Short and Conceptual answers of Chapter 2 | Kinematics | Physics Class 9 Notes

Physics Notes of Short and Conceptual answers on Kinematics
From the Notes Library of H.E.S (Health, Education, and Skills)

Define the following.
i. Mechanics    ii.    Kinematics    iii.    Dynamics    
iv.   Motion         v.          Rest               

i. Mechanics

The branch of physics that deals with the study of the motion of bodies is called mechanics.

ii. Kinematics

The branch of mechanics that deals with the motion of bodies without reference of force is called Kinematics.

iii. Dynamics

The branch of mechanics that deals with motion as well as with the force that causes the motion of a body is called dynamics.

iv. Motion

When a body (let A) changes its position with respect to another body (let B) then body A is said to be in a state of motion with respect to body B.

For example, a person walking in the garden will be in motion with respect to another person who is not moving but is static (stop).

v. Rest

When a body (let A) does not change its position with respect to another body (let B) then body A is said to be in a state of rest with respect to body B.

For example, in the above example, the person that is static and is not changing his position is said to be in a state of rest with respect to the person that is walking.

Define motion? 

Motion

When a body (let A) changes its position with respect to another body (let B) then body A is said to be in a state of motion with respect to body B.

For example, a person walking in the garden will be in motion with respect to another person who is not moving but is static (stop).

Describe that motion is relative. How two observers in relative motion can have conflicting views about the same object?

Motion and rest are not absolute, but relative

An object can be at rest and in motion at the same time. For example, consider that there are two persons A and B in a bus and a third person C is sitting on the floor outside the moving bus. Both A and B are in the state of rest with respect to each other but are in motion with respect to person C outside the bus. Thus while telling about the rest or motion of the body, we have to specify the observer. Otherwise, two observers may have conflicting views about the same object. This can be explained under the following examples

Example 1:

When a teacher changes his/her position with respect to students sitting on their chairs. According to student observation, the teacher is in motion. while under the teacher’s observation, the students are also in motion.

Example 2:

A passenger in a moving train observes the outside objects in the state of motion. In the same way, the person(s) outside the train observes that passengers are in motion. So both observers have conflicting views about the same object.

What is the position?

Position

The location of an object relative to some reference point (called origin) is known as position.

Explanation

In order to describe the motion of an object, we have to describe its position- where it is at any particular time. For example, if someone says that his house is 10 km away towards the east from the mosque (reference point), then actually he is showing the position. The position of an object is usually represented by x, r, s, l, or d.

Distance	Displacement	Displacement magnitude
The length of the path traveled between two positions is called distance.	The shortest directed distance between two positions is called displacement.

Define and explain the scalar with suitable examples.

Scalar quantities

Those physical quantities which can be completely specified by magnitude (number with the proper unit) are called scalar quantities.

Explanation

Scalar quantities are non-directional. For example, let's say a person buys 10 kg of sugar from a shop; here he is dealing with scalar quantity.

Arithmetic operations with scalar quantities

Arithmetic operations like addition, subtraction, multiplication, and division of scalar quantities are similar to simple algebraic arithmetic e.g.10 kg and 15 kg will be a total of 25 kg.

Examples

Speed, Volume, Time, Length, Mass, Energy, Temperature, Charge, Power, Density, and Distance are some of the scalar quantities.

Define and explain the vector quantities with suitable examples.

Vector quantities

Those physical quantities which can be completely specified by magnitude, as well as direction, are called vector quantities

Explanation

If we say that an airplane is moving with the speed of 50 km/hr (kilometer per hour) towards the North, here we are dealing with vector quantity because 50 km/hr represents the magnitude and the North represents direction.

Arithmetic operations with vector quantities

There are special rules for vector addition (head-to-tail rule), subtraction, and multiplication as well as vectors can be split into their components (Resolution of vectors).
Examples

Velocity, Acceleration, Displacement, Force, Weight, Momentum, and Torque are some vector quantities.

How will you represent vectors Symbolically?

The symbolical representation of Vectors

By an arrow above or below the letter: e.g. A or A. This type of vector representation is the most common.

By modulus: To represent the magnitude of a vector, it can be written as the symbol of a vector with no arrow e.g. A, B, or C.

How will you represent vectors Graphically?

Graphical representation of vectors

Graphically a vector is represented by the arrow. The length of the arrow shows the magnitude (under a certain scale) and the direction of the arrow indicates the direction of a vector.

Use of the coordinate system in vector representation

To represent vectors, we place them on a coordinate system. In the case of the geographical coordinate system, we have different directions as North (N), East (E), West (W), and South (S) i.e. (NEWS). On the other hand, in the Cartesian coordinate system, we have horizontal lines (XOX’), vertical line (YOY’), and their point of intersection “O” (origin). Vectors are considered with reference to coordinate axes. 

