Mardan board (KPK) Physics class 9 Chapter 2 (Kinematics) notes are written in easy words. Important questions are also given in Kinematics Chapter 2 notes.

## Physics class 9 Chapter 2 (Kinematics) Conceptual Questions

### Is it possible that displacement is zero but not the distance ? Under what condition displacement is equal to the distance?

For example, a student leaves school from point A. He moves 3m north and reaches point B, 5m in the west to reach point C. Then 3m in the south to reach point D and finally 3m to east and comes the same initial point A. Here the path traveled is 3 + 5 + 3 + 5 = 16m. Here path traveled is distance which is 16 m. But the student is in the same point where he was initially hence his displacement is zero as shown in figure below.

The distance and displacement will be equal when a motion of a body from one point to another point is in a straight line.

For example, Suppose a body only moves from ‘O’ to ‘C’, the distance is the actual path covered by the body, while displacement is the shortest distance between ‘O’ and ‘C’. In both cases, it is 25 m.

**Q.2) Does a speedometer measures a car’s speed or velocity?**

**Answer:**

Car speedometer only measures speed and doesn’t give any information about direction. The difference between speed and velocity is that velocity has a direction (the direction of the instantaneous speed) associated with it. Suppose a car is moving with 50 km/h, it is the speed of a car, but when you specify 50 km/h towards West, then it is the velocity of a car.

### Q.3) Is it possible for an object to be accelerating and at **rest at the same time? Explain with example.**

**Answer:**

We know that the motion and rest are not absolute but relative.

As, acceleration = **a** = change in velocity / elapsed time

and velocity = **v** = displacement / elapsed time

Since, acceleration is indirectly dependent on displacement if the body is at rest with respect to some observer, it’s displacement is zero with respect to that observer, in the situation acceleration should be zero with respect to that observer, in this case it is not possible for an object to be accelerating and at rest at the same time.

But if we have a relative motion then for the same event two observers can have different observations. For example, a body in the bus is accelerating with respect to an observer on the ground. Whereas the same body is at rest and have acceleration zero with respect to another observer sitting inside the bus.

**Q.4) Can an object have zero acceleration and non zero velocity at the same time? Give example.**

**Answer:**

Yes we can have zero acceleration and non zero velocity in case of uniform motion in which an object is moving in straight line with constant velocity, here object has non zero velocity but there is no change in velocity so no acceleration.

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**Q.5) A person standing on the roof a building throws a rubber ball down with a velocity of 8.0m/s. what is the acceleration (magnitude and direction)of the ball?**

**Answer:**

As we know that all bodies falling toward earth with a constant acceleration of g = 9.8 m/s^{2}. Now a person standing on the roof a building throws a rubber ball down with a velocity of 8 m/s. The acceleration of the ball will be g = 9.8 m/s^{2} directed toward the earth.

**Q.6) Describe the situation in which the speed of an object is constant while the velocity is not.**

**Answer:**

Suppose an object moving in uniform circular motion. The speed of the body will be uniform while the direction of body change at every point, therefore the velocity of the body is changing at each point. In this situation, the speed of an object is constant while the velocity is not constant.

**Q.7) Can an object have a northward velocity and a ****southward acceleration? Explain.**

**Answer:**

Yes, an object can have northward velocity and a southward acceleration, this is possible when a body is moving towards the north and gradually its velocity decreases. Now the acceleration which is actually the deceleration produced will be in the southward direction.

**Q.8) As a freely falling object speeds up, what is happening to its acceleration-does it increases, decreases, or stay the same? **

**Answer:**

The acceleration of a freely falling object remains constant and is 9.8 m/s^{2}, therefore as a freely falling object speeds up, its acceleration will not increase or decrease but will remain the same i.e. 9.8 m/s^{2}.

**Q.9) A ball thrown vertically upward with an initial speed of 5 ms**^{-1}. What will its speed be when it returns to its starting point?

^{-1}. What will its speed be when it returns to its starting point?

**Answer:**

In case of no air resistance, the ball will reach its initial position at the same speed of 5 m/s which is the speed with which the ball was thrown vertically upward.

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**Physics Class 9th Notes Comprehensive Questions Chapter 2 Kinematics**

**Q.1) What is motion? Describe that motion is relative. How two observers in relative motion can have conflicting views about the same object? **

**Motion**

“*A body is in a state of motion with respect to an observer if it changes its position with respect to that observer*“.**Motion is relative**

For the same event, two observers can have different observations. For example, a body in a train is in motion with respect to an observer on the ground. Whereas the same object is at rest with respect to another observer in the train moving with the object. Thus the **motion** and rest are not absolute but relative, This means that we have to specify the observer while telling about the rest or motion of the body.

