### Contents of Science for Ninth Class Part 1 Physics

This book contains chapter-wise solutions, topic-wise solutions, exercise-wise solutions, and videos.

#### 6 Chapters

#### 6 Topics

Distance Travelled and Displacement

Speed

Velocity

Acceleration

Graphical Representation of Motion

Uniform Circular Motion

#### 3 Topics

Effects of Force

Newton's Laws of Motion

Conservation of Momentum

#### 4 Topics

Universal Law of Gravitation

Mass

Thrust and Pressure

Pressure in Fluids

#### 2 Topics

Work

Energy

#### 7 Topics

Sound Travels in the Form of Waves

Characteristics of a Sound Wave

Production of Sound

Reflection of Sound

Ultrasound

Characteristics of Sound

The Human Ear

#### 1 Topics

Model Test Paper (As per the Latest Marking Scheme)

## Experience Tests Tailored to Match the Real Exam

## Practise Questions with Solutions from the Science for Ninth Class Part 1 Physics

- (a)Magnitude of acceleration being produced.
- (b)Distance covered by the sprinter per second.
- (c)Direction in which the sprinter is running.
- (d)Centripetal force acting on the sprinter.

**Hint**

The circular motion is an accelerated motion even when the speed of the body remains constant.

**Solution:**

Since the sprinter is running with a constant speed which shows that the motion will be a uniform circular motion and only the direction will change. Due to the change in direction, velocity also changes.

- (a)$ST$
- (b)$QR$
- (c)$RS$
- (d)$PQ$

**Hint**

In a velocity-time graph, straight line sloping downwards indicates uniform deceleration.

**Solution:**

In a velocity-time graph, a straight line with a negative slope indicates uniform deceleration. In the given graph only the part RS is a straight line with a negative slope and hence, indicates uniform deceleration.

- (a)The scooter has uniform speed in this section.
- (b)The distance travelled by scooter is the maximum in this section.
- (c)The distance travelled by the scooter is the minimum in this section.
- (d)The distance travelled by the scooter is zero in this section.

**Hint**

When a body does not change its position, the distance-time graph will be parallel to the time axis.

**Solution:**

If the distance time graph is parallel to the time axis, it implies that the object has not travelled any distance in that time period. Hence, the distance travelled by the scooter is zero in the given case.

- (a)Motion of the Earth around the Sun.
- (b)Motion of a toy train on a circular track.
- (c)Motion of a racing car on a circular track.
- (d)Motion of hours’ hand on the dial of the clock.

**Hint**

Recall the characterstics of uniform circular motion.

**Solution:**

In uniform circular motion, speed of the object along the circular path is constant. For a racing car, speed of the car is not constant (its either accelerating or decelerating). Hence, it is not uniform circular motion.

- (a)
- (b)
- (c)

**Hint**

Recall that the ball possesses both deceleration and acceleration during its motion.

**Solution:**

By observing the graph carefully, it can be seen that the speed of the ball goes on decreasing uniformly as it moves upward and becomes zero at the highest point and then increases uniformly as it moves downward.

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- (a)balanced forces
- (b) centripetal forces
- (c) unbalanced forces
- (d)centrifugal forces

**Hint**

Recall the force in which the resultant of all the forces acting on a body is zero.

**Solution:**

When we squeeze the tooth-paste, the upper finger exerts the force downwards and the lower finger exerts the force upwards and these two forces change the shape of the tooth-paste tube and the two forces are balanced, since the state of the tooth-paste tube remains the same. A body under the action of balanced forces does not change its position of rest.

- (a)to increase its speed
- (b)to decrease its speed
- (c)to resist a change in its state of motion
- (d)to decelerate due to friction

**Hint**

Recall Newton's first law of motion.

**Solution:**

When a body is at rest or in uniform motion, it has a tendency to resist the change in its state and this tendency is called inertia.

- (a)electrical force
- (b)balanced force
- (c)unbalanced force
- (d)nuclear force

**Hint**

Recall which forces can move a body from rest or stop a moving body.

**Solution:**

When we talk of a force acting on a body, it usually means unbalanced forces.

If the resultant of all the forces acting on a body is non-zero, then the forces are said to be unbalanced forces. Unbalanced forces can move a body from rest or stop a moving body.

- (a)Newton's third law of motion
- (b)Newton's law of gravitation
- (c)Newton's first law of motion
- (d)Newton's second law of motion

**Hint**

When the carpet is hanging, the dust particles present on the carpet are at rest. Recall the law of motion which says about states of motion.

**Solution:**

When the carpet is hanging, the dust particles present on the carpet are at rest. At the moment when we beat the carpet with a stick, the carpet moves in a to and fro manner while the dust particles resist the motion due to inertia of rest. So the dust particles get detached from the surface.

- (a)moves backward
- (b)moves forward
- (c)rises upwards
- (d)remains unaffected

**Hint**

Recall Newton's first law of motion.

