LAST MINUTE REVISION SET 2
55 IMPORTANT GIVE REASONS:
FORCE:
Screw jack is provided with longer arm
Ans: [i] Screw jack is used to lift heavy loads such as cars
[ii] longer the effort arm less would be the effort required as force
is inversely proportional to perpendicular distance of point of
application of force and fulcrum.
Steering wheel for huge trucks has larger diameter as compared to
smaller cars.
Ans: [i] Moment of force is directly proportional to perpendicular
distance and force applied.
[ii] hence trucks with larger inertia would require larger force
hence larger diameter of steering wheel.
When water is dropped on a turning wheel it tends to move
outwards.
Ans: [i] A turning object results in two forces acting one radially
inwards while other radially outwards.
[ii] when a water droplet is dropped on the rotating wheel a
tangential force acting vertically outwards namely centrifugal
force appears to move the drop in the outward direction.
SIMPLE MACHINES:
Efficiency of practical machine is less than 100%
Ans: [i] Since the machine is a practical machine certain losses are
incurred.
[ii] Some part of work input is utilised in overcoming frictional
resistance plus in overcoming weight of machine parts which results
in reducing the efficiency of the machine.
Single fixed pulley is a modified form of class one lever
Ans: [i] Since fulcrum is between load and effort
[ii] Effort arm is equal to load arm equal to the radius of the pulley its M.A
is equal to one. Hence modified form of class 1 lever.
Single movable pulley is a modified form of class two lever.
Ans: [i] Since load is between fulcrum and effort
[ii] Effort arm is twice[diameter] the load arm[radius] hence M.A is
greater than1 as a result modified form of class 2 lever.
REFRACTION OF LIGHT AT PLANE SURFACES:
Coin appears to be raised when dipped in water
Ans: [i] This is because of refraction of light
, [ii] As, ray of light travels from optically denser medium to
optically rarer medium bends away from normal hence appears
to be raised.
Stick appears to be bent when dipped in water
Ans: [i] This is because of refraction of light.
[ii] As, the ray travels from optically denser medium to optically
rarer medium bends away from normal hence appears to be
bent.
Stars twinkle in the night
Ans: [i] This is because of refraction of light
[ii] Star being a luminous body emits light of its own. Hence when
this ray of light enters different layers of atmosphere having
different optical densities suffer successive refraction. When
these refracted rays are extended backwards apparent shift of
image results in a twinkling effect
Face of a person sitting near a campfire appears to shimmer.
Ans: [i] This is because of refraction of light
[ii] Light rays from campfire fall on face of a person and reflects to
the observer. In doing so it passes through different layers of
optical density causing successive refraction resulting in
shimmering effect.
Fisherman points towards the tail while spear hunting.
Ans: [i] This is because of refraction of light
[ii] A ray of light from the tail of the fish travels from optically
denser medium water to optically rarer medium air this bending
away from normal. When this refracted ray is extended
backwards it shows the spear would strike the head of the fish.
Fishes have a larger eye view while in water.
Ans: [i] This is because of refraction of light
[ii] As, ray of light travels from water [where the eye of fish is
present] to air it would bend away from normal thus increasing
the angle of view.
Date palm tree when observed by a swimmer from inside water
appear taller than original height.
Ans: [i] This is because of refraction of light
[ii] A ray of light would travel from the tree to person inside water
thereby travelling from optically rarer medium to optically
denser medium this bend towards the normal. When these
refracted rays would be extended backwards it would give an
apparent increased size of image of the date palm tree.
Diamond sparkle in the dark
Ans: This is because of total internal reflection.
, [ii] It has a tetrahedral structure with large number of cut
surfaces reducing its critical angle to as low a value as 24o. Thus,
when light enters diamond and strikes the diamond air interface at
an angle greater than its critical angle it suffers successive total
internal reflection entrapping light inside diamond causing the
sparkling effect.
Lower surface of water appears to be silvery when viewed from
below.
Ans: [i] This is because of total internal reflection.
[ii] As, the ray of light travels from water to air at an angle
greater than critical angle it reflects into the same optically
denser medium appearing to be silvery
Crack in a window pane appears to be silvery
Ans: [i] This is because of total internal reflection
[ii] As, the ray travels from glass to air at an angle greater than
critical angle it reflects into the same optically denser medium
thus appearing to be silvery.
Mirage is a consequence of TIR followed by successive refraction.
Ans: [i] Mirage is an optical illusion generally occurring on hot
sunny days and in deserts due to total internal reflection.
[ii] When a nomadic traveller is in search of water to quench his
thirst tries to locate an oasis in the desert. The ray from the palm
tree would undergo refraction till at a particular angle of
incidence where the value exceeds critical angle and TIR
occurs followed by successive refractions. These refracted rays
extended backwards leads to the optical illusion of Mirage
Empty test tube dipped in beaker containing water appears silvery
Thicker the glass slab more is the lateral displacement.
Ans: [i] Lateral displacement is directly proportional to thickness
of glass slab.
[ii[ Greater the thickness of glass slab more the perpendicular
shift between emergent ray and incident ray extended.
REFRACTION THROUGH LENS:
A spherical lens disappears when dipped in liquid of a specific
optical density
Ans: [i] This is because optical density of liquid is equal to
optical density of spherical lens
[ii] No refraction occurs as a result lens appears invisible.
A convex lens at times behaves as a concave lens and vice versa.
Ans: [i] This would happen if the refractive index is so selected
that a converging lens behaves as diverging lens and vice versa.
