^Displacement versus distance
| Displacement | Distance | |
| Between two points | Length of shortest path | Length of actual path |
| Sign | +ve or –ve | +ve always |
| Can be zero | Yes | No |
| Direction | Initial to final | Meaningless |
| SI unit | meters | meters |
| Are equal | Only for motion along straight line with no reversal | |
^Displacement
Suppose a body is moving along a straight line. Let the straight line is along x – axis. Let at a time t1 it was situated at point A at a distance x1 from a fixed point O. Let it moves & at time t2 reaches point B situated at a distance x2 from O.
Here the distance AB is called displacement of the body along x axis (Sx) in the time interval t2 – t1 (= Dt). In terms of position coordinates it can be expressed as
Displacement, AB = OB – OA = x2 – x1 or Δx.
i.e. for motion along displacement is equal to the change in position coordinates along that line.
Position
^Position of an arbitrary point P at any time is described using cartesian coordinates i.e. P (x,y, z).
For 1 D motion along x axis, y & z coordinates are zero, where as x coordinate may be + ve or – ve.
1, 2 & 3 – D motion
If coordinate required to specify the position of a moving object at any instant are;
^Point object
An object is said to be a point object if it covers distance much greater than its size.
^Kinematics
Kinematics is a branch of physics which deals with the study of motion of the objects without considering their cause. In kinematics we ignore the size of body & assume it to be a point particle.
^Description of motion
Three parameters used to describe the motion of a body are; position, velocity & acceleration. Usually a reference point plus a clock is used to describe motion of an object.
Dynamics
^Dynamics is a branch of physics which deals with the study of motion of the objects considering their cause also.
Statics
^Statics is a branch of physics which deals with the conditions of objects at rest (i.e. static equilibrium).
^Mechanics
It is a branch of physics which deals with the conditions of rest or motion of the objects.
It is divided in three parts
1. Statics 2. Kinematics 3. Dynamics
^Rest & motion
If position of body changes w.r.t. surroundings with the time then it is said to be moving otherwise at rest. Rest & motion are relative. A person sitting in a moving vehicle is at rest w.r.t. other persons in sitting same vehicle while is in motion w.r.t. other persons standing on road. If the position of a body doesn’t change with respect to all objects in the universe, then it is said to be in absolute rest. Absolute rest is impossible. If the position of a body doesn’t change with respect to one or more things in the universe, then it is said to be in relative rest. Relative rest is possible.
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