2. What is the force between the Earth and a
body called?
Solution
The
force between the Earth and the body is known as the force of gravity or
gravitational force. It is also called the weight of the body.
Q 3. State Archimedes' principle. Give two applications
based on the principle.
Solution
Archimedes'
principle: When an object is wholly (or partially) immersed in a liquid, it
experiences a buoyant force (or upthrust) which is equal to the weight of
liquid displaced by the object. Buoyant force on an object = weight of liquid
displaced by that object Applications of Archimedes’ principle: 1. In
determining the relative density of a substance 2. In determining the
density of liquids by a hydrometer
Q 4.
Why do buildings have wide foundations?
Solution
Buildings
have wide foundations to reduce the pressure on the ground, as pressure is
inversely proportional to area. If they have narrow foundations,then large
pressure will be exerted on the ground by them making the ground susceptible to
collapse.
Q 5.
(a) Define weight. (b) How does the gravitational force between the two bodies
change if the distance between them is tripled?
Solution
Q 6.
The dimensions of a wooden block are 2 m × 0.25 m × 0.10 m. If
relative density of wood is 0.6, calculate mass of the block in kg. Density of
water = 103 kg/m3
Solution
V
= 2 m × 0.25 m × 0.10 m = 0.05 m3 R.D
= 0.6
Q 7.
Do all bodies immersed in a given fluid experience the same buoyant force?
Explain.
Solution
No.
According to Archimedes' principle, bodies with different weight will displace
different weight of the fluid. Hence, the buoyant force on different objects
will be different.
Q 8.
Give the units for Force, Acceleration due to gravity, Gravitational Constant
and Pressure.
Solution
Units
of Force, Acceleration due to gravity, Gravitational Constant and Pressure are:
Force: newton(N) Acceleration due to gravity: m/s2 Gravitation
Constant: Nm2kg-2 Pressure: pascal (Pa)
Q 9.
(a) Name the two factors on which the buoyant force depends. (b) State the
relationship between
the
buoyant force on an object and weight of the liquid
displaced by it?
Solution
(a)
The factors on which buoyant force depends are: (i) Volume of the object
immersed. (ii) density of the fluid (b) Buoyant force on an object
= weight of the liquid displaced by the object
Q 10.
State the source of centripetal force that a planet requires to revolve around
the Sun. On what factors does that force depend? Suppose this force
suddenly becomes zero, then in which direction will the planet begin to move if
no other celestial body affects it?
Solution
The
source of centripetal force is the gravitational force between the planet and
Sun. Gravitational force depends on the mass of the planet, mass of the Sun and
distance between them. The planet will begin to move in a straight line
tangential to the point of circle in which it was moving at the instant force
became zero.
Q 11.
What is S.I Units? State the units of Force and G.
Solution
International
System of Units, that is, SI unit stands for Système International d'unités and
is the modern form of the metric system. Unit of Force, F is newton (N).
Unit of universal constant of gravitation, G is N m2 kg-2.
Q 12.
What is meant by relative density of a substance? What is its significance? The
relative density of mercury is 13.6. What is its density in SI unit? (Density
of water=1000 kg/m3)
Solution
Relative
density = Density of the substance/density of water It gives an idea as to how
many times a substance is heavier than water 13.6 = Density of mercury/103 Density
of mercury = 13.6 x 103 kg/m3
Q 13.
On what factors does the gravitational force between the two bodies depend?
Solution
Gravitational
force between two bodies depends on: 1. Masses of the two
bodies. The greater an object's mass,
the greater its gravitational force. 2. Distance of separation
between them. The smaller the distance greater is the gravitational force.
Q 14.
Give two applications of pressure.
Solution
1.
The edge of knife is made sharp so that with little force it cuts better. This
is because, on sharpening the knife the area reduces, and hence the pressure
increases. 2. Foundation of dams and building are made on large surface area so
as to decrease the effect of force by increasing area on which it acts. Thus,
pressure is reduced.
Q 15.
The density of turpentine at 293 K is given as 870 kg/m3. Identify
and write the names of substances that sink in turpentine at the same
temperature. S. No. Substances Density kg/m3 1 Wood 690 2 Ice
920 3 Rubber 970 4 Paraffin Wax 900 5 Cork 240 6 Bone 1850
Solution
Ice,
rubber, paraffin wax and bone will sink in turpentine as their densities are
larger than turpentine.
Q 16.
Explain the following:(a) Swimmers are provided with an inflated rubber
jacket/tube. Why?(b) It is easier to swim in sea water than in river water.
Why?
Solution
(a)
Swimmers are provided with an inflated rubber jacket or rubber tube. The jacket
tube has low weight and large volume. Hence, it displaces large volume of
water. As a result, up thrust due to water increases and the person remains
afloat, i.e., there is no chance of drowning of the swimmer in such case. (b)
It is easier to swim in sea water because density of sea water is more due to
added salts. Hence, up thrust acting on the swimmer in sea water is more than
in fresh water. So, it is easier to swim in sea water.
