What is Motion ? Points and Explanation

Main points:-

A ref­er­ence point or ori­gin is required to explain the posi­tion of an object. To one observ­er, an object may appear to be mov­ing while to anoth­er, it appears to be motion­less.
For instance, a pas­sen­ger on board a bus per­ceives the oth­er pas­sen­gers to be at rest, yet an observ­er out­side the bus per­ceives the peo­ple to be mov­ing.
A con­ven­tion, or a stan­dard ref­er­ence point or frame, is required to make obser­va­tions eas­i­er. The ref­er­ence frames of all objects must be the same.

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Distance and Displacement

The mag­ni­tude of the length cov­ered by a mov­ing object is called dis­tance. It has no direction.

Dis­place­ment is the short­est dis­tance between two points or the dis­tance between the start­ing and final posi­tions with respect to time. It has mag­ni­tude as well direction.

Dis­place­ment can be zero, but dis­tance cannot.

Magnitude

Mag­ni­tude is the size or extent of a phys­i­cal quan­ti­ty. In physics, we have scalar and vec­tor quantities.

Scalar quan­ti­ties are only expressed as mag­ni­tude. E.g: time, dis­tance, mass, tem­per­a­ture, area, volume

Vec­tor quan­ti­ties are expressed in mag­ni­tude as well as the direc­tion of the object. E.g: Veloc­i­ty, dis­place­ment, weight, momen­tum, force, accel­er­a­tion, etc.

Time, Average Speed and Velocity

Time and speed

Time is the dura­tion of an event that is expressed in sec­onds. Most phys­i­cal phe­nom­e­na occur with respect to time. It is a scalar quantity.

Speed is the rate of change of dis­tance. If a body cov­ers a cer­tain dis­tance in a cer­tain amount of time, its speed is giv­en bySpeed=DistanceTimeSpeed=DistanceTime

Uniform motion and non-uniform motion

When an object cov­ers equal dis­tances in equal inter­vals of time it is in uni­form motion.

When an object cov­ers unequal dis­tances in equal inter­vals of time it is said to be in non-uni­form motion.

Aver­age speed: For non-uni­form motion, the aver­age speed of an object is obtained by divid­ing the total dis­tance trav­elled by an object by the total time taken.

Veloc­i­ty: Veloc­i­ty is the speed of an object mov­ing indef­i­nite direc­tion. S.I. unit is m/s.

Accel­er­a­tion: Change in the veloc­i­ty of an object per unit time.

Graph­i­cal rep­re­sen­ta­tion of motions

(i) Dis­tance-time graph
For a dis­tance-time graph, time is tak­en on x‑axis and dis­tance is tak­en on the y‑axis.
[Note: All inde­pen­dent quan­ti­ties are tak­en along the x‑axis and depen­dent quan­ti­ties are tak­en along the y‑axis.]

(ii) Veloc­i­ty-time graph

Equa­tion of motion by graph­i­cal methods

(i)velocity-time rela­tion:

(ii) The equa­tion for posi­tion-time relation:

(iii) Equa­tion for posi­tion-veloc­i­ty relation:

Uni­form cir­cu­lar motion: When a body moves in a cir­cu­lar path with uni­form speed, its . motion is called uni­form cir­cu­lar motion.