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8. Motion

(Science - IX)

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Motion: when an object changes its position respect to its surrounding with time, it is said to be the object is in motion. 

Example: motion of flying birds, motion of car, motion of blood in arteriesand veins etc. 

Most motions are complex. Some objects may move in a straight line, others may take a circular path. Some may rotate and a few others may vibrate. There may be situations involving a combination of these.

Origin: we need to specify a reference point that describes the position of an object is called the origin. 

Distance: This is total path covered by the body between its initial and final position is called distance.

Displacement: The shortest distance measured from the initial to the final position of an object is known as the displacement.

Magnitude: The numerical value of a physical quantity is its magnitude.

Physical Quantities: Quantities in physics which can be measured are called physical quantities like mass, time, area, velocity, area, density, volume, force etc. 

scalars  and vectors quantities:

Scalars quantities: The quantities that require magnitudes only to specify them are called scalars quantities or scalars. e.g Mass, time, area, speed, distance, temperature, density, volume etc. 

Vector quantities: Quantities that require both magnitudes and direction to specify them are called vector quantities or vectors. e.g Displacement, velocity, force, momentum, weight etc. 

Differences between Displacement and Distance. 

 

Displacement	Distance
(i)  It is vector quantity. (ii) It can be negative and positive. (iii) It can be zero.  (iv) It is the shortest distance between two points.	(i) It is scalar quantity. (ii) It cannot be negative. (iii) It cannot be zero. (iv) It is total path covered between to point.

Use of distance and displacement:

(i) The displacement and distance are used to describe the overall motion of an object.

(ii) They are used to locate its final position with reference to its initial position at a given time.

Motion along a straight line: 

The simplest type of motion is the motion along a straight line.

When an object moves on a straight path, such a motion is called motion along a straight line or Linear motion. 

Examples:

1. A bus moving on a straight road.
2. A train moving on straight road.                         
3. A ball rolling along a straight path.

Type of Motion: 

(i) Uniform Motion: As the object covers equal distance in equal intervals of time it is said to be in uniform motion.

Example: 

1. Motion of car.
2. Flying of a bird.
3. Motion of a sparing.

(ii) Non-uniform Motion: As the object does not covers equal distance in equal intervals of time it is said to be in uniform motion.

Uniform Motion:

When an object covers equal distance in equal intervals of time, it is said that the object is in uniform motion. 

e.g: See table

Time	Distance travelled by object A in m	Distance travelled by object B in m
9:30 am   9:45 am   10:00 am  10:15 am  10:30 am  10:45 am  11:00 am	10 20  30 40 50 60 70	12 19 23 35 37 41 44

 Object A covers equal distance in equal time interval, while object B does not cover equal distance in equal time interval. Therefore it is said that object A is in uniform motion and object B is in non-uniform motion. 

Finding rate of motion:

Rate of motion is found by the speed of an object. 

Speed: The distance travelled by the object in per unit time is called speed. 

The SI unit of speed is metre per second. symbolically it is written as m s^-1 . 

Avarage speed: The average speed of an object is obtained by dividing the total distance travelled by the total time taken.

If an object travels a distance s in time t then its speed v is,

                                       

 

Acceleration : 

Acceleration is changing in the velocity of an object per unit time is called acceleration. 

In other words: 

The rate of changing in the velocity is known as acceleration. 

• The SI unit ofacceleration is m s^-2.
• It is denoted by a.
• Acceleration is a vector quantity. 

Accelerated motion: If the velocity of an object changes from an initial value u to the final value v in time t, the acceleration is a, and this kind of motion is known as accelerated motion.

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Example: A bus starts from rest and attains a speed of 72 km h^-1 in 5 minutes while moving in uniform motion. Find the acceleration of the bus. 

Solution: 

Time (t) = 5 minites = 5 × 60 = 300 seconds

Initial velocity u = 0    [∵ bus was in rest] 

Final velocity v = 72 km h^-1 

       

                     

                         = (20 - 0)/300 

                         = 0.066 ms^-2

Acceleration of object = 0.066 ms^-2

Type of accelerations are based on direction of acceleration: 

These are:

(I) Positive acceleration

(II) Negative acceleration

(I) Positive acceleration : If the velocity of an object increases with time in the direction of motion of the object, this is known as positive acceleration. 

The direction of acceleration is same as the direction of motion. 

 

Main points: 

• The direction of acceleration is same as the direction of motion. 
• When the velocity increases with time in the same direction. 
• When an object starts from rest. 
• The final velocity is greater than initial velocity i.e (v > u).

(II) Negative acceleration : If the velocity of an object decreases with time in an opposite direction of motion of the object, this is known as negative acceleration. 

Retardation: Negative accelaration is known as retardation. 

Main points: 

• The direction of acceleration is opposite to the direction of motion. 
• Negative accelaraion produces when the velocity decreases with time.
• When an object is going to rest. 
• The final velocity is smaller than initial velocity i.e (v < u).

Uniform accelaration: When the velocity of an object changes by an equal amount in equal intervals of time such a accelaration of the object is known as uniform accelaration. 

Uniformly accelarated motion: The motion of an object having uniform accelaration is known as uniformly accelarated motion. 

Examples: 

(i) motion of a rolling ball on smooth surface. 

(ii) The motion of a freely falling body.

(iii) Motion of the earth round the sun. 

(iv) A high speed lift starts from multi-storied building. 

(v) The motion of the moon round the Earth.

Non-uniform accelaration: When the velocity of an object changes by an unequal amount in equal intervals of time such a accelaration of the object is known as non-uniform accelaration. 

Non-uniformly accelarated motion: The motion of an object having different accelaration is known as non-uniformly accelarated motion. 

Examples: 

(i) A racing car. 

(ii) The motion of vehicles moving at differents speeds.

(iii) The ball is thrown upward at top it get retarded. 

(iv) The motion of a bus passing through rough road. 

The Page is under construction. 

प्रश्न: 

Q1. दुरी-समय ग्राफ हमें क्या बताता है ? 

Q2. वेग-समय ग्राफ हम  क्या दर्शाते है ? 

Q3. यदि वस्तु के वेग-समय ग्राफ x-अक्ष के समांतर एक सीधी रेखा है तो वेग की प्रकृति बताइए ? 

Q4.  दिए गए वेग -समय ग्राफ में छ्यांकित भाग का क्षेत्रफल किस भौतिक राशि को दर्शाता है ? 

 

 

 

 

Q5. किसी वस्तु के वेग में परिवर्तन को किस ग्राफ के द्वारा दर्शाया जायेगा ? 

Q6. वेग-समय ग्राफ के नीचे के क्षेत्र से मापी गयी  राशी क्या होती है ? 

Q7. गति के समीकरण को ग्राफीय विधि से प्राप्त कीजिये |

Q8.