3. Force and Pressure
1. Write proper word in the
blank space.
a. The SI unit of force
is Newton
(Dyne, Newton, Joule)
b. The air pressure on our body is equal to
Atmospheric pressure
(Atmospheric, Sea bottom, Space)
c. For a given object, the buoyant
force in liquids of different density is the same
(the same, density, different, area)
d. The SI unit of pressure is N/m2 .
(N/m3, N/m2, kg/m2, Pa/m2)
2. Make a match.
A group B group
1. Fluid a.Higher pressure
2. Blunt knife b. Atmospheric Pressure
3. Sharp needle c. Specific gravity
4. Relative density d.
Lower pressure
5. Hecto Pascal e. Same pressure in all directions
Ans:
|
A group |
B group |
|
1. Fluid |
Same pressure in all directions |
|
2. Blunt knife |
Lower pressure |
|
3. Sharp needle |
Higher pressure |
|
4. Relative density |
Specific gravity |
|
5. Hecto Pascal |
Atmospheric Pressure |
3.
Answer the following questions in brief.
a. A plastic cube is released in water. Will
it sink or come to the surface of water?
Ans: The plastic cube is going to float on the
surface of water as its density is less than that of water. This is because its density is less
than that of water. When it floats, the unbalanced force acting on it is zero
b. Why do the load
carrying heavy vehicles have large number of wheels?
Ans: We know, Pressure = Force/Area
Greater
the area of contact between two surfaces, lesser will be the pressure.
So, the load carrying heavy vehicles have large number of wheels so that
the pressure on the road is reduced due to larger contact area. Also, using
large number of wheels ensures that the force due to the load is shared among
the tyres and no single tyre is under stress.
c. How much
pressure do we carry on our heads? Why don’t we feel it?
Ans: We
carry atmospheric pressure of about 105 Pa on our heads. We
don't feel it because this atmospheric pressure is balanced by the pressure
created by the air and blood inside our body. Hence, we don’t feel the atmospheric pressure.
4. Why
does it happen?
a. 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 freshwater is less than that of marine water.
Hence, the buoyant force on a body in freshwater is less than that in marine
water. Therefore, a ship dips to a larger depth in freshwater as compared to
marine water.
b. Fruits can
easily be cut with a sharp knife.
Ans: It is
easy to cut vegetables fruits with a sharp knife. A blunt knife does not work
here. The force exerted perpendicularly on a unit area is called ‘pressure’
The sharp edged knife makes
lesser contact with the fruits to be cut, thus the pressure exerted by it on
the fruit is very large. Because of this large pressure, fruits are easily cut
with a sharp knife.
c. The wall of a
dam is broad at its base.
Ans: We know pressure of liquid increases with depth. So, the
wall of a dam is made broader at its base so that it is able to withstand the
heavy pressure exerted by the river water.
d. If a stationary bus suddenly speeds up,
passengers are thrown in the backward direction.
Ans: If a stationary bus suddenly speeds up, passengers are
thrown in the backward direction. This is because initially the whole body of a
passenger inside the bus was in the state of rest. But, when the bus suddenly
starts or speeds up, the lower half of the passenger's body comes in motion in
the forward direction but the upper half still remains at rest due to inertia
of rest. Hence, the passengers are thrown backward when a stationary bus suddenly
speeds up.
5.
Complete the following tables.
Ans
By using the formula,
density = mass/volume:
|
Mass (kg) |
Volume (m3) |
Density (kg/m3) |
|
350 |
175 |
2 |
|
750 |
190 |
4 |
By using the formula
relative density = density of a metal/density of water
|
Density of Metal (kg/m3) |
Density of water (kg/m3) |
Relative density |
|
5××103 |
103 |
5 |
|
8.5x103 |
103 |
8.5 |
By using the formula
pressure = weight/area
|
Weight (N) |
Area (m2) |
Pressure (Nm-2) |
|
800 |
0.04 |
20,000 |
|
1500 |
500 |
3 |
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 × 103 kg/m3
density of water = 103 kg/m3 We know, density of water = 1000 kg/m3
relative density of the metal = ?
Relative density of substance 
Density
of metal /Density of water
Relative density of substance = 10.8 × 103 kg/m3 / 1000 kg/m3
Relative density of substance =10.8
7. Volume
of an object is 20 cm3 and the mass in 50 g. Density of water is 1 gcm-3. Will
the object float on water or sink in water?
Solution:
Given:
Volume of object = 20 cm3
Mass of object = 50 g
Density of object = Mass of object / Volume of object
Density of object = 50 g / 20 cm3
Density of object is 2.5 gcm3
We
know that Density of water is 1gcm3
It is greater than the density of water.
Hence, the object will sink in water.
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?
Given:
Volume of box = 350 cm3
Mass of box = 500 g
Density of object = Mass of object/Volume of object
=500/350
=1.43 g/cm3
Now, we
know density of water = 1 g/cm3
Since, density of box > density of water, therefore the object is going to
sink in water.
Now,
volume of liquid displaced = Volume of the object = 350 cm3
⇒Mass of liquid displaced /Density of liquid displaced
= 350 cm3
⇒Mass of liquid=350×1=350 g
⇒Mass of liquid displacedDensity of liquid displaced= 350 cm3
⇒Mass of liquid=350×1=350 g