INTRODUCTION OF MOTION

In our blog for the students of 9th,10th,11th,12th, NEET and IIT , this is a post dealing with one of the most complicated and a deemed-to-be very tough topic. We have made it very simple and can be useful for both RBSE and CBSE students

INTRODUCTION OF MOTION:

• In the physical world, one of the most common phenomena is motion. The branch of Physics, which deals with the behavior of moving objects, is known as mechanics.
• Mechanics is further divided into two sections namely Kinetics and Dynamics.
• Kinematics deals with the study of motion without taking into account the cause of motion.
• Dynamics is concerned with the cause of motion, namely force.

Motion: If an object changes its position with respect to its surroundings (reference point) in a given time, the object is said to be in motion.

Rest: When an object does not change its position with respect to its surroundings (reference point) in a given time, the object is said to be in rest.

Position: It is the location of the object with respect to another object or stationary surroundings. Motion or rest of any object is defined as its position w.r.t. the reference point.

Reference point: It is the point from which the location of the object is measured. It is often called as origin.

Example:

When a person changes its position with respect to traffic signal (a reference point) with elapse of time, then person is called in motion. And if the same person does not change its position with respect to that electric pole, the person is called in the state of rest.

Thus to observe the motion of any object; two things are necessary – a reference point and time. In the above example; person is the object and traffic signal is the reference point. A building, tree, or any other stationary thing can be taken as reference point to observe the motion of an object.

Types of Motion:

Motion of objects is categorized in mainly three branches-

• Translatory motion
• Rotatory motion
• Vibratory motion

Translatory Motion

 In translatory motion the object moves from one point in space to another. Motion along a straight line. Example: A car moving on a straight road.

 Rotatory Motion

 In rotatory motion, the particles of the body describe concentric circles about the axis of motion. This motion is along curved or circular path. Example: A car moving on a curved path.

 Vibratory Motion

 In vibratory motion the particles move to and fro about a fixed point. Example: A simple pendulum performing to and fro motion about central position.

SCALERS AND VECTORS:

Scalars are quantities that are fully described by a magnitude (or numerical value) alone. Ex : Temperature, Distance, Mass, Speed etc.

Vectors are quantities that are fully described by both a magnitude and a direction. Ex : Velocity, Displacement, Force etc.

Motion along a straight line:

When an object moves along a straight line, the motion of the object is called rectilinear motion (translator motion). For example; motion of a vehicle along a straight road.

DISTANCE AND DISPLACEMENT:

 Distance: Total path length covered by an object during its motion between any two points.

 Displacement: The length of shortest path measured from the initial to the final position of an object.

 Distance is a scalar quantity (it only has magnitude no direction) whereas Displacement is a vector quantity (it has both magnitude and direction)

SI unit of distance and displacement is meter (m).

Kilometer is used to measure bigger distance and it is abbreviated as ‘km’.

EXAMPLE 1:

Suppose, a ball is rolling along a straight line.

Case 1:

Suppose, the ball starts moving from point A and reaches at point B.

Thus, distance covered by ball = 10 m

Displacement of ball = 10 m towards west.

Case 2:

Suppose, ball starts moving from point A and reaches to B. Again it returns on the same path from point B and reaches at A.

Thus, distance covered by the ball = distance from A to B + Distance from B to A

 = 10 m + 10 m =20m

In this condition, distance covered by ball = 20 m.

Since, ball returns at point A, thus displacement of the ball = 0

Case 3:

Suppose, the ball starts moving from point A, reaches point B and returns back to point C.

Then, the distance covered by ball = distance from A to B + Distance from B to C

= 10 m + 7 m = 17 m

Displacement of ball = Distance of point C from A = 3 m towards west.

EXAMPLE: 2

Distance can only be either positive or zero whereas Displacement can be positive, negative or zero depending on the reference point and direction.

Uniform & Non-Uniform Motion

UNIFORM MOTION: As the object covers equal distances in equal intervals of time, it is said to be in uniform motion.

Note: The time interval in this motion should be small.

Example- Motion of planets around Sun, they revolve at a constant speed. So the distance travelled in every equal time period is equal

Note: If rate of change of any quantity with time is constant, it is said to be Uniform.

NON UNIFORM MOTION: Motions where objects cover unequal distances in equal intervals of time are considered non-uniform.

Example- Athletes running, they are slow initially in their race but as the race progresses their speed increases. Let us say they cover 1 km in the first hour and after 1 hour they cover 3 km, so the distance covered in 1 hour is changing.

Speed:

Distance covered by a moving object in unit time is called speed of that object.

Speed is a scalar quantity. It has only magnitude (numerical value).

Where, v = speed, d = distance, t = total time.

SI unit of speed is meter per second (m/s).

In most cases, objects will be in non-uniform motion. Therefore, we describe the rate of motion of such objects in terms of ‘average’.

Average speed:

The average distance covered in unit time by a moving object is called average speed.

Average speed is the ratio of total distance covered and total time taken.

Where, v = Average Speed, d = Total distance covered, t = total time taken.

SI unit of average speed is meter per second (m/s).

Velocity:

The speed of a moving object in particular direction is called velocity. 

Velocity has both magnitude and direction while speed has only magnitude and no direction.

Where, v =velocity, s = displacement, t = total time.

SI unit of velocity is meter per second (m/s).

Velocity of an object is the distance covered in particular direction in unit time.

Uniform velocity:

Uniform speed of an object in same direction is called uniform velocity.

Non-Uniform velocity:

Velocity of an object changes in following three conditions.

1. When only speed changes.
2. When only direction changes.
3. When both speed and direction change.

Thus, non-uniform speed of a moving object in same direction or uniform speed in different directions or non-uniform speed in different directions is called non-uniform velocity.

Example: If a vehicle is moving on a circular path with uniform speed, then its velocity is said to be non-uniform, because on a circular path the direction of moving body changes along with direction of curve.

Average Velocity:

Average displacement of an object in unit time is known as average velocity.

In case of constant acceleration (rate of change of velocity is same per unit time), then arithmetic mean of velocity of an object moving along a straight line is called the average velocity.

u=Initial velocity,v=final velocity

Acceleration:

Acceleration: It is a measure of the change in the velocity of an object per unit time.

Acceleration can be caused either by change in direction of motion or change in speed or both.

Acceleration=(Change in velocity)/(Time taken)

SI unit is m/s²

It is a vector quantity. the acceleration is taken to be positive if it is in the direction of velocity and negative when it is opposite to the direction of velocity.

a=acceleration; u=initial velocity; v=final velocity; t=time taken,

When you apply brakes on a car the velocity is in forward direction but acceleration is in backward direction, so acceleration will be negative. It is also called Deceleration or Retardation.

An object retards when velocity and acceleration are in opposite directions.

When you are increasing the speed of a car the acceleration is positive as velocity and acceleration both are in the same direction.

An object accelerates when velocity and acceleration are in the same direction.

Uniform Acceleration:

It is the acceleration in which the velocity of an object changes equally (increases or decreases) over equal intervals of time.

Eg .the motion of a freely falling body is uniformly accelerated.

Non-uniform Acceleration:

It is the acceleration in which the velocity of an object changes unequally over equal intervals of time.

For example: The motion of a vehicle on a crowded road is with variable acceleration.

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