The kinematics of a particle is characterized by specifying, at any given instant, the particle’s
position, velocity, and acceleration.
Position
The straight-line path of a particle will be defined using a single coordinate axis s, Fig. 12–1a. The
origin O on the path is a fixed point, and from this point the position coordinate s is used to specify
the location of the particle at any given instant. The magnitude of s is the distance from O to the
particle, usually measured in meters (m) or feet (ft), and the sense of direction is defined by the
algebraic sign on s.
Displacement
The displacement of the particle is defined as the change in its position. For example, if the particle
moves from one point to another, Fig. 12–1b, the displacement is s = s - s
Velocity
If the particle moves through a displacement s during the time interval t, the average velocity of the
particle during this time interval is vavg = s t If we take smaller and smaller values of t, the
magnitude of s becomes smaller and smaller. Consequently, the instantaneous velocity is a vector
defined as v = lim tS0 (s>t), or (S+ ) v = ds dt
Acceleration
Provided the velocity of the particle is known at two points, the average acceleration of the particle
during the time interval t is defined as aavg = v t Here v represents the difference in the velocity
during the time interval t, i.e., v = v - v, Fig. 12–1e. The instantaneous acceleration at time t is a
vector that is found by taking smaller and smaller values of t and corresponding smaller and smaller
values of v, so that a = lim tS0 (v>t), or (S+ ) a = dv dt
Constant Acceleration, a = ac .
Velocity as a Function of Time. Integrate ac = dv>dt, assuming that initially v = v0 when t = 0. L v v0 dv
= L t 0 ac dt
Position as a Function of Time. Integrate v = ds>dt = v0 + act, assuming that initially s = s0 when t = 0.
L s s0 ds = L t 0 (v0 + act) dt
Velocity as a Function of Position. Either solve for t in Eq. 12–4 and substitute into Eq. 12–5, or
integrate v dv = ac ds, assuming that initially v = v0 at s = s0 . L v v0 v dv = L s s0 ac ds