**Solution:**

When the tank is moving with uniform speed, the water in the tank also moves with same speed of the tank, but when the brakes are applied, the tank stops moving, but the water tends to move forward due to inertia of motion.

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- (a)is the same on equtor and poles
- (b)is the least on poles
- (c)is the least on equator
- (d)increases from pole to equator

**Hint**

Analyse the change in the value of radius while going from poles to the equator.

**Solution:**

As we know, the earth is not a perfect sphere. Due to the flattening of the surface at poles, the radius of the earth is minimum at poles and maximum at equator. Therefore, $g$ should be least at the equator as $g$ is inversely proportional to the square of the radius of the earth.

- (a)the earth and a point mass only
- (b)the earth and the sun only
- (c)any two bodies having some mass
- (d)any two charged bodies only

**Hint**

Recall that the law of gravitation is also known as Newton's universal law of gravitation.

**Solution:**

Newton's law of gravitation states that everybody in the universe attracts every other body with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

Hence, we can conclude that the law of gravitation gives the gravitation force between any two bodies of some mass.

- (a)depend on the value of g on the place of observation
- (b)is used only when the earth is one of the two masses
- (c)is the greatest on the surface of the earth
- (d)is of the same value irrespective of the place of the observation

**Hint**

$G$ is also known as the universal gravitational constant.

**Solution:**

In Newton's law of gravitation, the quantity $G$ is the universal constant and does not depend on any unit. So, $G$ is of the same value irrespective of the place of observation.

- (a)depends on mass of earth only
- (b)depends on radius of earth only
- (c)depends on both mass and radius of earth
- (d)depends neither on mass nor on radius of earth

**Hint**

$G$is also known as universal gravitational constant.

**Solution:**

In Newton's law of gravitation,

$F=\frac{G{m}_{1}{m}_{2}}{{d}^{2}}$

where $G$ is a proportionality constant. A constant does not depend on any values. Therefore, $G$ depends neither on mass nor on radius of earth.

- (a)$\frac{1}{4}$ times
- (b)$\frac{1}{2}$ times
- (c)$4$ times
- (d)$2$ times

**Hint**

The universal law of gravitation states that the gravitational force is directly proportional to the product of the masses and inversely proportional to the square of the distance between them.

**Solution:**

Let $F$ be the gravitational force between the two particles of masses ${m}_{1}$ and ${m}_{2}$ and distance between them be $d$.

Therefore,

$F=\frac{G{m}_{1}{m}_{2}}{{d}^{2}}....\left(1\right)$

Now, let ${F}_{1}$ be the gravitational force between the two particles of masses $2{m}_{1}$ and $2{m}_{2}$ and distance between them be $d$.

Therefore,

${F}_{1}=\frac{4G{m}_{1}{m}_{2}}{{d}^{2}}\phantom{\rule{0ex}{0ex}}{F}_{1}=4F....(from1)$

Therefore, the gravitational force will become 4 times.

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- (a)increases
- (b)decreases
- (c)remains constant
- (d)first increases and then decreases

**Hint**

Energy cannot be created or destroyed but can be changed from one form into another.

**Solution:**

If we assume that a freely falling object constitute an isolated system, then its total energy is always conserved. Here, the sum of the body's potential energy and kinetic energy remains constant at all points. Isolated system means that the system does not interact or exchange with its surrounding.

- (a)chemical energy to heat energy
- (b)kinetic energy to heat energy
- (c)potential energy to sound energy
- (d)chemical energy to kinetic energy

**Hint**

Rubbing both hands together to create heat is an example of presence of frictional force which involves both movement and heat generation.

**Solution:**

When an object having some kinetic energy stops somehow or tries to stop, for example by applying brakes, it slows down. As it slows down, its kinetic energy decreases. At the same time if we observe around the brakes, there will be some increase in temperature. We conclude that friction has caused kinetic energy to be converted into heat energy.

- (a)hydroelectric power stations use water to drive turbines
- (b)in a power station, turbines drive generators
- (c)electricity from thermal power stations differs from that produced in hydroelectric power stations
- (d)in hydroelectric power stations and thermal power stations, alternators produce electricity

**Hint**

Hydro-electric power stations use water to produce electricity, whereas thermal power plants make electricity by using fossil fuels.

**Solution:**

A power station or power plant is a facility for the generation of electric power. Electricity produced in hydro electric power station is similar to electricity produced in thermal power stations. The thermal power is produced by burning coal, petroleum or natural gas in thermal generator. But Hydro-electricity or hydel power is derived from the force of running water by means of dynamo driven by hydro-turbine.

- (a)only potential energy
- (b)only kinetic energy
- (c)half potential and half kinetic energy
- (d)less potential and more kinetic energy

**Hint**

Energy cannot be created or destroyed, but it changes from one form into another.

**Solution:**

When the object was resting at a height, the only energy it possessed was potential energy due to gravity of earth. When the object starts falling, energy conversion starts taking place. The potential energy decreases while kinetic energy increases.

At the halfway between, potential energy will be decreased to half of the initial value. This decrease will be equal to the increase in kinetic energy.

