## Q1: Write the unit of pressure?

Ans. Newton per square metre.

## Q2: Do liquids also exert pressure?

Ans. Yes, liquids also exert pressure. When a liquid is put in a vessel it exerts pressure on its bottom and wall.

## Q3: What is gas pressure?

Ans. The outward force per unit area of the walls is called gas pressure.

## Q4: What is atmospheric pressure?

Ans. The pressure exerted by the atmospheric air is known as atmospheric pressure.

## Q1. In what direction does the buoyant force on an object immersed in a liquid act?

Ans. The buoyant force on an object immersed in a liquid acts in upward direction (i.e., opposite to weight of the object).

## Q2. Why does a block of plastic released under water come up to the surface of water?

Ans. The density of plastic is less than that of water, so the force of buoyancy on plastic block(V$$\rho$$plastic g) will be greater than the weight of plastic block displaced, i.e., (V $$\rho$$ water  g). Hence, the acceleration of plastic block will be in upward direction, and comes up to the surface of water.
Mathematically,
acceleration, a = B — W

= $$\frac { V\quad { \rho }_{ water }\quad g\quad -\quad V\quad { \rho }_{ plastic }\quad g }{ m }$$

= $$\frac { V\quad g\quad ({ \rho }_{ water }\quad \quad -\quad \quad { \rho }_{ plastic })\quad }{ m }$$

is positive in upward direction.

## Q3. The volume of 50 g of a substance is 20cm . If the density of water is 1 g cm-3 , will the substance float or sink?

Ans. Density of substance,  $$\rho$$ =$$\frac{mass}{volume}$$ = $$\frac { 50\quad g }{ 20\quad { cm }^{ 3 } }$$ = 2.5 g cm-3
As density of substance is greater than the density of water, therefore, the substance will sink in water. Alternatively, the buoyant force exerted by water ($$(V\quad { \rho }_{ water }\quad g)$$) on substance, when fully immersed, is less than the weight of substance, i.e., B< W; hence, the substance will sink in water.

## Q4. The volume of a 500 g sealed packet is 350 cm3. Will the packet float or sink in water if the density of water is 1 g cm-3 ? What will be the mass of the water displaced by this packet?

Ans. Density of packet, $$\rho =\frac { mass }{ volume }$$

=$$\frac { 500\quad g }{ 350\quad { cm }^{ 3 } }$$

=$$\frac { 10\quad }{ 7 } g\quad { cm }^{ -3 }$$

= l.43 g cm-3
As density of packet is more than the density of water, so the packet will sink in water.
Weight of water displaced by the packet
= volume of packet x density of water
= (350 cm3) x (l g cm-3) = 350 g
Obviously, the force of buoyancy, B = 350 g wt and weight of packet, W = 500 g wt i.e., B < W; so the packet sinks in water.

## Q1. Why is it difficult to hold a school bag having a strap made of a thin and strong string?

If a school bag has a strap made of thin and strong string, the whole weight of school bag will fall of over a very small area (equal to that string) on the shoulder. So pressure on shoulder P =$$\frac{Weight of the bag}{Area of string}$$ of child will be very large and hence painful for the child. Hence, it is difficult to hold a school bag having a strap made of thin string.

## Q2. What do you mean by buoyancy?

Ans. When an object is placed in a liquid, the liquid exerts an upward force on it. The tendency of liquid to exert such an upward force on the object is called the buoyancy and the upward force exerted by liquid on the object is called the buoyant force (or the force of buoyancy).

## Q3. Why does an object float or sink when placed over the surface of water?

Ans. When an object is immersed in water, the water exerts an upward force on the object. This upward force equal to the weight of water displaced by object is called the buoyant Force.
If on completely immersing the object, the buoyant force is more than the weight of object will float on water. In other words, if the density of object is less than that of water, then the object will float on water.
On the other hand if buoyant force, on completely immersing the object in water, is less than the weight of the object, the object will sink in water. In other words, if the density of object is greater than water, then the object will sink in water.
In brief, if  p object < p water   , the object will float and if p object > p water, the object will sink.

## Q4. You find your mass to be 42 kg on a weighing machine. Is your mass more or less than 42 kg?

Ans. When we stand on a weighing machine, we replace air equal to volume of our body. So measured weight, W = actual weight — upthrust
= mg — V p air g
Therefore, Measured mass, m’ = $$\frac { W }{ g }$$ = (m-V air )
That is the measured mass will be less than the actual mass. Therefore, our mass is more than 42 kg.

