# Physics Class 9 Chapter 7 Properties of Matter questions, answers Mardan board

Properties of Matter Physics Class 9 Chapter 7 Mardan board (KPK) questions, answers, Conceptual Questions, Comprehensive Questions, and Numerical Questions.

## Conceptual Questions Physics Class 9 Chapter 7

### Q.1) If you climbed a mountain carrying a mercury barometer, would the level of mercury in the glass tube of the barometer increases or decreases as you climb the mountain? Explain.

As we climb up the mountain the level of mercury in the glass tube of the barometerdecreases.
The height of the mercury column depends on the atmospheric pressure. A higher atmospheric pressure results in an increase of the height of the mercury column in the barometer and vice versa.
The atmospheric pressure decreases as we go towards the higher altitude, hence, the level of the barometer also decreases.

### Q.2) Walnuts can be broken in the hand by sequeezing two together but not one, why?

When two walnuts are placed in the hand, the area of the palm of the hand is greater than the area of surfaces of walnuts in contact.

As we know                                        Pressure = Force/ Area

When a force is applied to squeeze these walnuts together, the pressure between the surfaces in contact increases as the contact area is small. Hence, by the above equation, a decrease in the area results in a higher pressure which causes the walnuts to break at the surface in contact.

### Q.3) Why is the cutting edge of the knife made very thin?

The cutting edges of the knife are made very thin because the smaller the area more will be the pressure that is exerted on the area and vice versa.
Mathematically, we know the relation between pressure and applied force

where ‘P’ is pressure, ‘F’ is the applied force and ‘A’ is area. Hence, a small area with large pressure is easy for things like cutting, slicing or stabbing etc.

### Q.4) Why water tanks are constructed at the highest level in our houses?

As we know that                                         p = ρgh

The water tanks are constructed at the highest level in our houses because the pressure depends upon the height.
From the above equation, the pressure of water increases with height. So as the pressure applied by the water on pipes increases it results in an easy flow of water in a pipe system.

### Q.5) Why a small needle sinks in water and huge ships travel easily in water without sinking?

A small needle sinks in water while a huge ship travels easily in water without sinking because the density of needle is greater than water.
F (buoyant force of fluid) = W (displaced fluid) = m (mass of fluid) x g
ρ  = m/V
The mass of needle is more than the mass of water having same volume. Hence, the density of the needle is greater than the density of water displaced by it. As a result the buoyant force of fluid is less than the weight of the needle, so the needle sinks in water.

In the case of a huge ship, the weight of water displaced is greater than the weight of the ship due to which the ship does not sink and floats on water.

### Q.6) Explain how and why camels have adapted to allow them to walk more easily in desert conditions?

Camels can walk easily in the desert because of their wide feet. Their wide feet allow the weight of their body to act on a larger surface of land reducing the pressure exerted on the land by the camel. we know that pressure exerted and area are inversely proportional.

### Q.7) You would have probably experienced your ears ‘popping’ while driving in mountains. Why ears’ pop’?

In our ears a fluid maintains the internal and external pressure normally. While driving to an elevation the change in the atmospheric pressure results in changing the pressure on the ear drums, so a net pressure arise on ear drums which cause poping.
It can be relieved in a few minutes or by swallowing, chewing or blowing out nose.

### Q.8) If you filled an airtight balloon at the top of a mountain, would the balloon expand or contact as you descend the mountain? Explain.

The air balloon will contract as the atmospheric pressure increase going down the mountain. So the external pressure is more as compared to the pressure on the top of the mountain.

### Q.9) A rowboat is floating in a swimming pool when the anchor is dropped over the side. When the anchor is dropped, will the water level in the swimming pool increase, decrease, or remain the same? Explain.

When the anchor is dropped, the water level in the swimming pool increases as the anchor displaces a volume of water resulting in the water level to rise. In this case, the swimming pool attains more volume of the anchor as it is dropped in it.

### Q.10) Which material is more elastic, steel or rubber and why?

Elasticity is defined as the ability to resist a change.
Steel is more elastic than rubber because steel produces more resistance against the deformation than rubber. So, more deforming force is required to deform steel while rubber can be easily deformed.

