# CBSE Class 11th Physics Notes: Motion in a Straight Line (Part - II)

CBSE Class 11 Physics Notes on Chapter 3, Motion in a Straight Line are available here. The topics covered in these notes are Speed, Uniform Speed, Variable Speed, Average Speed, Instantaneous Speed, Velocity, Uniform Velocity, Variable Velocity, Average Velocity, Uniform Motion in a straight line and Relative Velocity.

Class 11 Physics chapter notes on Chapter 3, Motion in a Straight Line are available here. These notes are helpful for quick revision of the chapter. This article is continuation of C**lass 11th Physics Notes: Motion in a Straight Line (Part - I)** where basic concepts like Rectilinear motion, Frame of Reference, Path Length, Displacement etc are covered. Topics covered in this part are:

Speed |

Uniform Speed |

Variable Speed |

Average Speed |

Instantaneous Speed |

Velocity |

Uniform Velocity |

Variable Velocity |

Average Velocity |

Instantaneous Velocity |

Uniform Motion in a straight line |

Relative Velocity |

The notes are as follows:

**Speed**

Speed of an object is defined as the ratio of path length and the corresponding time taken by the object

i.e., Speed = Distance travelled/Time taken

Speed is a scalar quantity. Its SI unit is ms^{‒1}. Its dimensional formula is [M^{0}L^{1}T^{‒1}].

**CBSE Class 11th Physics Notes: Units and Measurement (Part - I)**

**Uniform Speed**

An object is said to be travelling with uniform speed if it covers equal distances in equal interval of time.

**Variable Speed**

An object is said to be travelling with uniform speed if it covers equal distances in unequal interval of time.

**Average Speed**

Average speed is defined as the total path length travelled divided by the total time interval during which the motion has taken place:

Average speed = (Total path length)/(Total time interval)

It is a scalar quantity and its SI unit is ms^{‒1}.

Suppose a body covers distances S_{1}, S_{2}, S_{3}… with speeds V_{1}, V_{2}, V_{3}… respectively, in same direction then average speed is given by

V_{avg} = {Total Distance Travelled}/{Total Time Taken} = {S_{1} + S_{2} + ….}/{(S_{1}/ V_{1}) + (S_{2}/ V_{ 2}) +...}

**Instantaneous Speed**

The average velocity tells us how fast an object has been moving over a given time interval but does not tell us how fast it moves at different instants of time during that interval. For this, we define instantaneous.

*v *= Δ*x*/Δ*t* [where, Δ*t* → 0]

⇒ *v *= d*x*/d*t*

**Class 11th Physics NCERT Solutions**

**Velocity**

Velocity of an object is defined as ration of displacement and the corresponding time interval taken by object i.e., velocity = (displacement)/(time interval), It is a vector quantity.

**Uniform Velocity**

A body is said to be moving with uniform motion if it undergoes equal displacement in equal interval of time.

**Variable Velocity**

It is defined as the ratio of displacement and the corresponding time interval taken by the object.

Velocity = (displacement)/(time taken)

It is a vector quantity.

**Average Velocity**

Average velocity is defined as the change in position or displacement (Δ*x*) divided by the time intervals (Δ*t*), in which the displacement occurs. It is a vector quantity.

Average Velocity = (Δ*x*/Δ*t*).

**Instantaneous Velocity**

The velocity of an object at a given instant of time is called its instantaneous velocity. When a body is moving with a uniform velocity, its instantaneous velocity = average velocity = uniform velocity.

**Uniform Motion in a straight line**

An object is said to be in uniform motion if its velocity is uniform, i.e., it undergoes equal displacements in equal interval of time.

**Relative Velocity**

The relative velocity of an object with respect to another is the velocity with which one object moves with respect to another object.

It is defined as the time rate of change of relative position of one object with respect to another object.

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