- (a)watt
- (b)watt-hour
- (c)kilowatt-hour
- (d)kilowatt

**Hint**

It is the amount of energy you would use by keeping a $1000$ watt appliance running for one hour.

**Solution:**

The commercial unit of energy is kilowatt-hour.

Note: $1\mathrm{kW}=1000\mathrm{W}$ and $1\mathrm{hour}=3600\text{seconds}$.

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- (a)The maximum disturbance caused by a wave.
- (b)The number of complete vibrations per second.
- (c)The distance between one crest of a wave and the next one.
- (d)The distance travelled by a wave per second.

**Hint**

Frequency is inversely proportional to the time period.

**Solution:**

The number of oscillations per unit time is called the frequency of the sound wave. The number of compressions or rarefactions that cross a given point in per unit time is the frequency of the wave. It is denoted by $f$ and its unit is $\mathrm{Hz}$ (Hertz).

- (a)Stretched strings
- (b)Stretched membranes
- (c)Metal plates
- (d)Air columns

**Hint**

These are made of alloys.

**Solution:**

A musical note is produced by the cymbal when metal plates vibrate in an orchestra. Cymbal is a musical

instrument that produces sound when being struck.

- (a)$2$
- (b)$0.2$
- (c)$20$
- (d)$200$

**Hint**

The wavelength is the distance between two successive crests or troughs.

**Solution:**

Given that,

Speed of wave, $\mathrm{v}=340{\mathrm{ms}}^{-1}$

Frequency, $\mathrm{f}=1700\mathrm{Hz}$

Therefore, $\mathrm{\lambda}=\frac{\mathrm{v}}{\mathrm{f}}=\frac{340}{1700}=0.2\mathrm{m}=20\mathrm{cm}$

- (a)Water
- (b) Wax
- (c)Vacuum
- (d)Empty vessel

**Hint**

Remember, sound needs a medium to travel.

**Solution:**

Sound is a mechanical wave as it needs a material medium such as solid, liquid and gas for propagation. Sound cannot travel through a vacuum. Hence, the note must be passing through a vacuum; that's why it is not audible to the person.

- (a)Light emitted by a CFL
- (b)TV signals from a satellite
- (c)Ripples on the surface of a pond
- (d)Musical notes of an orchestra

**Hint**

The sound wave from musical instruments does not have crest and trough.

**Solution:**

Musical notes of an orchestra are sound waves generated by vibration of the air inside the orchestra pipe, and the sound wave is an example of a longitudinal wave therefore musical notes of an orchestra consisting of longitudinal waves.

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- (a)$30\raisebox{1ex}{$\mathrm{m}$}\!\left/ \!\raisebox{-1ex}{${\mathrm{s}}^{2}$}\right.$
- (b)$3\raisebox{1ex}{$\mathrm{m}$}\!\left/ \!\raisebox{-1ex}{${\mathrm{s}}^{2}$}\right.$
- (c)$18\raisebox{1ex}{$\mathrm{m}$}\!\left/ \!\raisebox{-1ex}{${\mathrm{s}}^{2}$}\right.$
- (d)$0.83\raisebox{1ex}{$\mathrm{m}$}\!\left/ \!\raisebox{-1ex}{${\mathrm{s}}^{2}$}\right.$

**Hint**

Recall that, $1\mathrm{km}/\mathrm{h}=\frac{5}{18}\mathrm{m}/\mathrm{s}$.

**Solution:**

Given,

Initial velocity, $u=20\raisebox{1ex}{$\mathrm{km}$}\!\left/ \!\raisebox{-1ex}{$\mathrm{h}$}\right.\times \frac{5}{18}=5.55\raisebox{1ex}{$\mathrm{m}$}\!\left/ \!\raisebox{-1ex}{$\mathrm{s}$}\right.$

Final velocity, $v=50\raisebox{1ex}{$\mathrm{km}$}\!\left/ \!\raisebox{-1ex}{$\mathrm{h}$}\right.\times \frac{5}{18}=13.88\raisebox{1ex}{$\mathrm{m}$}\!\left/ \!\raisebox{-1ex}{$\mathrm{s}$}\right.$

As we know,

Acceleration, $a=\frac{v-u}{t}$.

Hence, $a=\frac{13.88-5.55}{10}=0.83\raisebox{1ex}{$\mathrm{m}$}\!\left/ \!\raisebox{-1ex}{${\mathrm{s}}^{2}$}\right.$

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### Importance of Science for Ninth Class Part 1 Physics

Practice Now#### Question-wise Difficulty Level Summary

Additionally, we tag all questions of any book to difficulty, ranging from 1 to 10. This tagging helps create personalised learning paths based on the performance of the student.

The table given below shows the difficulty tagged to the questions of this textbook -

Difficulty | No. of Questions |
---|---|

Easy | 795 |

Medium | 433 |

Hard | 2 |

## Why is the Science for Ninth Class Part 1 Physics Important?

Exam Name |
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