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Motion in a Straight Line-CBSE CLASS 11

Laws of motion-CBSE CLASS 11


Rest & Motion

Motion is one of the significant term in physics. If an object changes its position with time than it is said to be in motion. Everything in the universe moves. It might be very fast or very very slow.

If position of object don't change with time than it is said to be in rest.

There are two branch of physics that deals the motion of object.
1. Kinematics
2. Dynamics

Kinematics : Kinematics deals the motion of objects without knowing the cause of motion . 

Dynamics: Dynamics relates motion with the force that is the cause of motion.

Reference system 

The motion of a particle is well described with respect to a reference system. It  is made by taking an arbitrary point as origin and imagining a co-ordinate system to be attached  with it. This co-ordinate system taken for a given problem constitutes the reference system for it

Generally reference system is taken that  is attached with earth to solve many problems .It is also known as a frame of reference.

The position of particle is described as by giving : 
  • Its distance from fixed point called origin.
  • Its direction from the origin .

Point object 

If the path length covered by the objects are very large in comparison to the size of the objects, the objects  are considered point objects. 

Earth is considered as a point object for examining the motion of the earth around the sun.

Engine of train is considered as a point object for examining the motion of the train between two railway stations.

Type of motion

Motion of object is described by the position of object in motion. For this we need to position coordinates of object. Based on these motion is classified as 

One dimensional motion : A particle moving along a straight-line or a straight path is said to be  undergo one dimensional motion. For example, motion of a train along a straight line,  freely falling body under gravity etc. 

Two dimensional motion : A particle moving in a plane is said to undergo two dimensional  motion. For example, motion of aa ant on wall, carrom board coins etc.

Three dimensional motion :A particle moving in space is said to undergo three  dimensional motion. For example, motion of a kite in sky, motion of aero plane etc. 

Distance & Displacement 

Distance : If a particle in motion then total length of the actual path traversed between initial and final  positions of the particle is known as the ‘total path length’ or distance covered by it. 

 Displacement : Displacement of a particle in a given time is defined as the change in the position of particle in  a particular direction during that time. It is a vector physical quantity can be  find by drawing a straights line from its initial position to its  final position.
Difference between Displacement from Distance :
  • Displacement has direction. Distance does not have direction. 
  • Distance is always positive. It never decreases with time.  
  • The magnitude of displacement can be both positive, negative and zero.   
  • Distance between two points depends on the path of motion of object where as displacement is independent from the path of motion.
  • Distance is the longest length of the path of motion, where as displacement is the shortest length between initial and final position.

Speed & Velocity 

Speed : 

Rate of change of position or distance of an object is called speed. 

Average Speed : 
Total distance upon total time taken is called average speed.

Instantaneous Speed : 
Instantaneous Speed is the speed of an object at an instant/ time.

Velocity : 

Rate of change of displacement of an object is velocity. 

Average Velocity: 
Total displacement upon total time taken is called average velocity.

Instantaneous velocity: 
Instantaneous velocity is the velocity of an object at an instant/ time.


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