## Notes

#### Q1.      Describe briefly about the matter?

Ans:     Matter exists in three states, solid, liquid and gas. There are many properties associated with matter. For example, the matter has weight and occupies space.

There are some other properties which are associated with one state of matter but not with others. For example, solids have the shape of their own while liquids and gases do not. Liquids on the other hand have definite volume while gases do not have. Various materials differ in their hardness, density, solubility, flow, elasticity, conductivity and many other qualities. The kinetic molecular model helps in understanding the properties of matter in a simplified way.

#### Q2.      How the kinetic molecular model of matter helps differentiate various states of matter?

Ans:     See Q # 7.2 from Exercise.

##### USEFUL INFORMATION

 Density of various substance Substances Density in kgm – 1 Air 1.3 Foam 89 Petrol 800 Cooking Oil 920 Ice 920 Water 1000 Glass 2500 Aluminum 2700 Iron 7900 Copper 8900 Lead 11200 Mercury 13600 Gold 19300 Platinum 21500

##### USEFUL INFORMATION

1 metre cube(1 m3)    = 1000 litre

1 litre                       = 10 – 3

1 cm 3                                 = 10 – 6

1000 kgm – 3                   = 1 gcm – 3

OR

#### Why is 1cm cubed of wood lighter than 1 cm cubed of iron?

Ans:    Because centimeters cubed is a unit of volume, not weight. The two might take up the same amount of space, but iron is much dense and as such weighs more.

Density = Mass of a substance / volume of the substance   OR  D = m/v

#### Q4.     What is meant by density? What is its SI unit?

Ans:    See Q # 7.4 from Exercise

#### Q5.     Calculate the density of 5 litres of water?

Solution:

The mass of 5 litres of water is 5 kg.

Since                                 1 litre  =      10 – 3 m3

5 litre  =      5 x 10 – 3 m3

Density = mass of a substance / volume of the substance  OR  D = m / v

Density = 5 kg / 5*10^-3 m^3

= 1000 kg m – 3

The density of water is 1000 kg m – 3

#### Q6.      Write down the density equations?

Ans:     Density Equations:

• Density =
• Mass = Density x Volume
• Volume =

##### DO YOU KNOW?

Earth’s atmosphere extends upward about a few hundred kilometers with continuously decreasing density. Nearly half of its mass is between sea level and 10 km. Up to 30 km from sea level contains about 99% of the mass of the atmosphere. The air becomes thinner and thinner as we go up.

#### Q7.      Define the term pressure?

Ans:     See Q # 7.6 from Exercise.

#### Q8.      Show that atmosphere exerts pressure.

Ans:     See Q # 7.7 from Exercise.

#### Q9.       How soap bubbles are produced. Why the soap bubbles so formed have spherical shapes?

Ans:     Formation of soap bubbles:

A soap bubble is a very thin sheet of water sandwiched between two layers of soap molecules. The film of soapy water surrounds a bubble of air. Soap molecules have one end that repels water, and one that attracts it, and these molecules move to the inner and outer surfaces, thrusting their water-repelling ends out in the air, and their “heads” inwards. Without such molecules on the surface, the bubble would spontaneously break apart into tiny water droplets.

The spherical shape of soap bubbles:

Surface tension causes the bubble to be spherical, which is the shape that gives the minimum surface energy – the lowest ratio of surface area to volume.

Soap bubbles expand till the pressure of air in them is equal to the atmospheric pressure.

#### Q10.     Write about the factors affecting pressure?

Ans:      Factors Affecting Pressure P  =  F/A

1. The size of the force – the greater the force the greater the pressure.
2. The area of contract – the smaller the area the greater the pressure.

#### Q11.     Write any three examples of pressure?

Ans:      Examples of Pressure:

• Skis have a large area to hold up the weight of the skier on the snow.
• Flat bottomed shoes are comfortable to wear due to reduced pressure acting on our feet.
• A sharp knife can cut easily because the very high pressure under the cutting surface is more than the object can withstand.
• In some situations, a highly concentrated force is useful so a small area of contact is desirable e.g. scissors, knife-edge, nail point, cheese wire, spikes on golf shoes, the sharp edge of the spade.
• In other situations, low pressure is better and so a large area of contact is part of the design e.g. wide tyres, schoolbag straps, suitcase handle, broad feet of elephant and camel.

OR

#### Write the procedure for measuring atmospheric pressure with the help of barometer?

Ans:      See Q # 7.9 from Exercise.