## Q5. You have a bag of cotton and an iron bar, each indicating a mass of 100 kg when measured on a weighing machine. In reality, one is heavier than other. Can you say which one is heavier and why?

Ans. Actually mass of cotton is more than the mass of iron bar. This is because
Weight measured = Actual weight — buoyant force
Therefore,  Actual weight =  weight measured + buoyant force
The volume of cotton is much more than the volume of iron bar, so buoyant force (B = volume of object x density of air x g) of air on cotton is much more than that of air on iron bar, so mass of cotton will be more than the mass of iron bar.

## Q1: Define amplitude.

Ans. It is the maximum displacement of a vibrating body on one side.

## Q2: What is called time period?

Ans. It is the time taken by a vibrating body to complete one oscillation.

## Q3: Define frequency.

Ans. It is the number of vibrations produced by a vibrating body in one second.

## Q4: What are ultrasonic waves?

Ans. It is the sound of frequencies more than 20000 hertz. We cannot hear such sounds.

Ans. Amplitude.

Ans. Frequency.

## Q7: What do you mean by the term loudness?

Ans. The sound of high amplitude is called loudness.

## Q8: Pendulum A makes 14 oscillations in five seconds and pendulum B makes 10 oscillations in three seconds. Which has a higher frequency? Express the frequency of each pendulum in hertz.

Ans.

Frequency of A = $$\frac{14}{5}$$ = 2.8 hertz

Frequency of B = $$\frac{10}{3}$$ = 3.33 hertz

Therefore,    Pendulum B has higher frequency.

## Q9: Define Hertz.

Ans. Frequency of a vibration or oscillation is measured in hertz. It is the unit of frequency.

## Q1. Why does a sharp knife cut better than a blunt knife?

Ans. A sharp knife has a very thin edge to its blade. A sharp knife cuts objects better because due to its very thin edge, the force of our hand falls over very small area of the object producing a large pressure. And this large pressure cuts the object easily. On the other hand, a blunt knife has a thicker edge. A blunt knife does not cut an object easily because due to its thicker edge, the force of our hand falls over a larger area of the object and produces lesser pressure. This
lesser pressure cuts the object with difficulty.

## Q2. Why is the tip of the needle sharp?

Ans. The tip of the needle is sharp so that due to its sharp tip, the needle may put the force on a very small area of the cloth, producing a large pressure sufficient enough to pierce the cloth being stitched.

## Q3. Why is the tip of the nail pointed?

Ans. A nail has a pointed tip, so that when it is hammered, the force of hammer falls on a very small area of wood creating a large pressure which pushes the nail into wood.

## Q4. Why is the pressure on ground more when a man is walking than when he is standing?

Ans. When a man is walking, then at one time only his one foot is on the ground. Due to this, the force of weight of the man falls on a smaller area of the ground and produces more pressure on the ground. On the other hand, when the man is standing both his feet are on the ground and produces lesser pressure on the ground.

## Q5. Why is depression much more when a man stands on the cushion than when he lies down on it?

Ans. When a man stands on a cushion then only his two feet are in contact with the cushion. Due to this, the weight of man falls on a small area of cushion producing a large pressure. This large pressure causes a big depression in  cushion. On the other hand, when the same man is lying on the cushion, then his whole body is in contact with the cushion. In this case, the weight of man falls on a much larger area of the cushion producing much smaller pressure. And this smaller pressure produces a very little depression in the cushion.

## Q6. Why is it easier to walk on soft sand if we have flat shoes rather than shoes with sharp heels?

Ans. This ¡s because flat shoes have a greater area in contact with the soft sand due to which there is less pressure on the soft ground. Due to this the flat shoes do not sink much in soft sand and it is easy to walk on it. On the other hand, a sharp heel has a small area in contact with the soft sand and so exerts greater pressure on the soft sand. Due to this greater pressure, the sharp heels tend to sink deep into soft sand making it difficult for the wearer to walk on sand.

## Q7. Why do tractors have broad tyres?

Ans. The tractors have broad tyres so that there is less pressure on the ground and the tyres do not sink into comparatively soft ground in the fields.

## Q8. Why do snow shoes stop you from sinking into snow?

Ans. The snow shoes have large, flat soles so that there is less pressure on the soft snow and stop the wearer from sinking into it.

## Q9. Why does a ship made of iron and steel float in water whereas a small piece of iron sinks in it?

Ans. Ship is not a solid block of iron and steel. A ship is a hollow object made of iron and steel which Contains a lot of air in it. Air has a very low density. Due to the presence of a lot of air in it, the average density of the ship becomes less
than the density of water, therefore, a ship floats in water.

