## Class 11 Physics Motion In A Straight Line

• • December 16, 2020

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

## Class 11 Physics Motion In A Straight Line Notes

Motion

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.

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.

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

Average Velocity and Average Speed

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.

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

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