## Comprehensive Questions Physics 9th Notes Chapter 7

### Q.1) Using kinetic molecular model of matter, explain three states of matter.

Matter has three states solids,liquid and gases. These three sates of matter are explained not he basis of kinetic molecular theory. It is assumed that:

Solids:
1.    Solids are made up of molecules which are arranged closely in a fixed pattern.
2.    Molecules in solids vibrate about their mean position.
3.    The attractive forces between the molecules are strong.
Liquids:
1.    Liquids are also made up of molecules which are close together.
2.    The pattern of molecules is not fixed and does not extend far. The molecules in a pattern keep changing their position.
3.    Molecules are able to move about, which means that a liquid is able to change its shape and can adopt the shape of the container.
4.    The attractive forces between the molecules of liquid is less than the solid.
Gases:
1.    A gas is made up of molecules which are in constant random motion.
2.    The distance between molecules id larger as compared to the size of molecules.
3.    The molecules are constantly colliding elastically with each other and with the walls of the container.
4.    Forces between molecules are negligible, except during collisions.

### Q.2) Define  and explain density and pressure.

Density:
“Mass per unit volume of a body is called density”. It is a scalar quantity.
Consider a body of mass ‘m’ has a volume ‘v’. The density of an object is;

Explanation:
Let’s take the example of iron and wood. The same volume of iron is heavier than wood due to the reason that the density of iron is more than that of wood. In other words, the mass of iron for 1 m3 is more than the mass of wood of 1 m3 volume.
Molecules of iron are held closely together, whereas the molecules of wood are arranged in a different manner and have voids in the structure so the number of molecules in the same volume is more in case of iron than the wood.
Pressure:
“The normal force applied per unit area is called pressure”. It is denoted by “P”.
Mathematically:

The SI unit of pressure is pascal (Pa).
Pascal:
It can be defined as “when a one newton force acts on a body of area one meter square, then the pressure is one pascal”. Mathematically

Explanation:
Let’s take an example of a drawing pin. Hold the pin between a finger and the thumb, and the pointed part directed towards the thumb. By applying pressure we feel pain at the contact point at the thumb but not at the finger. This is because the pressure at the thumb is much higher than the pressure at the finger.
The reason is that the pressure is inversely proportional to the area. The pressure increase as the area decreases keeping the force constant. The area of the pointed part is very small as compared to the head of the drawing pin, so a pain is felt on the thumb.

### Q.3) What is atmospheric pressure? How is it measured by using a mercury barometer? Also, describe how weather changes with atmospheric pressure?

Atmospheric Pressure:
“The pressure exerted by the gases in the atmosphere is known as atmospheric pressure”.
Barometer:Atmospheric pressure is measured by using mercury barometer.
Construction:
A long tube opened at one end is filled with mercury and inverted in a dish of mercury. The mercury does not empty into the bowl. Instead, the atmospheric pressure pushes the mercury in the tube to some height ‘h’ above the bowl. In this way the force exerted on the bowl of mercury by the atmosphere is equal to the weight of the column of mercury in the tube.
A change in the height means a change in the atmospheric pressure.

Weather changes:
When we keep a barometer at some height at sea-level, it shows some variation in atmospheric pressure from day to day.
The winds move from higher to lower pressure regions and the strength of the wind depends upon the pressure gradient. There will be strong winds if the pressure gradient is high and vice versa in this way weather changes with atmospheric pressure.

### Q.4) State Pascal’s principle and explain with example?

Pascal’s principle

This principle states that ″whenever an external pressure is applied in a liquid, the pressure is transmitted equally to every point of the liquid in all directions.”
Or
“All liquids exert the same pressure in all directions.”
Explanation:
There are so many systems which work on the principle of Pascal’s law. For example, hydraulic press, hydraulic brakes in vehicles and the hydraulic lifts etc. Here, we take a simple example of a hydraulic press to understand it. Consider two interconnected cylinders having different diameters. The chamber is filled with incompressible fluid, and their top ends fitted with pistons as shown in figure.

### Q.5) How pressure varies with depth in liquids?  Explain.

The pressure in liquid increases with depth. Because the further down you go, the greater the weight of the liquid above. It can be observed in the figure below that the water spurts out fastest and furthest from the lowest hole. A simple set-up in the following figure shows how water pressure increases with depth.
As we know                                            P = ρgh
Consider a container is filled with water and having three opening ‘P1‘, ‘P2‘ and ‘P3‘ as shown in figure

The water form opening ‘P1‘, shoots out with the least force because the pressure at ‘P1‘ is very small due to smaller depth ‘h1‘.

At opening ‘P2‘, the water shoots out with medium force because, the pressure at ‘P2‘ is greater due to greater depth ‘h2‘ as compared to ‘h1‘.