55 IMPORTANT GIVE REASONS:
FORCE:
Screw jack is provided with longer arm
Ans: [i] Screw jack is used to lift heavy loads such as cars
[ii] longer the effort arm less would be the effort required as force
is inversely proportional to perpendicular distance of point of
application of force and fulcrum.
Steering wheel for huge trucks has larger diameter as compared to
smaller cars.
Ans: [i] Moment of force is directly proportional to perpendicular
distance and force applied.
[ii] hence trucks with larger inertia would require larger force
hence larger diameter of steering wheel.
When water is dropped on a turning wheel it tends to move
outwards.
Ans: [i] A turning object results in two forces acting one radially
inwards while other radially outwards.
[ii] when a water droplet is dropped on the rotating wheel a
tangential force acting vertically outwards namely centrifugal
force appears to move the drop in the outward direction.
SIMPLE MACHINES:
Efficiency of practical machine is less than 100%
Ans: [i] Since the machine is a practical machine certain losses are
incurred.
[ii] Some part of work input is utilised in overcoming frictional
resistance plus in overcoming weight of machine parts which results
in reducing the efficiency of the machine.
Single fixed pulley is a modified form of class one lever
Ans: [i] Since fulcrum is between load and effort
[ii] Effort arm is equal to load arm equal to the radius of the pulley its M.A
is equal to one. Hence modified form of class 1 lever.
Single movable pulley is a modified form of class two lever.
Ans: [i] Since load is between fulcrum and effort
[ii] Effort arm is twice[diameter] the load arm[radius] hence M.A is
greater than1 as a result modified form of class 2 lever.
REFRACTION OF LIGHT AT PLANE SURFACES:
Coin appears to be raised when dipped in water
Ans: [i] This is because of refraction of light
, [ii] As, ray of light travels from optically denser medium to
optically rarer medium bends away from normal hence appears
to be raised.
Stick appears to be bent when dipped in water
Ans: [i] This is because of refraction of light.
[ii] As, the ray travels from optically denser medium to optically
rarer medium bends away from normal hence appears to be
bent.
Stars twinkle in the night
Ans: [i] This is because of refraction of light
[ii] Star being a luminous body emits light of its own. Hence when
this ray of light enters different layers of atmosphere having
different optical densities suffer successive refraction. When
these refracted rays are extended backwards apparent shift of
image results in a twinkling effect
Face of a person sitting near a campfire appears to shimmer.
Ans: [i] This is because of refraction of light
[ii] Light rays from campfire fall on face of a person and reflects to
the observer. In doing so it passes through different layers of
optical density causing successive refraction resulting in
shimmering effect.
Fisherman points towards the tail while spear hunting.
Ans: [i] This is because of refraction of light
[ii] A ray of light from the tail of the fish travels from optically
denser medium water to optically rarer medium air this bending
away from normal. When this refracted ray is extended
backwards it shows the spear would strike the head of the fish.
Fishes have a larger eye view while in water.
Ans: [i] This is because of refraction of light
[ii] As, ray of light travels from water [where the eye of fish is
present] to air it would bend away from normal thus increasing
the angle of view.
Date palm tree when observed by a swimmer from inside water
appear taller than original height.
Ans: [i] This is because of refraction of light
[ii] A ray of light would travel from the tree to person inside water
thereby travelling from optically rarer medium to optically
denser medium this bend towards the normal. When these
refracted rays would be extended backwards it would give an
apparent increased size of image of the date palm tree.
Diamond sparkle in the dark
Ans: This is because of total internal reflection.
, [ii] It has a tetrahedral structure with large number of cut
surfaces reducing its critical angle to as low a value as 24o. Thus,
when light enters diamond and strikes the diamond air interface at
an angle greater than its critical angle it suffers successive total
internal reflection entrapping light inside diamond causing the
sparkling effect.
Lower surface of water appears to be silvery when viewed from
below.
Ans: [i] This is because of total internal reflection.
[ii] As, the ray of light travels from water to air at an angle
greater than critical angle it reflects into the same optically
denser medium appearing to be silvery
Crack in a window pane appears to be silvery
Ans: [i] This is because of total internal reflection
[ii] As, the ray travels from glass to air at an angle greater than
critical angle it reflects into the same optically denser medium
thus appearing to be silvery.
Mirage is a consequence of TIR followed by successive refraction.
Ans: [i] Mirage is an optical illusion generally occurring on hot
sunny days and in deserts due to total internal reflection.
[ii] When a nomadic traveller is in search of water to quench his
thirst tries to locate an oasis in the desert. The ray from the palm
tree would undergo refraction till at a particular angle of
incidence where the value exceeds critical angle and TIR
occurs followed by successive refractions. These refracted rays
extended backwards leads to the optical illusion of Mirage
Empty test tube dipped in beaker containing water appears silvery
Thicker the glass slab more is the lateral displacement.
Ans: [i] Lateral displacement is directly proportional to thickness
of glass slab.
[ii[ Greater the thickness of glass slab more the perpendicular
shift between emergent ray and incident ray extended.
REFRACTION THROUGH LENS:
A spherical lens disappears when dipped in liquid of a specific
optical density
Ans: [i] This is because optical density of liquid is equal to
optical density of spherical lens
[ii] No refraction occurs as a result lens appears invisible.
A convex lens at times behaves as a concave lens and vice versa.
Ans: [i] This would happen if the refractive index is so selected
that a converging lens behaves as diverging lens and vice versa.
