Motion in a Straight Line — Class 11 Physics

This chapter describes rectilinear motion — the motion of an object treated as a point along a straight line — without enquiring into its causes (kinematics). Position is measured from a chosen origin and may be positive or negative. Average velocity is displacement divided by time interval, while instantaneous velocity is the limit of average velocity as the interval shrinks to zero, ; graphically it is the slope of the tangent to the position-time graph. Speed is the magnitude of velocity, and average speed (path length over time) is always greater than or equal to the magnitude of average velocity. Acceleration is the rate of change of velocity, , and may be positive, negative or zero. The area under a velocity-time graph equals the displacement. For uniformly accelerated motion the three kinematic equations , and relate displacement, time, initial and final velocities and acceleration, and can be derived graphically or by calculus. Free fall under gravity, with , is an important special case, illustrated by Galileo's law of odd numbers, stopping distance and reaction time. Relative velocity describes the motion of one object as seen from another.