As position needs a reference, therefore rest and motion also need the specification of an observer.

For example when a teacher changes her position in the classroom while students are sitting on their chairs. According to students observation, the teacher is in motion. Interestingly, teacher while moving also observes the students move as well.

Similarly, when Sara Leaves in train and her cousin John sees her off. As the train starts moving Sara see John moving to the right with the same speed as John see Sara moving to the left.

**Q.3) Define scalar and vector quantities. Explain with examples the graphical representations of vector quantities.**

**Answer:****Scalars quantities**

In physics, scalar are those quantities, which are completely described by their magnitudes and with a proper unit are called scalars.

For example, speed, volume, time, work, energy, power, and density etc.**Explanation**

There is one characteristic is associated with scalar quantities, that is their magnitude. When comparing two scalar quantities of the same type, you have to compare only their magnitude.**Vectors quantities**

In physics, vectors are those quantities, which are completely described by their magnitudes and with a proper unit and direction are called vectors.

For example, velocity and acceleration.**Explanation**

There are two characteristics of vector quantities, a magnitude, and a direction. When comparing two vector quantities of the same type, you have to compare both the magnitude and as well as their direction. When doing any mathematical operation on a vector quantity (like adding, subtracting, multiplying) you have to consider both the magnitude and the direction. This makes dealing with vector quantities a little more complicated than scalars.

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**Q.4) What is a position. Explain the difference between distance traveled, displacement, and displacement magnitude. **

**Answer:****Position**

*“A Position is the location of an object relative to some observer”*.

Earth is usually taken as a reference, and we often describe the position of an object as it relates to stationary objects on earth.**Distance traveled**

* “The length of a path traveled between two positions is called distance”*.

Distance has no direction and therefore it is a scalar quantity and its SI unit is a meter.**Displacement**

*“The shortest directed distance between two positions is called displacement”*. Straight distance from one point to another is called displacement.

Displacement has direction, therefore, it is a vector quantity. Its SI unit is meter same as distance.

**Q.6) Use velocity-time graph to prove the following equations of motion **

**a) v _{f} = v_{i} + at b) S = v_{i} t + 1/2 at^{2 }c) 2aS = v_{f}^{2} – v_{i}^{2}**

**Answer:****First equation of motion**

First equation of motion gives the relation of final velocity ‘**v**_{f}‘ in terms of initial velocity ‘**v**_{i}‘ and acceleration ‘**a**‘ in time t.

From the graph it is clear that

DB = DC + CB ……….. (i)

In figure:

Line segment DB represents, final velocity v_{f } DB = **v _{f}**

Line segment DC represents, initial velocity v

_{i }DC =

**v**

_{i}

Line segment CB in terms of the slope givesCB =

**a**t

Putting these value in equation (i) from the graph we get

**v**t

_{f }= v_{i}+ a**……….. (A)**

**Second equation of motion**

Second equation of motion relates displacement ‘

**s**‘ with the initial velocity ‘

**v**

_{i}‘ and acceleration ‘

**a**‘ in time ‘t’. As the area under the velocity-time curve represents the displacement ‘

**s**‘ as shown in the figure below.

**Q.7) What is free-fall, what is its value near the surface of the earth. Explain with examples that rock and sheet of paper will fall at the same rate without air resistance.**

**Answer:**

Everyone has observed the effect of gravity as it causes objects to fall downward. In the absence of air resistance, it is found that all bodies at the same location above the earth fall vertically with the same acceleration. Furthermore, if the distance of the fall is small compared to the radius of the earth, the acceleration can be considered constant throughout its fall. **The motion, in which air resistance is neglected and the acceleration is nearly constant, is known as free-fall**.

The acceleration of a freely falling body is called the acceleration due to gravity, and its magnitude (without any algebraic sign) is denoted by the symbol **g**. The acceleration due to gravity is directed downward, toward the center of the earth. **Near the earth’s surface, g** is approximately

** g = 9.80 m/s ^{2} ** or

**32.2 ft/s**

^{2}

A well-known phenomenon of a rock falling faster than a sheet of paper in which under the effect of air resistance the rock will fall faster than the paper, the air resistance is responsible for the slow fall of paper. If we have a tube in which the air is removed then the rock and air have exactly the same acceleration due to gravity and in the absence of air, the rock and the paper both fall freely.

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