## (c) What happens to the buoyant force on the boat?

Ans. (a) The boat floats lower in water.
(b) The weight of water displaced increases.
(c) The buoyant force on boat increases.

## Q11. A 0.5 kg sheet of tin sinks in water but if the same sheet is converted into a box, it floats. Why?

Ans. The average density of hollow box made up of sheet of tins becomes less than the density of water, so it floats.

## Q12. Why does the driver wear metallic cover while going into the sea?

Ans. The pressure increases with increase of depth of water. The increase in pressure is 1 atm by 10 m increase of water. If this pressure falls on the driver directly, he will not tolerate it. Therefore, if a driver wants to go to large
depth, he should wear the metallic cover. The pressure of water column falls on the metallic cover and the pressure inside the cover remains nearly equal to atmospheric pressure. Thus, by wearing metallic cover the driver remains quite safe.

## Q13. Why are the walls of a dam made thick at the bottom and thin upwards?

Ans. The pressure at a point inside a liquid depends on the depth of point from the free surface, therefore, the pressure is very high at the bottom of the dam. To tolerate this pressure, the walls of dam are made thick at the bottom. As
we move upwards, the pressure goes on decreasing: so the thickness of wall is made smaller and smaller. That is why the walls of dam are made thick at the bottom and thin at the top.

## Q14. Why is it easier to float in sea water than in river water?

Ans. The body of man is lighter than water but the head is heavier than water. Therefore, total weight of man happens to be greater than the weight of water displaced: therefore the man can sink in water. Hence, for floating in water, the man has to displace more water by moving his arms and legs. By doing so when the weight of water displaced by man becomes equal to the weight of man, the man begins to float. The density of sea water is greater than the fresh water of river, therefore the weight of sea water displaced becomes easily equal to the weight of man. Hence, it is easier to float in sea water than in river water.

## Q1: Fill in the blanks:

(a) Friction opposes the ………. between the surfaces in contact with each other.

(b) Friction depends on the ………. of surfaces.

(c) Friction produces ………. .

(d) Sprinkling of powder on the carrom board ………. friction.

(e) Sliding friction is ………. than the static friction.

Ans.

(a) relative motion

(b) smoothness (or irregularities or nature)

(c) heat

(d) reduces

(e) smaller.

## (d) sliding, static, rolling

Ans. (c) static, sliding, rolling.

## (d) wet marble floor, dry marble floor, towel, newspaper.

Ans. (a) wet marble floor, dry marble floor, newspaper and towel.

## Q4: Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show the direction of frictional force acting on it.

Ans. Frictional force will act upward i.e., direction opposite to that of sliding book.

## Q5: Explain why sportsmen use shoes with spikes.

Ans. It is done to provide the shoes better grip on the ground.

## Q6: Name the force responsible for wearing out of bicycle tyres.

Ans. Frictional force.

Ans.

(a) South

(b) North.

## Q8: What is the cause of friction?

Ans. The cause of friction is the irregularities on the two surfaces in contact.

## Q9: On which kind of surface the friction will be greater (i) smooth surface (ii) rough surface.

Ans. Friction will be greater in case of a rough surface.

## Q10: Does the use of ball bearing reduce or increase friction?

Ans. Use of ball bearing reduces the friction.

## Q11: Why do we sprinkle fine powder on the carrom board?

Ans. We want to reduce friction in order to increase efficiency.

## Q12: What are lubricants?

Ans. The substances which reduce friction are called lubricants.

## Q13: What is drag?

Ans. The frictional force exerted by fluids is also called drag.

## Q1: A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.

Ans.
Given that,
Number of oscillation    = 40

Total time taken        = 4 seconds

Time period                = time taken in one oscillation

= $$\frac{Total time}{Total number of oscillations}$$

= $$\frac{4 seconds}{40}$$

= $$\frac{1}{10}$$ = 0.1 second

Again, frequency       = number of oscillations per second

= $$\frac{Number of vibrations }{ Time taken}$$

= $$\frac{40}{4}$$

= 10 per second = 10 Hz.

## Q2: The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per second. What is the time period of the vibration?

Ans.

Number of vibrations per second = 500

Time period = time taken for one vibration

= $$\frac{Total time}{Total number of vibrations}$$
= $$\frac{1}{500}$$ Hz
= 0.002 Hz.

## (a) Dholak            (b) Sitar            (c) Flute

Ans.

(a) Stretched membrane

(b) String of sitar

(c) Air column.

