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Gravitation · Bihar Board (BSEB) STD 9 (BSEB - English Medium). 62 questions.

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Question 11 Mark
lire relative density of mercury is $13.6.$ What does this statement mean$?$
Answer
The relative density of mercury is $13.6,$ this means that mercury is $13.6$ times as heavy as an equal volume of water.
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Question 51 Mark
What is the weight of a $1$ kilogram mass on the earth$? (g = 9.8 \mathrm{~m} / \mathrm{s}^2).$
Answer
Weight, $W=m \times g$
$1 \mathrm{~kg} \times 9.8 \mathrm{~m} / \mathrm{s}^2=9.8 \mathrm{~N}$
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Question 61 Mark
Name the scientist who gave he magnitude of buoyant force acting on a solid object immersed in a liquid.
Answer
Archimedes gave the magnitude of buoyant force acting on a solid object immersed in a liquid.
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Question 81 Mark
Explain why, if a stone held in our hand is released, it falls towards the earth.
Answer
This is because the earth exerts a force of attraction (called gravity) on the stone and pulls it down.
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Question 91 Mark
What is the value of gravitational constant $G\ (i)$ on the earth, and $(ii)$ on the moon$?$
Answer
Value of gravitational constant $G$ on the earth and the moon is $=6.67 \times 10^{-11} \mathrm{Nm}^2 / \mathrm{kg}^2$ Note that the value of $G$ always remains constant irrespective of the location.
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Question 101 Mark
On what factor/factors does the weight of a body depend?
Answer
The weight of a body is directly proportional to its mass. It also depends on the acceleration due to gravity which varies from place to place.
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Question 111 Mark
Why does ice float in water?
Answer
The density of ice is less than that of water, so ice floats in water.
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Question 121 Mark
What name has been given to the force with which two objects lying apart attract each other?
Answer
Gravitational force causes two objects lying apart attract each other.
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Question 131 Mark
State the units in which pressure is measured.
Answer
Pressure is measured in newton per square meter $\left(\mathrm{N} / \mathrm{m}^2\right)$ i.e., Pascal $( Pa)$.
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Question 141 Mark
A stone resting on the ground has a gravitational force of $20\ N$ acting on it. What is the weight of the stone$?$ What is its mass? $\left(g=10 \mathrm{~m} / \mathrm{s}^2\right)$.
Answer
Weight of the stone $=$ Gravitational force acting on it $=20 \mathrm{~N}$
Weight, $W=m \times g$
$20=\mathrm{m} \times 10$
$\mathrm{~m}=2 \mathrm{~kg}$
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Question 151 Mark
A body has a weight of $10\ kg$ on the surface of earth. What will be its weight when taken to the centre of the earth$?$
Answer
Its weight will be zero as value of g is zero at the centre of the earth.
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Question 161 Mark
Name the property of earth which is responsible for extremely small acceleration being produced in it as a result of attraction by other small objects.
Answer
Extremely large mass of the earth.
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Question 171 Mark
During a free fall, will heavier objects accelerate more than lighter ones?
Answer
No, acceleration is independent of the mass of the body during free fall.
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Question 181 Mark
As the altitude of a body increases, do the weight and mass both vary?
Answer
Weight of the body varies with altitude; mass of an object is constant.
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Question 191 Mark

Why does a block of wood held under water rise to the surface when released?

