Given below are the Class 9 Science Important Questions for Gravitation
a. Concepts questions
b. Calculation problems
c. Multiple choice questions
d. Long answer questions
e. Fill in the blank's
Multiple choice Questions
Question 1. Which of the following is true?
a. The acceleration due to gravity acting on a freely falling body is directly proportional to the mass of the body
b. Mass of the object is same on Moon and Earth
c. G value is always constant
d. The weight of an object at the center of earth will be zero
Solution (b), (d), (c)
Question 2. A big stone and small are dropped from the roof of the house at the same time. Which one will reach the ground first?
a. Big Stone
b. Small stone
c. Both at the same time
d. Not able to determine with the given data
Solution (c)
Question 3. The value of acceleration due to gravity of earth
a. Same on equator and poles
b. Is the least at equator
c. Is the least on poles
d. Increase from pole to equator
Solution (c)
Question 4. An object is thrown vertically upwards and rises to a height of 10 m. Calculate the velocity with which the object was thrown upwards? Take g=9.8 m/s
^{2}
a. 14m/s
b. 16m/s
c. 10m/s
d. 9.8 m/s
Solution
Distance traveled, s = 10 m
Final velocity, v = 0 m/s
Acceleration due to gravity, g = 9.8 m s
^{2}
Acceleration of the object, a = -9.8 m s
^{2}
$v^2= u^2+ 2as$
$0 = u^2+ 2 \times (-9.8) \times 10$
$u^2= 2 \times 9.8 \times 10$
u = 14 m/s
Question 5. The time taken by the object to reach the highest point in the above question
a. 1.42s
b. 1.5 s
c. 1 s
d. 1.43 s
Solution
$v = u + a t$
$0 = 14 - 9.8 \times t$
t = 1.43 s.
Question 6. Which of the them is true for two bodies separated by some distance?
a. When the distance between them is halved, Gravitational force becomes 4 times
b. When one of the mass becomes halved, Gravitational force becomes halved
c. When the distance between them is increased four times, Gravitational force becomes 1/16 times
d. None of the above
Solution (a) (b) (c)
The above can be simply calculated from the below formula
$F=\frac{Gm_{1}m_{2}}{r^{2}}$
Question 7.The Weight of the body at a certain place is 30 N. The acceleration due to gravity at that point is 10 m/s
^{2}. Find out the mass and weight of the object at the place where acceleration due to gravity is zero?
a. 3 kg ,0 N
b. 3 Kg,30 N
c. 3 Kg,3 N
d. None of these
Solution:
Mass of the body=$ \frac {30}{10}=3 Kg$
Since Mass remains same everywhere, Weight varies as per acceleration due to gravity which is zero at g=0
Hence (a)
Question 8. The acceleration due to gravity at three point A,B and C are 9.8 m/s
^{2} ,10m/s
^{2} and 5 m/s
^{2} on the earth surface?
Which of the following is true?
a. B is at least distance out of three point from the center of the earth
b. C is at farthest distance out of three point from the center of the earthv
c. Weight of the object is lowest at point C out of three point
d. The weight of the object varies as
$W_B > W_A > W_C$
Solution :All are correct and self explanatory
Short Questions
Question 1. What is the difference between mass and Weight?
Question 2. Derive the inverse square of Newton.
Question 3. Define ‘G’ and give its value.
Question 4. A ball is thrown up with speed of 10 m/s. How high will it go before it begins to falls? Take g=10m/s
^{2}
Solution:
v=0 ,u=10m/s a=-10m/s
^{2}, h=?
$v^2=u^2 +2ah$
Substituting the above values
H=5 m
Question 5.
The weight of the man on earth is 150 N and on certain planet is 25 N.
Take g=10m/s
^{2} on earth
a.Find the mass of the man on earth and planet
b. Find the acceleration due to gravity on the planet
Solution
Weight on earth =150 N
So, mass of man on earth= $ \frac {150}{10}=15 kg$.