##### DO YOU KNOW

The fan in a vacuum cleaner lowers air pressure in its bucket. The atmospheric air rushes into it carrying dust and dirt with it through its intake port. The dust and dirt particles are blocked by the filter while air escapes out.

#### Q13.     Why does the atmospheric pressure vary with height?

Ans:      Variation in atmospheric pressure:

The atmospheric pressure decreases as we go up. The atmospheric pressure on mountains is lower than at sea level. At a height of about 30 km, the atmospheric pressure becomes only 7 mm of mercury which is approximately 1000 Pa. It would become zero at an altitude where there is no air. Thus, we can determine the altitude of a place by knowing the atmospheric pressure at that place.

Atmospheric pressure may also indicate a change in the weather. On a hot day, the air above the Earth becomes hot and expands. This causes a fall of atmospheric pressure in that region. On the other hand, during cold chilly nights, the air above the Earth cools down. This causes an increase in atmospheric pressure.

#### Q14.     What does it mean when the atmospheric pressure at a place falls suddenly?

Ans:      See Q # 7.13 from Exercise.

#### Q15.      What change is expected in the weather if the barometer reading shows a sudden increase?

Ans:       See Q 7.14 from Exercise.

#### Q16.      Why does atmospheric pressure changes with altitude?

Ans:       See Q # 7.12 from Exercise.

##### DO YOU KNOW?

When air is sucked through a straw with its other end dipped in a liquid, the air pressure in the straw decreases. This causes the atmospheric pressure to push the liquid up the straw.

OR

#### Prove that (P = rgh)

Ans:      Pressure in Liquids:

Consider a surface of area A in a liquid at a depth h. The length of the cylinder of liquid over this surface will be h.

The force acting on this surface will be the weight w of the liquid above this surface. If r is the density of the liquid and m is mass of liquid above the surface, then

Mass of the liquid cylinder = m = volume x density

= (A x h) x p

Force acting on Area         F   =       w    =      mg

=   A h p g

As                 Pressure                     P        =  F/A

=  Ahpg/A

Liquid pressure at depth h  =     P    = p g h    ………(i)

Equation i) gives the pressure at a depth h in a liquid of density r. It shows that its pressure in a liquid increase with depth. (P h)

##### DO YOU KNOW?

The piston of the syringe is pulled out. This lowers the pressure in the cylinder. The liquid from the bottle enters into the piston through the needle.

#### Q18.     State Pascal’s law.

Ans:      See Q # 7.15 from Exercise.

OR

#### A small force applied on the smaller piston, results into a large force on the larger piston. Why?

Ans:      See Q # 7. 16 from Exercise.

#### Q20.     Explain the braking system in a vehicle?

Ans:      The braking systems of cars, buses, etc. also work on Pascal’s law. The hydraulic brakes allow equal pressure to be transmitted throughout the liquid. When the brake pedal is pushed, it exerts a force on the master cylinder, which increases the liquid pressure in it. The liquid pressure is transmitted equally through the liquid in the metal pipes to all the pistons of other cylinders. Due to the increase in liquid pressure, the pistons in the cylinders move outward pressing the brake pads with the brake drums. The force of friction between the brake pads and the brake drums stops the wheels.

#### Q21.     State Archimedes principle.

Ans:      See Q # 7.18 from Exercise.

##### DO YOU KNOW?

Hydrometer:

A hydrometer is a glass tube with a scale marked on its stem and heavyweight in the bottom. It is partially immersed in a fluid, the density of which is to be measured. One type of hydrometer is used to measure the concentration of acid in a battery. It is called acid meter

#### Q22.     Determine the density of an object using the Archimedes principle?

Ans:      Density of an object:

Archimedes principle is also helpful to determine the density of an object. The ratio in the weights of a body with an equal volume of liquid is the same as in their densities.

Let                    Density of the object       =      D

Density of the liquid         =       r

Weight of the object       =      w1

Weight of equal volume of liquid = w = w1 – w2

Here w2 is the weight of the solid in liquid.  According to Archimedes principle, w2 is less than its actual weight w1 by an amount w.

Since      D/p  = w1 / w

= w1 / w x p

Or                D = w1 / w1 – w2  x p    ………………(i)

Thus, finding the weight of the solid in air w1 and its weight in water w 2, we can calculate the density of the solid by using equation (i) as illustrated in the following example.

#### Q23.     What is upthrust? Explain the principle of floatation.

Ans:      See Q # 7.19 from Exercise.

