# Class 9 Science Gravitation test questions

Question 1
Define
a.Buoyancy
b.Factor of buoyant force
c.Archimedes Principle and its use

Question 2
Explain why
a. Pressure on ground more when man is walking than he is standing?
b. A bucket of water is lighter when in water than when it is taken out of water?
c. An iron nail floats on mercury but sink in water
d. It is easy to walk on sand with flat shoes than with high heel shoes?
Question 3
A iron cube of side 10 cm is kept on a horizontal table. If the density of iron is 8000 kg/m3.Find the pressure on the portion of the table where cube is kept
Take g=10m/s2

Question 4
The dimension of wooden block is $2m \times .25 m \times .10 m$. If relative density of wood is 0.6 calculate the mass of the block in kg
given density of water is 1000 kg/m3
Question 5
Differentiate between density and relative density
Question 6
A 100 cm3 block has a mass of 395g. Find its relative density
given density of water is 1000 kg/m3
Question 7
A block of wood is kept on the table top. The mass of wooden block is 5 kg and its dimension are $40 \; cm \times 20 \; cm \times 10 \; cm$. Find the pressure exerted by the wooden block on the table if it made it lie  on the table top with its sides of dimension
a. $20 \; cm \times 10 \; cm$
b. $40 \; cm \times 20 \; cm$
Take g=9.8 m/s2

Question 8.
Why are railway tracks laid on large sized concrete sleepers?
Question 9.
While drawing water from a well, the bucket of water appears to be heavier as it comes out of the water?
Question 10.
Why does a block of plastic releases underwater come up to the surface of water?
Question 11.
Two particles A and B of mass $m_1$ and $m_2$ respectively are placed at some distance. If the mass of each of the two particles is doubled, keeping the distance between them unchanged, the value of gravitational force between them will be
a. 4 times
b. 2 times
c. 1/2 times
d. 1/4 times
Question 12.
A boy is whirling a stone tied with a string in an horizontal circular path. If the string breaks, the stone
(a) will continue to move in the circular path
(b) will move along a straight line towards the centre of the circular path
(c) will move along a straight line tangential to the circular path
(d) will move along a straight line perpendicular to the circular path away from the boy
Question 13.
An object weighs 20 N when measured on the surface of the earth. What would be its weight when measured on the surface of the moon?
Question 14.
Earth Gravitational pull causes an acceleration $7 m/s^2$ in 2 kg of body somewhere in the space. How much will be the acceleration of 10 Kg mass due to earth gravitational pull at the same place
Question 15.
A child sitting along a swimming pool feels lighter while lowering his legs in water. Why?

## Solutions

2.
a. Pressure is inversely proportional to area. $P = \frac {F}{A}$
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.
b. A bucket of water feels heavier when taken out of water because when immersed in water, an buoyant force(upward force) acts on it which is equal to the weight of water displaced by the bucket.So net force on the bucket is zero .
c. An iron nail floats on mercury because the density of mercury is greater than the density of iron.
It sinks in water as the density of water is less than that of iron so it sinks in water.
d. Flat shoes has more surface area in contact with sand that high heel shoes.
Now Pressure is inversely proportional to area. $P = \frac {F}{A}$. So less pressure is experienced in case if flat shoes and we feel comfortable

3.
Side of cube=10 cm=0.1 m
Area of plane of cube=$(side) ^2=0.01 m^2$
volume of cube=$(side)^3=0.001 m^3$
given density=8000 kg/m3.
Now ,
$density= \frac {mass}{volume}$
or
$mass= density \times volume$
Substituting the values above
m=8kg
Now force on the surface of table by iron block=$mg=8 \times 10=80$ N
Now $P = \frac {F}{A}$
$P= \frac {80}{0.01}=8000 N/m^2$

4.
The volume of the wooden block= $l \times b \times h =2 \times 0.25 \times 0.10 =0.05 \; m^3$
Now $Relative \; density = \frac {density \; of \; the object}{density \; of \; water}$
or
$density \; of \; the \; object = (Relative \; density) \times (density \; of \; water)$
$\text{density of the object}= 0.6 \times 1000 =600 \; kg/m^3$
now,
$density \; of \; the \; wood= \frac {mass}{volume}$
or
br>$mass = density \times volume$
mass=30kg

5.
Density: Mass per unit volume of a substance is called density. SI unit is kg/m3
$density= \frac {mass}{volume}$

Relative Density: The ratio between density of a substance and density of water is called relative density.Relative density has no units because it is the ratio between the similar physical quantities
$Relative \; density = \frac {density \; of \; the object}{density \; of \; water}$

6.
$density= \frac {mass}{volume}$
$density =\frac {.395}{100 \times 10^{-6}} = .395 \times 10^4$ kg/m3
$Relative \; density = \frac {\text{density of the object}}{\text{density of water}}$
or
$Relative \; density = \frac {.395 \times 10^4}{1000} = 3.95$

7.
Mass of the wooden block = m= 5 kg
Dimensions = $40 \; cm \times 20 \; cm \times 10 \; cm$
Thrust on the table top is due to the weight of the wooden block.Now,
Weight of the box = $F = mg= 5 kg \times 9.8 m/s^2= 49 N$
a. Area of a side = $length \times breadth = 20 \times 10 = 200 cm^2 = 0.02 m^2$
$Pressure = \frac {weight}{ area}= \frac {49 }{0.02}= 2450 N/m^2$
b. Area= $length \times breadth= 40 \times 20= 800= 0.08 m^2$
$Pressure = \frac {weight}{ area}= \frac {49 }{0.08}= 612.5 N/m^2$
8.
Since concrete sleeper has very large size ,it helps to reduce the pressure on the ground.
9.
When the bucket is in water, an upthrust is acting on it,so it reduces the net gravitational pull and it appears lighter
10.
Density of plastic block is less than of water,So the upward thrust exerted equals to displaced water is greater then its weight,so it come up to the surface of water
11.
$F=\frac{Gm_{1}m_{2}}{r^{2}}$
So Force will becomes 4 times
12.
(c)

13.
We know that,
$\text{ Weight of object on the moon} = \frac {1}{6} \times \text{body weight on the earth}$.
$W= \frac {1}{6} \times 20 = 3.34 N$.
14.
$7 m/s^2$ only as $g= \frac {GM}{r^2}$ is independent of the mass of the object
15.
It is because of the upward force experience due to buoyancy