CBSE Class 11 Physics Kinematics Exam Notes

INTRODUCTION

Mechanics deals with motion of various types of bodies, relations of forces, mechanical properties of bodies and matter etc. Kinematics deals with the motion of particles and rigid bodies without accounting for the forces responsible for their motion. If the body is, however, of a small size such that its motion can be described by a point mass moving along a straight line, such a motion is called rectilinear motion or motion in one dimension.

MOTION

When the body changes position w.r.t. observer, the body is said to be in motion. If the body doesn’t change position then it is said to be at rest w.r.t. observer. For example if two passengers are travelling in moving train they are at rest w.r.t. each other while they are in motion w.r.t. an observer standing on the ground.

A PARTICLE

A particle is physical analogue of a point. A body of finite size may be considered as a particle only if all parts of the body undergo same displacement and have same velocity and acceleration. Thus, for a particle like body, its motion can be described by studying the motion of any point on the body

DISTANCE AND DISPLACEMENT

As a particle moves in space with respect to time it follows a curve or a straight line, which is called its path or trajectory. Distance is the actual length covered along the path, while displacement is the difference between the final and initial positions of the particle. The position of a particle is given by a position vector drawn from the origin to the particle.
The figure illustrates the difference between distance and displacement. The curve length AB is called the distance; the vector rAB = rB – rA is called the displacement.

(a) Distance is a scalar; displacement is vector.
(b) The magnitude of displacement is always less than or equal to distance.
(c) For a moving body, displacement can be zero but distance cannot be zero.

For a body moving in a straight line from point A to point B along x-axis such that its initial and final position is at a distance xi and xf respectively, the minimum distance between final and initial position is called displacement, Δx

Displacement is a vector from initial position to final position. If the path of the body is not along a straight line, the total length of the actual path covered by the body is called the distance covered by the body and is a scalar quantity. The displacement is independent of the actual path covered by the body.

AVERAGE SPEED AND AVERAGE VELOCITY

The average speed in a time interval is defined as the total distance travelled by the particle divided by the time interval.

The distance travelled by a body per unit time in a specific direction is called the velocity. It is a vector quantity measured in m/s or similar units. The distance covered by a body per unit time is without any reference to its direction of motion.