#### Q24.     Explain how a wooden block, ships and boats move up the water surface.

Ans:      A wooden block floats on water. It is because the weight of an equal volume of water is greater than the weight of the block. According to the principle of floatation, a body floats if it displaces water equal to the weight of the body when it is partially or completely immersed in water.

Ships and boats are designed on the same principle of floatation. They carry passengers and goods over water. It would sink in water if its weight including the weight of its passengers and goods becomes greater than the upthrust of water.

#### Q25.     Explain how a submarine moves up the water surface down into the water.

Ans:      See Q # 7.20 from Exercise.

#### Q26.      What is meant by deforming force?

Ans:       Deforming force:

We know that the length of a rubber band increases on stretching it. Similarly, the pointer of a spring balance is lowered when a body is suspended from it. It is because the length of the spring inside the balance increases depending upon the weight of the suspended body.

The applied force that changes shape, length or volume of a substance is called deforming force. In most of the cases, the body returns to its original size and shape as soon as the deforming force is removed.

#### Q27.     What is meant by elasticity?

Ans:      Elasticity:

The property of a body to restore its original size and shape as the deforming force ceases to act is called elasticity.

Due to elasticity, we can determine the strength of a material and the deformation under the action of a force.

#### Q28.     Differentiate between stress and strain?

Ans:      Stress:

The force acting on unit area at the surface of a body is called stress.

Thus     stress    =  Force / Area

Unit:    In SI, the unit of stress is newton per square metre (Nm – 2).

Strain:

A comparison of such a change caused by the stress with the original length, volume or shape is called as strain.

If stress produces a change in the length of an object then the strain is called tensile strain.

Tensile strain    =  Change in length / Original length

Strain has no units as it is simply a ratio between two similar quantities.

#### Q29.     What is Hook’s law? What is meant by elastic limit?

Ans:      See Q # 7.22 from Exercise

#### Q30.     What do you know about Young’s modulus? How would you determine young’s modulus of an object?

Ans:      Young’s modulus:

The ratio of stress to tensile strain is called Young’s modulus.

Determination of young’s modulus:

Consider a long bar of length L0 and cross-sectional area A. Let an external force F equal to the weight w stretches it such that the stretched length becomes L. According to Hooke’s law, the ratio of this stress to tensile strain is constant within the elastic limit of the body.

Young’s modulus        Y      =  stress / Tensile strain

Let L be the change in length of the rod, then

△L   =      L – L0

Since                           Stress      =   Force / Area   =   F/A

And                             Tensile strain  =  L – L0 / L0  =  L / L0

As                               Y                  =  Stress / Tensile strain

= F/A  x L0 / L

Y             =   FL0 / AL

Unit:  SI unit of Young’s modulus is Newton per square metre (Nm – 2).

##### SUMMARY
1. Kinetic molecular model: Kinetic molecular model explains the three states of matter assuming that
• matter is made up of particles called molecules.
• the molecules remain in continuous motion.
• molecules attract each other.
1. Plasma: At very high temperature, the collision between atoms and molecules tears off their electrons. Atoms become positive ions. This ionic state of matter is called the plasma-the fourth state of matter.
2. Density: Density is the ratio of mass to volume of a substance. Density of water is 1000 kgm – 3.
1. Pressure: Pressure is the normal force acting per unit area. Its SI unit is Nm – 2 or pascal (Pa).
2. Atmospheric pressure acts in all directions.
3. Barometer: The instruments that measure atmospheric pressure are called barometers.
4. The atmospheric pressure decreases as we go up. Thus, knowing the atmospheric pressure of a place, we can determine its altitude.
5. The changes in atmospheric pressure at a certain place indicate the expected changes in the weather conditions of that place.
6. Liquids also exert pressure given by: P = r g h
7. Pascal’s law: Liquids transmit pressure equally in all directions. This is called Pascal’s law.
8. Archimedes principle: When a body is immersed wholly or partially in a liquid, it loses its weight equal to the weight of the liquid displaced. This is known as Archimedes principle.
9. For an object to float, its weight must be equal or less than the upthrust of the liquid acting on it.
10. Elasticity: The property of matter by which matter resists any force which tries to change its length, shape or volume is called elasticity.
11. Stress: Stress is the deforming force acting per unit area.
12. Tensile strain: The ratio of change of length to the original length is called tensile strain.
13. Young’s modulus: The ratio between stress and tensile strain is called Young’s modulus. 