Answer
Because the weight of the block of wood is less than the weight of an equal volume of water. So when it is completely submerged in water, the upward buoyant force on it is greater than the downward gravitational force on it. Hence, the lock rises to the surface.
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Question 201 Mark
Which of the Kepler’s laws of planetary motion led Newton to establish the inverse-square rule for gravitational force between two bodies?
Answer
Kepler’s third law of planetary motion led Newton to establish the inverse-square rule for gravitational force between two bodies.
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Question 211 Mark
When an object is dropped from a height, it accelerates and falls down. Name the force which accelerates the object.
Answer
Gravitational force of the earth.
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Question 221 Mark
What is the usual value of the acceleration due to gravity of earth?
Answer
Usual value of acceleration due to gravity, $\mathrm{g}=9.8 \mathrm{~m} / \mathrm{s}^2$.
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Question 231 Mark
Name these force: The force which wears away two surfaces as they move over one another.
Answer
Force of friction.
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Question 241 Mark
An object has mass of $20\ kg$ on earth. What will be its $(i)$ mass, and $(ii)$ weight, on the moon$? (g$ on moon $= 1.6 \mathrm{m} / \mathrm{s}^2).$
Answer
  1. Its mass will be $20\ kg$ as mass is a constant quantity.
  2. Weight, $W = m × g =20 × 1.6 = 32N.$
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Question 251 Mark
What is the relation between mass and weight of a body$?$
Answer
Weight, $W = m × g,$ i.e. the weight of a body is directly proportional to its mass.
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Question 261 Mark
What is the density of water in $SI$ units$?$
Answer
Density of water $=1000 \mathrm{~kg} / \mathrm{m}^3$.
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Question 271 Mark
Define mass of a body. What is the $SI$ unit of mass$?$
Answer
The mass of a body is the quantity of matter contained in it. The $SI$ unit of mass is kilogram $(kg).$
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Question 281 Mark
The weight of a body is $50\ N .$ What is its mass? $\left(\mathrm{g}=9.8 \mathrm{~m} / \mathrm{s}^2\right)$.
Answer
Weight, $\mathrm W=m \times g$.
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Question 291 Mark
State the Kepler’s law which is represented by the relation $\text{r}^3\propto\text{T}^2.$
Answer
Kepler’s law of periods states that: The cube of the mean distance of a planet from the sun is directly proportional to the square of time it takes to move around the sun.
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Question 301 Mark
Name the scientist who gave the three laws of planetary motion.
Answer
Johannes Kepler gave the three laws of planetary motion.
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Question 311 Mark
Define weight of a body. What is the $SI$ unit of weight$?$
Answer
The weight of a body is the force with which it is attracted towards the centre of the earth. The $SI$ unit of weight is newton $(N).$
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Question 321 Mark
A mug full of water appears light as long as it is under water in the bucket than when it is outside water. Why?
Answer
A mug full of water appears light as long as it is under water because buoyant force acts on it which reduces its effective weight and makes it appear lighter.
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Question 331 Mark
Does the acceleration produced in a freely falling body depend on the mass of the body?
Answer
No, the acceleration produced in a freely falling body is independent of the mass of the body.
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Question 341 Mark
Which force is responsible for holding the solar system together?
Answer
Gravitational force of the sun holds the solar system together.
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Question 351 Mark
When a golf ball is lowered into a measuring cylinder containing water, the water level rises by $3cm^3$ when the ball is completely submerged. If the mass of ball in air is $33g,$ find its density.
Answer
Volume of golf ball $=$ rise in water level $= 30 cm^3$ Density of ball $=\frac{\text{Mass of ball}}{\text{Volume of ball}}=\frac{33}{30}=1.1\text{g}/\text{cm}^3$
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Question 361 Mark
What are fluids? Name two common fluids.
Answer
Those substances which can flow easily are called fluids. All the liquid and gases are fluids, like water, air etc.
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Question 371 Mark
In which direction does the buoyant force on an object due to a liquid act?
Answer
Buoyant force on an object due to a liquid act s in the vertically upward direction.
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Question 391 Mark
Give the formula for the gravitational force $F$ between two bodies of masses $M$ and $m$ kept at a distance d from each other.
Answer
The gravitational force $F$ between two bodies of masses $M$ and $m$ kept at a distance $d$ from each other is:
$\text{F}=\text{G}\times\frac{\text{m}\times\text{M}}{\text{d}^2}$
Here, gravitational constant, $\mathrm{G}=6.7 \times 10^{-11} \mathrm{Nm}^2 / \mathrm{kg}^2$
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Question 401 Mark
Name the scientist who explained the motion of planets on the basis of gravitational force between the sun and planets.
Answer
Newton explained the motion of planets on the basis of gravitational force between the sun and planets.
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Question 411 Mark
Write down the weight of a $50\ kg$ mass on the earth. $\left(\mathrm{g}=9.8 \mathrm{~m} / \mathrm{s}^2\right)$.
Answer
Weight, $W=\mathrm{m} \times \mathrm{g}=50 \times 9.8=490 \mathrm{~N}$
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Question 431 Mark
Name these force:
The force which pulled the apple off Isaac Newton’s tree.
Answer
Gravitational force.
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Question 441 Mark
Which force is responsible for the moon revolving round the earth?
Answer
Gravitational force is responsible for the moon revolving round the earth.
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Question 451 Mark
What causes buoyant force (or upthrust) on a boat?
Answer
The buoyant force on a boat is caused by the pressure of water ‘pushing up’ on the bottom of the boat.
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Question 461 Mark
If the same body is taken to places having different gravitational field strength, then what will vary: its weight or mass?
Answer
Its weight varies; mass of an object is constant.
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Question 471 Mark
What is the acceleration produced in a freely falling body of mass $10\ kg ? ($Neglect air resistance$)$
Answer
Acceleration produced in a freely falling body, irrespective of its mass, is $9.8 \mathrm{~m} / \mathrm{s}^2$
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Question 481 Mark
State the $SI$ unit of acceleration due to gravity.
Answer
$SI$ unit of acceleration due to gravity is $\mathrm{~m} / \mathrm{s}^2$.
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Question 491 Mark
The density of a body is $800 \mathrm{~kg} / \mathrm{m}^3$. Will it sink or float when dipped in a bucket of water? (Density of water $=$ $1000 \mathrm{~kg} / \mathrm{m}^3$ )
Answer
The body will float when dipped in a bucket of water as its density is less than that of water.
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Question 501 Mark
If the mass of an object be $10\ kg ,$ what is its weight? $\left(\mathrm{g}=9.8 \mathrm{~m} / \mathrm{s}^2\right)$.
Answer
Weight, $W=m \times g=10 \times 9.8=98 \mathrm{~N}$
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