Group A (1 mark questions)
1 What is focusing? [1]
Answer: Changing the lens position so as to obtain the sharp image on the screen is called focusing.
2 What is least distance of distinct vision? [1]
Answer: The closest or least distance at which an object can be seen clearly is called least distance of distinct vision.
3 What is weightlessness? [1]
Answer: A state of apparent loss of weight is called weightlessness.
4 What is concave lens? [1]
Answer: The lens which is thin at the middle and thick at the ends is called concave lens.
5 What is null point? [1]
Answer: The points in space where the gravitational forces acting on the body become equal and opposite and the net pull amounts to nothing are called null points.
6 What is heat? [1]
Answer: Heat is a form of energy that gives us the sensation of warmth.
7 Under what condition a falling parachute has no acceleration? [1]
Answer: A falling parachute has no acceleration when the gravity of the earth and the upthrust of the air acting on the parachute are equal in magnitude but opposite in direction.
8 What is convex lens? [1]
Answer: The lens which is thick at the middle and thin at the ends is called convex lens.
9 What is weightless? [1]
Answer: A body is said to be weightless when it appears to have no weight.
10 What is gravitational field intensity? [1]
Answer: Gravitational field intensity at a point is defined as the gravitational force experienced by a unit mass kept at that point.
Group B (2 marks questions)
11 Why does an astronaut feel weightless inside an artificial satellite orbiting around the earth? [2]
Answer: In an artificial satellite orbiting around the earth, there are two forces acting on it:
i) The force of gravity of the earth acting towards the centre of the orbit and
ii) The centrifugal force due to the motion of the satellite acting outward, opposite the centre.
These two forces are equal and opposite in direction. So, they balance each other.
Therefore, there will be no resultant force acting on the satellite and the astronaut inside.
The satellite is a small mass. It does not have its own gravity.
So, the astronaut feels weightless inside the satellite due to the lack of attracting force.
12 Why does water overflow from the beaker when the beaker full of water at 4oC is heated? [2]
Answer: This is because water at 4oC has minimum volume and when the beaker full of water at 4oC is heated, water expands for which it overflows from the beaker.
13 Why does a body fall faster in the polar region of the earth than in the equatorial region? [2]
Answer: When a body starts to fall, its initial velocity is zero.
While falling, velocity of the body increases constantly with time.
The increase in its velocity depends upon the acceleratio due to gravity at the place.
More the acceleration due to gravity, more will be the increase in velocily per second and the object will fall faster.
In the equatorial region, the radius of the earth is more.
Acceleration due to gravity is less in this region as it is inversely proportional to the square of the radius of the earth.
Due to less acceleration, the object falls slower.
While in the polar region, the radius of the earth is less and the acceleration due to gravity is more.
So, the object falls faster in the polar region due to more acceleration due to gravity.
14 In the moon, if the coin and feather are dropped simultaneously from equal height they reach on the ground together. Why? [2]
Answer: Because in the surface of the moon, there is no atmosphere and due to the lack of air resistance all the falling bodies gain the same acceleration due to gravity.
15 The value of gravitational constant is 6.67 x 10-11Nm2/kg2. What does it mean? [2]
Answer: The value of gravitational constant is 6.67 x 10-11Nm2/kg2.
It means the gravitational force of attraction between two unit(1kg) masses which are separated by unit distance(1m) is 6.67 x 10-11N.
16 Tides occur in oceans but not in lakes. Give reason. [2]
Answer: Tides occur due to the gravity of the moon. As the mass of oceans water is greater than the mass of water on the lakes, there is more force of gravity between the ocean water and the moon. It results in the pull of the water due to which the ocean level increases and tide occurs. But force of attraction between the moon and the lake is negligible. So, tides occur in oceans but not in lakes.
17 Water is not used as a thermometric liquid. Why? [2]
Answer: Water is not used as a thermometric liquid because water has not uniform expansion as it contracts on heating from 0oC to 4oC and then expands on further heating from 4oC.