Now mass does not varies and it will remain same on earth and planet
Now weight on Planet=25N
Now as mass =15 kg
Acceleration due to gravity on Planet= $ \frac {25}{15}=1.66$ m/s
^{2}
Question 6. Is acceleration due to gravity (g) constant?. Tell us how it is there at different places on earth?
What are these:
(i) Product Rule
(ii) Inverse Square rule
(iii) Universal gravitational constant
(iv) Universal law of gravitation
Solution
acceleration due to gravity (g) is not constant. It varies with different planet and it is different at different places on earth. It varies as per Inverse Square rule
Question 7.The pressure exerted by a cube of side 0.03m on a surface is 10Pa. Calculate the thrust exerted by the cube?
SolutionSide of cube = 0.03 m
Hence, area = $0.03 \times 0.03 = 0.0009 m^2$
Pressure = 10 Pa.
Now,
$thrust = pressure \times area$
So,
$
= 10 \times 0.0009
= 0.009N$
Question 8.
State the mathematical formula that relates thrust and pressure. List the factors on which the pressure depends and explain how it depends on them?
Question 9.Differentiate between thrust and pressure?
Question 10.The mass of a block made of certain material is 13.5 kg and its volume is $15 \times 10^{-3} m^3$. Will the block float or sink in water?
Solution
$Density = \frac {mass}{volume}$
$Density = \frac {13.5}{15 \times 10^{-3} }=900 kg/m^3$
Now density of water is 1000kg/m
^{3}. So block density is less than that of water. Hence block will float
Question 11.
State Archimedes principle?
Solution
It states that when an body is fully or partially immersed in a fluid, it experiences an upthrust, which is equal to the weight of water displaced by the immersed portion of the body
Question 12.Define buoyancy. State the factors on which buoyant force depends?
Solution
Buoyancy is the upward force acting on the body when it is submerged in a liquid
buoyant force Depends on
a.Volume of the fluid displaced.
b.Density of the fluid in which the body is immersed.
c. Acceleration due to gravity at the place.
Question 13.A solid weighs 50 gf in air(where gf is gram force) and 44 gf when completely immersed in water. Calculate:
a.the upthrust
b.the volume of the solid
c. the relative density of the solid
Given Density of water=$1000 kg/m^3$
Solution
a. Upthrust = Weight in air - Weight in water = 50 -44 = 6 gf = .0588 N
b. Let V be the volume
$Upthrust= V \rho_w g = V \times 1000 \times 9.8 = 9800V$
Now
$9800V=.0588$
or $V= .000006 m^3 = 6 cm^3$
c. Mass of Solid = 50g
$Volume= 6 cm^3$
$Density =\frac {mass}{volume} = 8.33 gm/cm^3$
Now $\text {relative density of the solid} = \frac {\text{Density of Solid}}{\text{Density of water}} = \frac {8.33}{1} = 8.33$
Question 14.The volume of a 350g sealed tin is 200 cubic.cm. Find the density of the tin in g/cc. Also find the density in SI unit?
Solution
$Density = \frac {mass}{volume}$
$Density = \frac {350}{200 }=1.75 g/cm^3$
Density in SI unit
$=1.75 \times 1000= 1750 kg/m^3$
Question 15.Why does a body lose weight when immersed in a liquid?
Solution
When a body is immersed in liquid, an upward force acts on the body which is equal to the weight of water displaced by the immersed portion of the body. So net gravtational force becomes less and body lose weight
Question 16.State the conditions under which a body will sink or float in a fluid?
Solution
a. if the density of the body is higher than the density of liquid,then it would sink.
b. if the density of body is less than the density of liquid,then it would float.
Question 17. Mona weighs 750 N on earth.
(i) What will be Mona’s mass on Earth ($g = 10 \ ms^{-2}$)
(ii) On the planet Mars, the force of gravity is 38% of that of Earth. How much will Mona weigh on Mars?
Question 18.Give three points of differences between mass and weight.
Question 19.If the earth attracts an apple, does the apple also attract the earth? If so, why does the earth not move towards the apple?
Question 20.The ratio of the gravitational force on Neptune to the gravitational force on earth is 9 : 8. Ramesh weighs 792 N on earth. Calculate his weight on Neptune.
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