## Q4: What do you understand by stringed instruments?

Ans. Those instruments have taut strings, which vibrate when they are plucked, struck, or played with a bow.

## (v) The sound of a bursting balloon.

Ans.

(i) Strings of sitar or veena vibrate to produce sound.

(ii) The skin of the tabla vibrate to produce sound.

(iii) The disc of the bell when beated with hammer vibrate to produce sound.

(iv) Wings of the mosquito or a bee vibrate to produce sound.

(v) Air vibrates to produce sound.

(vi) In the radio speakers vibrate to produce sound.

## Q6: What is the normal length of vocal cords in man?

Ans. The normal length of the vocal cord of a man is about 20 mm long.

## Q7: What is the unit of the loudness of sound? Explain giving the loudness of few sounds.

Ans. The loudness is expressed in a unit called decibel (dB). It is essentially a ratio if the actual loudness to some base loudness level. The base level is usually defined as that loudness of sound that the human ear can just perceive. The following table gives some idea of the loudness of sound coming from various sources:

At 80 dB the noise becomes physically painful.

## Q8: We hear sound only if its frequency is greater than ……. and lower than ……. .

Ans. 20 hertz, 20000 hertz.

Ans. Higher.

## Q1: Fill in the blanks in the following statements:

(a) To draw water from a well, we have to ………. at the rope.

(b) A charged body ………. an uncharged body towards it.

(c) To move a loaded trolley, we have to ………. it.

(d) The north pole of a magnet ………. the north pole of another magnet.

Ans.

(a) apply force (lift)                 (b) attracts

(c) pull / push                          (d) repels.

## muscular, contact, non-contact, gravity, friction, shape, attraction.

(a) To stretch the bow, the archer applies a force that causes a change in its ………. .

(b) The force applied by the archer to stretch the bow is an example of ………. force.

(c) The type of force responsible for a change in the state of motion of the arrow is an example of a ………. force.

(d) While the arrow moves towards its target, the forces acting on it are due to ………. and that due to ………. of air

Ans.

(a) shape

(b) muscular

(c) non-contact

(d) gravity, friction.

## Q3: In the following situations identify the agent exerting the force and the object on which it acts. State the effect of the force in each case.

(a) Squeezing a piece of lemon between the fingers to extract its juice.

(b) Taking out paste from a toothpaste tube.

(c) A load suspended from a spring while its other end is on a hook fixed to a wall.

(d) An athlete making a high jump to clear the bar at a certain height.

Ans.

 S. No. Agent exerting the force Object on which it acts Form of affect (a) Fingers (i) Lemon as whole (ii) Juice in side Change in shape Change in state of motion (b) Fingers (i) Tube as whole (ii) Paste in side Change in shape Change in state of motion (c) Load Spring Change in shape (d) Ground from which athlete takes jump Athlete itself Change in state of motion

## Q4: An inflated balloon was pressed against a wall after it has been rubbed with a piece of synthetic cloth. It was found that the balloon sticks to the wall. What force might be responsible for the attraction between the balloon and the wall?

Ans. Electrostatic force.

## Q5: When we press the bulb of a dropper with its nozzle kept in water, air in the dropper is seen to escape in the form of bubbles. Once we release the pressure on the bulb, water gets filled in the dropper. The rise of water in the dropper is due to

(a) pressure of water.

(b) gravity of the earth.

(c) shape of rubber bulb.

(d) atmospheric pressure.

Ans. (a) pressure of water.

## Q6: What is force?

Ans. A push or pull on an object is called a force.

## Q7: Has force only magnitude or both direction and magnitude?

Ans. Force has both magnitude and direction.

## Q8: What is the necessary condition for a force to come into play.

Ans. At least two objects must interact for a force to come into play.

Ans.

## Q10: Name the force responsible for the wearing out of bicycle tyres.

Ans. Force of friction.

## Q11: What causes a change in the state of motion of an object?

Ans. The force of friction is responsible for change in the state of motion of an object.

## Q12: What brings a ball rolling on the ground to rest gradually?

Ans. The force of friction between the surface of the ball and the ground brings the rolling ball to rest.

## Q13: What force will you use to sort out pins easily from garbage? Whether it is a contact force or non-contact force?

Ans. Magnetic force act on to sort out pins and, it is a non-contact force.

## Q14: When an object is thrown vertically upwards why does its speed decrease?

Ans. It’s speed decrease because the force of gravity opposes its upward motion.

## Q15: What is electrostatic force?

Ans. The force exerted by a charged body on another charged or uncharged body is called electrostatic force.