Q 17.
Ram throws a stone in the pond. It displaced 1.5 kg of water. Calculate the
buoyant force acting on the stone. (g = 9.8 m/s2)
Solution
Mass
of water displaced by the stone, m = 1.5 kg We know that, Weight
= Mass x Acceleration due to gravity
= m x
g Weight of the water displaced = 1.5 x 9.8 N = 14.7 N In
accordance with Archimedes' Principle, When an object is wholly or partially
immersed in a liquid, it experiences a buoyant force or upthrust which is equal
to the weight of liquid displaced by the object. The buoyant force
acting on the stone is 14.7 N.
Q 18.
When can the weight of an object be zero?
Solution
Weight
can be zero when acceleration due to gravity is zero. The value of 'g' is
maximum on the surface of the Earth and decreases on going inside the surface
or above the surface of the Earth. Thus, the weight of a body will be zero at
the centre of the Earth.
Q 19.
1 kg sheet of tin sinks in water but if the same sheet is converted into a box
or boat, it floats. Give reason.
Solution
The
sheet of tin sinks in water because the density of tin is higher than that of
water. When the same sheet of tin is converted into a box or a boat, then due
to the trapping of lot of 'light' air in the box or boat, the average density
of the box or boat made of tin sheet becomes lower than that of water and hence
it floats in water.
Q 20.
Define thrust and pressure and state the SI units in which they are measured.
Solution
THRUST:
Force acting perpendicularly on a surface is called thrust. Its SI unit is
newton (N). PRESSURE: The thrust acting on unit area of the surface is
called pressure. Its SI unit is pascal (Pa).
Q 21.
Why do objects feel lighter when in water than outside water?
Solution
When
we dip an object in water, there is an upward force known as buoyant force
acting on the object produced by the surrounding water in which it is fully or
partially immersed, due to the pressure difference of water between the top and
bottom of the object. It is this buoyant force that makes the object feel lighter
when placed in water than outside water.
Q 22.
(a) Distinguish between mass and weight of an object. (b) How will the weight
of a body of mass 100g change if it is taken from equator to the poles? Give
reason for your answer.
Solution
(a)
Mass Weight 1. The quantity of matter contained in a body is called mass of the
body. 1. The force with which the earth attracts a body towards its centre is
called the weight. 2. Mass is a scalar quantity. 2. Weight is a vector
quantity. (b) W = mg As 'g' is greater at the poles than at the equator, the
weight of the body increases from equator to poles.
Q 23.
State the relationship between the buoyant force on an object and the weight of
the liquid displaced by it.
Solution
Buoyant
force on an object is equal to the weight of the liquid displaced by the
object. This is given by Archimedes' principle.
Q 24. Define thrust and
pressure
Solution
Thrust:
Force acting on a body perpendicular to its surface. Pressure: Thrust per unit
area is called pressure.
Q 25.
Which force governs the motion of the Moon around the Earth?
Solution
The
gravitational force between the Earth and moon governs the motion of the
Moon around the Earth.
Q 26.
Why is the pressure on ground more when a man is walking than when he is
standing?
Solution
Pressure
is inversely proportional to area. P = F/A While walking, one foot is in
contact with the ground at a time. Surface area of one foot is less than two
feet. Therefore, pressure is more than the pressure exerted while standing, as
surface area of both feet is more.
Q 27.
A body of weight 20 N floats half submerged in a liquid. What is the buoyant
force on the body?
Solution
A
floating body displaces an amount of fluid equal to its own weight. So, FB=20N
Q 28.
What would be the apparent weight of an iron block of size 5cm x 5cm x 5cm when
it is completely immersed in water? (density of iron = 7.8 g/cc)
Solution
Volume
of the iron block = 125 cc = 1.25 x 10-4 m3 Mass
of the iron block = volume x density = 125 cc x 7.8 g/cc = 975g = 0.975 kg
Weight of the iron block = 0.975 kg x 10 m/s2 = 9.75 N Upthrust
= Weight of water displaced = volume of water displaced x density of water x g
= volume of iron bock x density of water x g = (1.25 x 10-4 m3)
x (1000 kg/m3) x (10 m/s2) = 1.25 N Thus, apparent weight
of iron block = 9.75 - 1.25 = 8.5 N
Q 29.
Explain the term ‘gravity’.
Solution
The
term gravity is used to denote the gravitational force acting on a body due to
earth (or moon, sun or any planet). If a body of mass m is at a distance ‘r’
from the earth’s centre, the gravitational force acting on the body by the
earth has magnitude of G(mM/r2), where M is the mass of the earth
and G is the universal constant of gravitation. It produces acceleration in the
body, called gravitational acceleration (g),It is given by the formula:
Q 30.
State any two phenomena related to the Universal Law of Gravitation.
Solution
(i)
It is the force that binds us to the earth. (ii) It explains the motion of the
moon around the earth. (iii) It explains motion of planets around the sun. (iv)
It explains the tides due to the moon and the sun. (Any two)
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