Group C (3 marks questions)
18 An iron ball of mass 60kg is attracted by another ball of mass 50 kg with a force of 3.16×10-6 N. Find the value of Gravitational constant if the centre of balls are 25 cm apart. [3]
Answer: Do yourself
19 The volume of Earth is compressed to make equal volume of the moon of radius 1.7×106m. If mass of earth is 6 ×1024kg, then how many times the acceleration due to gravity increases?
Answer do yourself
20 How much heat is supplied to raise the temperature of 100 gram of water from 5°C to 90°C? (Specific heat capacity of water = 4200 Jkg-1 °C-1) [3]
Answer do yourself Q = 35700 J
21 Hot water of mass 10 kg at 90°C cooled for taking bath by mixing 20 kg of water at 20°C. What is the final temperature of water? Specific heat capacity of water is 4200 J/kg°C? Neglect the heat taken by bucket? [3]
Solution:
Here,
Given,
For hot water,
Mass (m1) = 10 kg
Initial temperature (t1) = 90°C
For cold water,
Mass (m2) = 20 kg
Initial temperature (t2) = 20°C
Specific heat capacity (s) = 4200 J/kg°C
Final temperature of mixture (t) =?
According to the heat equation,
Q = msdt
where,
m = Mass of body
Q = Heat gained or lost
s = Specific heat capacity of body
dt = Change in temperature
Again,
According to the principle of calorimetry, the amount of heat gained is equal to the amount of heat lost.
So,
Heat lost by hot water = Heat gained by cold water
m1sdt1 = m2sdt2
Or, 10 х 4200 х (90 — t) = 20 х 4200 х (t — 20)
t = 43.33°C
Hence, the final temperature of the water is 43.33°C.
Group D (4 marks questions)
22 Give reason:
I. Water is used to cool the engine of vehicles.
II. A new quilt is felt warmer than old one.
Answer:
I. Water has very high specific heat capacity (4200 J/kg°C) as compared to other liquids. The substances which have more specific heat capacity change their temperature slowly. Thus, water can absorb large amount of heat from the hot engines without appreciable rise in temperature of water. So, it prevents the engines of vehicle being excessively hot. Due to this reason water is used to cool the engine of vehicles.
II. A quilt feels warm because the air trapped in between the cotton or woolen batting acts as an insulator and does not allow the heat of the body to escape. However, when the quilt gets old, the cotton/woolen batting gets compressed and air spaces decreases. It does not remain as good an insulator to heat as it was earlier. Hence, a new quilt is felt warmer than old one.
23 Draw a ray diagram to show the formation of image kept 2 cm far from the convex lens with focal length of 4 cm. Mention the nature of image with one application. [4]
Answer: In the figure below, F is the principal focus of the convex lens, O is the optical centre. Thus, the line OF represents focal length (f). The point 2F on the principal axis is at a distance of 2f from the optical centre O. If f = 4 cm, the point 2F is at a distance of 10 cm from O.
According to the question, the object is placed between focus (F) and optical centre (O). When an object is placed between F and O, the convex lens forms an image on the same side as the object. The image is:
• virtual
• erect
• magnified
This type of position of the object in the convex lens is used in the simple microscope.
24 A student sitting on the last bench of the class can see things written on the board but wears spectacles while reading a book. On the basis of this, answer the following questions:
I. What is the type of his defect?
II. What are the causes of a defect of vision?
III. What type of spectacles should be used to remove his defect of vision?
IV. Draw a diagram to show the remedy of this defect? [4]
Answer
I. The type of defect he has is hypermetropia or long sightedness. It is defined as the defect of vision in which a person cannot see nearby objects clearly but can see the distant object distinctly.
II. The causes of this type of defect of vision are as following:
a. As the eye lens becomes thin and the ciliary muscles fail to press the lens for the required thickness, it results in an increase in the focal length of the eye lens.
b. If the eyeball gets shortened, the distance between the eyes lens and retina becomes less. This also results in an increase in the focal length of the eye lens.
III. For the correction of this defect, a convex lens is suitable focal length is used in spectacles.
IV.
Fig: Use of the convex lens for the cure of hypermetropia.
