Std. 8. Force and Pressure

Question 1:

Write the word in the blank space.
A. The SI unit of force is_________ (Dyne, Newton, Joule)
B. The air pressure on our body is equal to __________ pressure (Atmospheric, sea bottom, space)
C. For a given object, the buoyant force in liquids of different ________ is__________ (the same, density, different, area)
D. The SI unit of pressure is_______ (N/m³ , N/m², kg/m², pa/m²) 

ANSWER:

A. The SI unit of force is Newton.
B. The air pressure on our body is equal to atmospheric pressure.
C. For a given object, the buoyant force in liquids of different density is different.
D. The SI unit of pressure is N/m².

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Question 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

ANSWER:

A group	B group
1. Fluid	e. Same pressure in all directions
2. Blunt knife	d. Lower pressure
3. sharp needle	a. Higher pressure
4. Relative density	c. Specific gravity
5. Hecto pascal	b. Atmospheric pressure

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Question 3:

Answer the following questions in brief.
A. A plastic cube is released in water.  Will it sink of come to the surface of watet?
B. Why do the load carrying heavy vehicles have large number of wheels?
C. How much pressure do we carry on our heads? why don't we feel it?

ANSWER:

(a) The plastic cube is going to float on the surface of water as its density is less than that of water.

(b) We know, $\mathrm{Pressure}=\frac{\mathrm{Force}}{\mathrm{Area}}$
So, 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) 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. 

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Question 4:

Why does it happen?
A. A ship dips to a larger depth in fresh water as compared to marine water.
B. Fruits can easily be cut with a sharp knife.
C. The wall of a dam is broad at its base.
D. If a stationary bus suddenly speeds up, passengers are thrown in the backward direction.

ANSWER:

A. The density of marine water is more than the fresh water due to which the buoyant force on the ship in marine water is more than the fresh water. Hence, the ship dips to a larger depth in fresh water as compared to marine water.

B. Since 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. 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. 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.

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Question 5:

Complete the following tables.

Mass(Kg)	Volume (m3)	Density(kg/m3)
350	175	-
-	190	4

 

Density of Metal (kg/m3)	Density of water (kg/m3)	Relative Density
	103	5
8.5$\times$103	103	-

 

weight(N)	Area (m2)	Pressure(Nm−2)
-	0.04	20,000
1500	500	-

ANSWER:

We know, $\mathrm{Density}=\frac{\mathrm{Mass}}{\mathrm{Volume}}$
So, using above formula, we can find one quantity if other two quantities are given.

Mass (kg)	Volume (m3)	Density (kg/m3)
350	175	2
760	190	4

 
We know, $\mathrm{Relative} \mathrm{density} \mathrm{of} \mathrm{substance}=\frac{\mathrm{Density} \mathrm{of} \mathrm{substance}}{\mathrm{Density} \mathrm{of} \mathrm{water}}$
So, using above formula, we can find one quantity if other two quantities are given.

Density of Metal (kg/m3)	Density of water (kg/m3)	Relative Density
5$\times$103	103	5
8.5$\times$103	103	8.5

We know, $\mathrm{Pressure}=\frac{\mathrm{Force}}{\mathrm{Area}}$
So, using above formula, we can find one quantity if other two quantities are given. 

Weight (N)	Area (m2)	Pressure (N m−2)
800	0.04	20,000
1500	500	3

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Question 6:

The density of a metal is $10.8\times {10}^{3 }\mathrm{kg}/{\mathrm{m}}^3$. Find the relative density of the metal.

ANSWER:

Given:
Density of metal = $10.8\times {10}^{3 }\mathrm{kg}/{\mathrm{m}}^3$
We know, density of water = 1000 kg/m³
$\mathrm{Relative} \mathrm{density} \mathrm{of} \mathrm{substance} =\frac{\mathrm{Density} \mathrm{of} \mathrm{substance}}{\mathrm{Density} \mathrm{of} \mathrm{water}}$
$\mathrm{Relative} \mathrm{density} \mathrm{of} \mathrm{substance} =\frac{10.8\times {10}^3}{1000}=10.8$

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Question 7:

Volume of an object is 20 cm³ and the mass in 50 g. Density of water is 1 g cm^$-$3. Will the object float on water or sink in water?

ANSWER:

Given:
Volume of object = 20 cm³
Mass of object = 50 g
Density of object = $\frac{\mathrm{Mass} \mathrm{of} \mathrm{object}}{\mathrm{Volume} \mathrm{of} \mathrm{object}}=\frac{50}{20}=2.5 \mathrm{g}/{\mathrm{cm}}^3$
Now, we know density of water = 1 g/cm³
Since, density of object > density of water, therefore the object is going to sink in water.

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Question 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?

ANSWER:

Given:
Volume of box = 350 cm³
Mass of box = 500 g
Density of object = $\frac{\mathrm{Mass} \mathrm{of} \mathrm{object}}{\mathrm{Volume} \mathrm{of} \mathrm{object}}=\frac{500}{350}=1.43 \mathrm{g}/{\mathrm{cm}}^3$
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 cm^{3
$$\begin{gathered} \Rightarrow \frac{\mathrm{Mass} \mathrm{of} \mathrm{liquid} \mathrm{displaced}}{\mathrm{Density} \mathrm{of} \mathrm{liquid} \mathrm{displaced}}= 350 {\mathrm{cm}}^3 \\ \Rightarrow \mathrm{Mass} \mathrm{of} \mathrm{liquid}=350\times 1=350 \mathrm{g} \end{gathered}$$}