# NCERT Solutions for Class 9 Physics Chapter 8 Free PDF Download

## NCERT Solutions for Class 9 Physics Chapter 8 – Motion

Physics is an interesting subject that deals with the behavior of the universe. And it is an important part of class 9 Science (Physics). In this, NCERT Solutions for Class 9 Physics Chapter 8 we are going to discuss the various pieces of the chapter Motion. These are explained in simple language.

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### CBSE Class 9 Physics Chapter 8 – Motion NCERT Solutions

In general terms, motion means to move forward. But according to physics, motion refers to the movement of an object in a definite interval of time with respect to its surroundings.

The beginning of the chapter talks about the movement and stationary objects. After that, it describes that everything living and non-living moves and their movement is motion. This can be plants, animals, birds, humans, objects, veins and arteries, atoms, molecules, stars, and even galaxies.

### Subtopics covered under NCERT Solutions for Class 9 Physics Chapter 8

#### 8.1 Describing Motion

This topic describes what is motion with the help of an example.

8.1.1 Motion Along A Straight Line- This simply refers to the motion of an object on a straight line. This is also explained with the help of an example.

8.1.2 Uniform Motion And Non-Uniform Motion- This topic discusses uniform and non-uniform motion

• Uniform Motion- It refers to the movement of an object on a straight path with a uniform speed and it covers equal distance in equal intervals of time.
• Non-Uniform Motion- It refers to the movement of an object that covers unequal speed in an equal interval of time.

#### 8.2 Measuring the Rate of Motion

This topic describes that the rate of motion can vary from object to object. While some object takes more time and some take lesser time. Its formula is given below:
Average speed = $$\frac{Total distance travelled }{total time taken}$$ or v = $$\frac{s}{t}$$

Where v= speed, s = distance, t = time taken.

8.2.1 Speed With Direction- This topic describes that an object moving in a specific direction has greater speed than any object moving in a different direction. Its formula is

Average velocity = $$\frac{initial velocity + final velocity}{2}$$

Mathematically, vav = $$\frac{u + v}{2}$$

#### 8.3 Rate of Change of Velocity

This refers to the change of speed of any object from zero and it remains non-constant then it is called the rate of change of velocity or acceleration.
Acceleration = $$\frac{change in velocity}{time taken}$$

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

#### 8.4 Graphical Representation of Motion

This topic tells that a convenient way to present motion is by using a graph.

8.4.1 Distance- Time Graphs- This topic describes how to make a distance and time graph of various units.

8.4.2 Velocity- Time Graphs- This topic describes how to make a velocity and time graph of different units.

#### 8.5 Equations of Motion by Graphical Method

This topic tells the formula or equation that helps in graphical representation. These are mentioned below:

v = u + at
s = ut + ½ at2
2 a s = v2 – u 2

8.5.1 Equation For Velocity- Time Relation- This with the help of a graph explains the equation of velocity.

8.5.2 Equation for Position-Time Relation- This topic explains the equation of position and time relation with the help of an example.

8.5.3 Equation For Position-Velocity Relation- With the help of an example the equation of position and velocity relation is also explained in this topic.

#### 8.6 Uniform Circular Motion

This refers to the movement of an object in a circular path at a constant speed.
Formula, v = $$\frac{2πr}{t}$$

Where v = velocity, 2πr = circumference of circle, t = time taken

### Solved Questions for You

Question 1: State which of the following situations are possible and give an example for each of these:
(a) An object with a constant acceleration but with zero velocity.
(b) An object moving in a certain direction with an acceleration in the perpendicular direction.

Answer: (a) Let’s say a stone is thrown in vertical direction with initial speed u. As the time passes its velocity decreases and at the maximum height it attained, its velocity is zero but it has an acceleration (9.8)m/s2 in the downward direction due to gravity.

(b)  When a pebble is tied with an end of string and it is whirled in horizontal circular motion with the other end of the string at the centre of circular path, then it has a constant magnitude of centripetal acceleration towards the centre. Hence the acceleration is in perpendicular direction to the motion of pebble.

Question 2: The angle between velocity and acceleration during the retarded motion is?

1. 180
2. 40
3. 45
4. 0

Answer: During the retarded motion accelerations acts exactly opposite to the direction of velocity. Thus, the correct option is A.

Question 3: Assertion- Retardation is directly opposite to the velocity.

Reason- Retardation is equal to the time rate of decrease of speed.

1. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
2. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
3. Assertion is correct but Reason is incorrect
4. Both Assertion and Reason are incorrect

Answer:  Acceleration is the time rate of increase of velocity.

But retardation is reverse of acceleration.

⟹ Retardation is time rate of decrease of velocity.

Also retardation acts in the direction opposite to that of velocity.

Hence option A is correct.

Question 4: The equations of motion are applicable only when the body moves with

1. uniform acceleration
2. variable acceleration
3. Both A & B
4. None of the above.

Answer:  The equation of motion are applicable only when the body moves with uniform acceleration. One such example is projectile motion.

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