At opening ‘P3‘, the water shoots out with maximum force because, the pressure at ‘P3‘ is greatest due to maximum depth ‘h3
Hence, it is proved that pressure varies directly with depth.

### Q.6) What is meant by buoyant force or upthrust in fluids?

Buoyant force:
The force exerted by liquids in opposite direction against the weight of the body is known as upward thrust.
Explanation:
When an object is immersed in a fluid, the pressure on the lower surface of the object is higher than the pressure on the upper surface. The difference in pressure leads to an upward net force acting on the object due to the fluid pressure called up-thrust or buoyant force and phenomenon is called buoyancy.
If you try to push a piece of cork underwater, you feel the buoyant force pushing the cork back up.

The buoyant force arise because the fluid pressure at the bottom of the cylinder is larger than at the top.

Net force Fnet of the fluid on the cork is the buoyant force Fb.
Fup > Fdown because the pressure is greater at bottom of the beaker, hence the fluid exerts a net upward force.

### Q.7) State and explain Archimedes principle .

Archimedes principle:
“The buoyant force acting on an object fully or partially submerged in a fluid is equal to the weight of the fluid displaced by the object”.
Explanation:
According to Archimedes principle every object experiences a buoyant force. The buoyant force will decrease the weight of the object which termed as apparent lose of weight. Whether it floats or sinks depends on the object’s density relative to the fluid. When we push a wooden block underwater and you will feel the upward buoyant force as in figure.

By Archimedes principle, the buoyant force equals the weight of water displaced; since water is denser than wood, the buoyant force is greater than the wood’s weight, and that is why wood floats. Now when submerged a coin. It’s denser than water, so the coin’s weight is greater than the weight of the displaced water. Hence the coin’s weight is greater than the upward buoyant force, and it sinks.

Whether an object will float or sink depends on the net force acting on it. This net force can be calculated as follows:
Fnet = FB – W (object)
Now we can apply Archimedes principle, using ‘mo‘ to represent the mass of the submerges object and ‘mf‘ as mass of fluid ‘g’ is acceleration due to gravity.
Fnet = mfg – mog
Remember that ‘m = ρV’, so the expression can be rewritten as follows:
Fnet = ρfVfg – ρoVog         or        Fnet = (ρfVf – ρoVo)g
A simple relationship between the weight ‘W’ of submerged object of mass mo and density ρo can be found by considering their ratio as follows:

for submerged object equal volume is displaced therefore
V= VB = V
therefore                                mo = ρoV            and                mB = ρBV
putting these values in eq. 1, we get

### Q.8) What is elasticity? Explain

Elasticity:
“The property of Solid materials to return to their original shape and size after removal of deforming force is called elasticity”.
For example, when we stretch a rubber band, its size increases, but after the removal of the force, the rubber band gets their original shape.
Similarly, when a racket hits a tennis ball, the shape of the ball is distorted or deformed, but it regains its original shape when it bounces off the tennis racket.
But the materials are elastic up to a certain limit known as the elastic limit. Beyond this limit, a material deforms and do not attain its original position.

### Q.9) State and explain Hooke’s law.

Hook’s law:

This law states that “within the elastic limit, the extension (or compression) is directly proportional to the restoring force”.
Explanation
Consider that a spring is connected with a firm support at its one end. And a force ‘F’ is applied on the object the other end of the spring to produce an extension ‘x’ Then release it. A restoring force Frestoring of spring act on the object to restore its original position. According to Hook’s law, we have
Fres ∝ -x
Fres = k x
Where ‘K’ is constant of proportionality and is known as spring constant or modulus of elasticity. Its unit is Nm-1. The value of ‘K’ depends upon the nature of the spring and system of units.

### Q.10) Define and explain, Stress, strain and Young’s modulus

Stress:
“The force applied per unit area of cross-section to produce deformation”.
Mathematically,

The SI unit of stress is  Nm-2 orpascal.
Strain:
The deformation produced in a body due to stress is called strain.
Consider a wire has an initial length “L”. After the applied deforming force, the length of the wire change to an amount “ΔL”. The linear strain can be defined as the change in length per unit original length is called linear strain.
It is denoted by a Greek letter Epsilon “ε”
Mathematically

The strain has no unit because it is the ratio of two similar quantities.
Young’s modulus:
The ratio of the tensile stress to the strain is called Young’s modulus or modulus of elasticity. It is represented by ‘Y’.
Mathematically,

The SI unit of Young’s modulus is Nm-2.