**Chapter 3 Motion in a Straight Line**

**Chapter 3 Motion in a Straight Line** Class 11 Physics chapter notes on Chapter 3 (Motion in a straight line) of NCERT textbook are available here. These notes are based on latest CBSE Class 11 Physics Syllabus 2017-18 and are important for CBSE school exams and other competitive exams like NEET, JEE Mains, WBJEE etc.

Topics covered in these notes are

Statics |

Kinematics |

Dynamics |

Rest and Motion |

Rectilinear motion |

Frame of Reference |

Path Length |

Displacement |

The notes are given below:

**Statics:**

It is branch of mechanic which deals with the study of object at rest.

**Kinematics:**

It is the branch of mechanics which deals with the study of motion of objects without considering the factors (i.e. force etc) which causes motion.

**Dynamics:**

It is the branch of mechanics concerned with the motion of bodies under the action of forces.

**Rest & Motion**

*Rest*

An object is said to be in rest if it does not change its position with respect to its surroundings.

*Motion:*

An object is said to be in rest if it changes its position with respect to its surroundings.

*Rest and motion are relative*

It means that an object in one situation can be at rest but in another situation the same object can be in motion.

For example: A* *person sitting in a moving bus is at rest with respect to his fellow passengers but is in motion with respect to the objects outside the bus.

**Rectilinear motion**

Rectilinear motion is that motion in which a particle or point mass body moves along a straight line.

**Frame of Reference:**

As we know, a rectangular coordinate system consisting of three mutually perpenducular axes, labelled X-, Y-, and Z- axes. The point of intersection of these three axes is called origin (O) and serves as the reference point.

The coordinates (x, y. z) of an object describe the position of the object with respect to this coordinate system.

To measure time, we position a clock in this system. This coordinate system along with a clock constitutes a frame of reference.

**Path Length**

Consider the motion of a car along a straight line.

We choose the *x*-axis such that it coincides with the path of the car’s motion and origin of the axis as the point from where the car started moving, i.e. the car was at *x* = 0 at *t* = 0.

Let P, Q and R represent the positions of the bike at different instants of time.

Consider two cases of motion.

*In the first case, the car moves from O to P.*

Then the distance moved by the car is OP = +360 m.

This distance is called the path length traversed by the car.

*In the second case, the car moves from O to P and then moves back from P to Q.*

During this course of motion, the path length traversed is OP + PQ = + 360 m + (+120 m) = + 480 m.

Path length is a scalar quantity.

**Displacement**

Displacement of an object in a given interval of time is defined as the shortest distance between two positions of the object in a particular direction during that time and is given by the vector drawn from the initial position to its final position. It is a vector quantity.

Displacement is the change in position: Δ *x* = *x*_{2} – *x*_{1}. Path length is greater or equal to the magnitude of the displacement between the same points.

In the previous diagram, the displacement of the car from P to Q is 240 m – 360 m = – 120 m.

The magnitude of displacement may or may not be equal to the path length traversed by an object.

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