Any sets of values that indicate the position of a point in a given reference system is called coordinate axes.

Steps for representing a vector graphically

The following steps are to be taken to represent a vector on a graph.

Steps to be taken to represent a vector on a graph.	Explanation through an example
	Al-Khalid tank moves to 50 m from origin 30° North of East. Represent it graphically.
	Graphical coordinate system	Cartesian coordinate system
Draw a coordinate system.	Draw NEWS on a piece of paper.	Draw XOX’ and YOY’ on a piece of paper.
Select a suitable scale.	Let 10 m=1cm then 50 m=5cm.	Let 10 m=1cm then 50 m=5cm.
Draw a line parallel to the direction of the vector.	Draw a line (of some length) making a 30° angle from North to East.	Draw a line (of some length) making a 30° angle from the x-axis (taken as East).
Cut the line equal to the magnitude of the vector scale-wise.	Cut the line of length 5cm.	Cut the line of length 5cm.
Put an arrow in the direction of a vector.	Put an arrowhead to show its direction.	Put an arrowhead to show its direction.

Define, explain, and give examples of the following terms
(i). Distance (ii). Displacement

I. Distance

The actual space between any two points is called distance.       OR

The product of time and speed is called distance.

Explanation

By distance, we simply mean how far or near a body is, from another body so we are dealing with the space between these bodies. For example, the distance between Karachi and Islamabad.              

Representation of distance 

Distance is usually represented by “∆x, ∆r, ∆s, ∆l, or ∆d”.

Formula of distance

As distance (∆s) is the product of time (∆t) and speed (v), so

Distance = time x speed                                                  

Or       ∆s=∆t x v                 

Unit of distance

In SI the unit of distance is a meter (m).

ii. Displacement

The shortest distance between two points is called displacement.                                       Or     

Distance in a straight line directed from one point to another is called displacement.

Explanation

Let one say that his home is 500 m away from the school, then he is talking about the distance (S). But when he says that his home is 500 m straight towards the East from school then actually he is telling about displacement (∆s). Displacement has direction, so it is a vector quantity.

Representation of displacement

In physical quantities, displacement is represented by ∆x, ∆r, ∆s, ∆l, or ∆d.

Formula of displacement

Displacement = Velocity x time

Or        ∆s = Vx∆t

Unit of displacement

In SI the unit of displacement is meter (m).

Define and explain the following terms.
(i)   Speed       (ii)   Uniform speed   

(iii)   Variable speed       (iv) Average speed       

(v)   Instantaneous speed

1. Speed

The distance covered by the body in unit time is called the speed of that body.

Explanation

Speed means, how fast or slow a body is moving. For example, let someone say that the car is moving at 80 km/h, by this he means that the car is covering 80 km distance (∆s) in one hour (∆t). Speed is a scalar quantity.

Representation of speed

Speed is represented by the letter “V ”.

Formula of speed

Speed = Distance covered / (Elapsed time)

Or   V =  ∆S / ∆t = (S_f  - S_i) / (t_f  - t_i )                   

Unit of speed

The unit of speed is ms¹ (meter-per-second) or km/hr (kilometer-per-hour).

2. Uniform speed

When a body covers an equal distance in equal intervals of time then its speed is called uniform speed.

Explanation

Let's say a body covers 5 km in the first hour, then in the second hour again it covers a 5 km distance, its speed will be uniform speed. At uniform speed the average and instantaneous speeds become equal.

3. Variable speed

When a body covers a different distance in different intervals of time then its speed is called variable speed.

Explanation

A Car moves at a different speed in a populated area. For some time, it will move slowly and for some time it will move fast. So such a speed that changes with time is called variable speed.

4. Average speed

The total distance covered by a body in total time is called the average speed of that body.

Explanation

Let us consider that a car moves at 20 km/hr in a populated area and then speeds up to 60 km/hr when there is no traffic. In such a case, we will find its average speed by the following method.

Average speed = (20 km + 60 km) / 2hr

Average speed = 80 km / 2hr

Average speed = 40 km/hr

Representation of Average speed

Average speed is represented by “˂V˃”.

Formula and unit of average speed

<V> =  S/t

The unit of speed is ms^-1 or kmhr^-1.

5. Instantaneous speed

The speed of a body at some instant (particular time) is called the instantaneous speed of that body.

Explanation

While driving near a school we are not interested in how fast the car moves in the last ten minutes, but we are careful about the speed of the car at the instance when it is near the school. This is what instantaneous speed is called.

Formula and unit of instantaneous speed

V = Limit _(∆t→0) ∆s/∆t

The Limit approaching ZERO (close to zero but not zero) indicates that we observe the change in distance in a time as small as possible close to zero.