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NCERT Solutions for Class 11 Physics Chapter 3 Motion In Straight Line (Click Here)

Section NameTopic Name
3Motion in a Straight Line
3.2Position, path length and displacement
3.3Average velocity and average speed
3.4Instantaneous velocity and speed
3.6Kinematic equations for uniformly accelerated motion
3.7Relative velocity
motion in straight line

Class 11 Physics Motion In A Straight Line Notes


Motion is change in position of an object with time.Motion of object along a straight line is called rectilinear motion. Examples include flying kite, moving train, earth’s rotation etc.

motion in straight line

Frame of Reference

In order to know the change in position of an object, a reference point is required. Point O in the figure is the reference point or Origin and together with three axes, this system is called the coordinate system. A coordinate system with time frame is called frame of reference.

  • Objects changing positions with time with respect to the frame of reference are in motion while those which do not change position are at rest.
  • For a moving car, for the frame of reference outside the car, it appears moving. While for the frame of reference inside the car, the car appears stationary.
motion in straight line
motion in straight line

Path Length (Distance) Vs. Displacement

Path Length: It is the distance between two points along a straight line. It is scalar quantity.

Displacement: It is the change in position in a particular time interval. It is vector quantity. Change is position is usually denoted by Δx (x2-x1) and change in time is denoted by Δt (t2-t1).

Position-Time, Velocity-Time and Acceleration-Time Graph

CriteriaP-T GraphV-T GraphA-T Graph
X and Y axisTime and PositionTime and VelocityTime and Acceleration
SlopeIt represents velocity of an objectIt represents acceleration of an object.It represents the jerk or push of a moving object.
Straight slopeUniform velocityUniform accelerationUniform jerk
Curvy SlopeChange in velocityChange in accelerationChange in the amount of push/jerk
motion in straight line
motion in straight line

Average Velocity and Average Speed

CriteriaAverage VelocityAverage Speed
DefinitionChange in position or displacement divided by time interval.Total path length travelled divided by total time interval regardless of direction.
Formula Avg speed = Total path length/Total time interval
Scalar or VectorVectorScalar
SignCan be positive or negativeAlways positive
motion in straight line
motion in straight line
motion in straight line

Instantaneous Velocity and Instantaneous Speed

Instantaneous velocity describes how fast an object is moving at different instants of time in a given time interval. It is also defined as average velocity for an infinitely small time interval.

Here lim is taking operation of taking limit with time tending towards 0 or infinitely small.

dx/dt is differential coefficient – Rate of change of position with respect to time at an instant.

motion in straight line
motion in straight line

Relative Velocity

This is the velocity of an object relative to some other object which might be stationary, moving slowly, moving with same velocity, moving with higher velocity or moving in opposite direction.

If initial position of two objects A and B are xA (0) and xB (0), the position at time t will be,

  • xA (t) = xA (0) + vA t
  • xB (t) = xB (0) + vB t
  • Displacement from object A to B, [ xB (0) – xA (0) ] + (vB -vA)t
  • Velocity of B relative to A = vBA = vB – vA
  • Velocity of A relative to B = vAB = vA – vB
motion in straight line
motion in straight line

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