3 – Force And Pressure
Q 1. Write proper word in the blank
space :
Ans.
(1) The SI unit of force is the newton.
(2) The air pressure on our body is equal to
the atmospheric pressure.
(3)
For a given object, the buoyant force in liquids
of different density is the same.
(4) The SI unit of pressure is N/m2.
Q 2 Make a match.
Ans.
(1) Fluid – Same pressure in all directions
(2)
Blunt knife - Lower pressure
(3)
Sharp needle - Higher pressure
(4)
Relative density - Specific gravity
(5)
Hectopascal - Atmospheric pressure.
Q 3. Complete the following tables -
Ans
Using the formula, Density =
mass/volume:
|
Mass (kg) |
Volume (m3) |
Density (kg/m3) |
|
350 |
175 |
2 |
|
760 |
190 |
4 |
Ans
Using the formula, relative density =
density of a metal/density of water
|
Density of a metal (kg/m3) |
Density of water (kg/m3) |
Relative density |
|
5 x 103 |
103 |
5 |
|
8.5 x 103 |
103 |
8.5 |
Ans
Using the formula, pressure = weight /
area
|
Weight (N) |
Area (m2) |
Pressure (N.m-2) |
|
800 |
0.04 |
20000 |
|
1500 |
500 |
3 |
Q
4. Answer the following.
(1)
A plastic cube is released in water. Will it sink or come to the surface of
water ?
Ans.
It will come to the surface of water. [Note:
This is because its density is less than that of water. When it floats, the
unbalanced force acting on it is zero.]
(2) Why do
the load carrying heavy vehicles have large number of wheels?
Ans.
The pressure produced by a given force depends on the area of the surface on
which the force acts. The greater the surface area, the less is the pressure
produced. Load carrying
heavy vehicles have large number of wheels so that the load (weight, force) is
distributed over large surface area of the wheels in contact with the road.
Hence, the pressure decreases and the tyres do not burst.
(3) How much pressure do we carry on
our heads ? Why don't we feel it ?
Ans.
The air pressure at the sea level is about 101 x 103 Pa. This is the pressure that we
carry on our heads. The cavities in our body are filled with air, and arteries
and veins are filled with blood. Their pressure balances the pressure due to
the atmosphere. Hence, we don't feel the atmospheric pressure.
Q 5. Why does it happen ?
(1) A ship dips to a larger depth in fresh
water as compared to marine water.
Ans.
The buoyant force acting on a body is proportional to the density of the fluid
in which the body is immersed. The density of fresh water is less than that of
marine water. Hence, the buoyant force on a body in fresh water is less than
that in marine water. Therefore, a ship dips to a larger depth in fresh water
as compared to marine water.
(2) Fruits can easily be cut with a sharp
knife
Ans.
The effect of a given force varies inversely as the area of the surface on
which the force is applied. The less the surface area, the greater is the
effect OI the force. The cutting edge of a sharp knife has less cross sectional
area relative to that of a blunt knife. Hence, it is easy to cut vegetables, fruits with a sharp
knife, rather than with a blunt knife. For a given force, pressure is inversely
proportional to the area of the surface on which the force acts.
(3) The wall of a dam is broad at its base.
Ans.
(1) The pressure at a point in a liquid is proportional to the height of the
liquid column above it. Hence, the pressure of water in a dam is much greater
at the bottom of the dam than at the top.(2) To withstand this high pressure,
the wall of a dam is made stronger and thicker (broad) at the base than at the
top.
(4) If a stationary bus suddenly speeds up,
passengers are thrown in the backward direction.
Ans.
(1) When passengers sit or stand in a stationary bus, they are in a state of
rest. When the bus suddenly speeds up, the lower parts of their body in contact
with the bus acquire the speed of the bus. (2) The upper parts of their body, however, continue to
be in the state of rest due to inertia. Hence, they are thrown in the backward
direction.
Q 6.
The density of a metal is 10.8 x 103
kg/m3. Find the relative density of the metal.
Solution
: Data : Density of the metal = 10.8 x 103 kg/m3,
density of water = 103 kg/m3,
relative density of the metal = ?
Relative
density of a substance
density of the substance
= density of water
10.8 x 103 kg/m3
= 10.8 103 kg/m3
The
relative density of the metal = 10.8.
Q 7.
The volume of an object is 20 cm3 and the mass is 50 g. The density of water is
1 gm 3. Will the object float on water or sink in water ?
Solution : For reference, see solved Ex. (7).
p(object)= 50 g
20 cm3
= 2.5 g /cm3
It is
greater than the density of water.
Hence,
the object will sink in water.
Q 8.
The volume of a plastic covered sealed box is 350 cm3 and the box has a mass
500 g. Will the box float on water or sink in water? What will be the mass of
water displaced by the box ?
Solution
: For reference, see solved Ex. (7).
P(box)= 500 g
p = 10
350cm3
7
It is
greater than that of water.
Hence,
the box will sink in water.
The
volume of water displaced by the box
(V)=
the volume of the box = 350 cm3
Now, density (p) = mass (m)
volume (V)
m=pV
The mass of water displaced by the box = 1
g.cm-3 x 350 cm3 = 350 g.
