MOTION

REST  When a body does not change its position with respect to time and its surroundings, the body is said to be at rest.

MOTION  When the position of a body continuously changes with respect to time and its surroundings, the body is said to be in motion.

RELATIVE MOTION  Motion is relative and not absolute: When you sit in a train, your distances from the wall, ceiling and the floor of the compartment remains constant. Your position is not changing as measured from the compartment but your distance from the platform is changing as time passes. So you are moving with respect to the platform but you are at rest with respect to the compartment.  So, motion is not absolute. It depends on the body with respect to which all distances are measured.

 THE EARTH IS MOVING

CHARACTERISTICS OF A MOVING BODY 1) There must be a reference point (stationary object) to describe the position of the given body. 2) The position of the given body must continuously change with time and with respect to the reference point.

DISTANCE  Distance is the actual length of the path covered by a moving body irrespective of the direction in which the body moves.  Distance is a scalar quantity. It is expressed by the magnitude only. It has no direction.  In SI system, the unit of distance is metre. However we may use a bigger unit called kilometre.

DISPLACEMENT  Displacement is the shortest distance of a moving body from the point of reference ( initial position of body), in a specified direction.  It is a vector quantity. It is expressed by magnitude and direction.  In SI system, the unit of displacement is metre and the bigger unit is kilometre.

A B Displacement E G F i) A-E-B = 20 kms ii) A-F-B = 10 kms iii) A-G-B = 25 kms Out of 3 cases which one is the displacement of the object from A to B ?

Here the distance covered is 29 kms and the displacement is 0

DIFFERENCES BETWEEN DISTANCE AND DISPLACEMENT DISTANCEDISPLACEMENT 1) It is the actual length of the path travelled by a moving object. 2) The direction need not be specified. 1) It is the shortest distance between the initial position ( point of origin) and the final position of the object 2) The direction has to be specified with respect to the reference line.

3) The distance covered in different directions can be added by simple arithmetic. 4) The distance covered by a moving object cannot be zero. 5) The distance is a scalar quantity. 3) The displacements in different directions cannot be added by simple arithmetic. 4) The displacements of a moving object can be zero. 5) The displacement is a vector quantity.

UNIFORM MOTION  When a body covers equal distances in equal intervals of time, however small may be time intervals, the body is said to describe a uniform motion. Examples –  An aeroplane flying at a speed of 800km/h  Light energy traveling at a speed of 3 x 10 8 m/s 3 x 10 8 m/s

NON- UNIFORM MOTION  When a body covers unequal distances in equal intervals of time, it is said to be moving with a non-uniform motion. Examples –  An aeroplane running on a runway before taking off.  A freely falling stone under the action of gravity.

SPEED  The rate of change of motion is called the speed or or  Speed of a body is the distance travelled by it in a unit time  The unit of speed in SI system is metre per second (m/s)  For measuring larger speeds, the unit is km/h.

QUESTIONS 1) What is rest ? 2) What is motion ? 3) What is difference between distance and displacement ? 4) What is speed ? 5) What is uniform motion ? Give an example. 6) What is non-uniform motion ? Give example.

7) With the help of an example clearly explain distance and displacement. 8) Supply a situation where displacement is zero but distance is not zero and both distance and displacement is same. 9) Why is motion not absolute ? Explain. 10) Write the characteristics of a moving body ? 11) The unit of speed is ) Displacement is a quantity. 13) The unit of distance used to measure the thickness of a book is

14) The motion of the earth around the sun is ( uniform, non-uniform) 15) When a stone falls from a height its motion is

AVERAGE SPEED  The average distance covered by a moving body per unit time, Or  It is the quotient of total distance travelled by a body divided by the total time taken to cover the distance. Total distance traveled Total distance traveled  Average speed = Total time taken Total time taken v =(s 1 +s 2 +s 3 ….)/ (t 1 + t 2 + t 3 ….) v =(s 1 +s 2 +s 3 ….)/ (t 1 + t 2 + t 3 ….)

Problems Problems  1) A train starts from rest and covers a distance of 450 m in 2 min. Calculate the speed of the train in i) m/s ii) km/h  2) A train travels a distance of 2o km with a uniform speed of 60 km/h. It travels another distance of 40 km with a uniform speed of 80 km/h. Calculate the average speed of the train.  3) A boy runs first 100m with a uniform speed of 8 m/s and the next 200m with a uniform speed of 5 m/s. Calculate the average speed of the boy.

VELOCITY  The distance covered by a body per unit time in a specified direction is called the velocity. Or  The speed of a body in a specified direction is called the velocity. Or  The rate of change of displacement is called the velocity.  The SI unit of velocity is m/s and the larger unit is km/h

UNIFORM VELOCITY  When a body covers equal distances in equal intervals of time ( however small the time intervals may be ) in a specified direction, the body is said to be moving with a uniform velocity.  Examples –  A train running towards south with a speed of 50 km/h  An aeroplane flying north with a speed of 300 km/h

NON- UNIFORM VELOCITY  When a body covers unequal distances in equal intervals of time in a specified direction or when a body covers equal distances in equal intervals of time, but its direction changes, then the body is said to be moving with variable velocity.

CONDITIONS FOR VARIABLE VELOCITY  Either of the following conditions should be satisfied by a moving body, with a variable velocity.  i) It should cover unequal distances in equal intervals of time in a specified direction.  ii) It should cover equal distances in equal intervals of time, but its direction must change.

EXAMPLE OF VARIABLE VELOCITY  A car running towards north on a busy road has a variable velocity, as distances covered by it per unit time changes with change in the road conditions.(change in distance)  The blades of a rotating ceiling fan, a person running around a circular track with constant speed, etc., are the examples of variable velocity, as the direction of the moving body changes in each case.( change in direction)

AVERAGE VELOCITY  Average velocity is the arithmetic mean of initial velocity ( starting velocity) and the final velocity for a given period of time.  If u is the initial velocity and v is the final velocity after time interval t, the average velocity  v av = (u+v)/2  Average velocity = (Initial velocity + final velocity)/2

DIFFERENCES BETWEEN SPEED AND VELOCITY SPEEDVELOCITY 1. The distance covered per unit time is called speed. 2. It is a scalar quantity. 1. The distance covered per unit time in a specified direction is called velocity. 2. It is a vector quantity.

ACCELERATION  The rate of change of velocity of a moving body is called acceleration. Or  The change of velocity of a moving body with respect to time is called acceleration.  The rate of change of velocity, when the velocity increases is called positive acceleration.  The rate of change of velocity, when the velocity decreases is called negative acceleration or decceleration.

change in velocity change in velocity  Acceleration = Time Time  The unit of acceleration = m/s 2 or ms -2  Acceleration is a vector quantity, as it is expressed in magnitude and direction.

UNIFORM ACCELERATION  When a body undergoes equal changes in velocity in equal intervals of time ( however small the time interval may be), the body is said to be moving with a uniform acceleration.  Examples – A freely falling body under the pull of the Earth has a uniform acceleration.  A marble rolling down an inclined plane has a uniform acceleration.

NON-UNIFORM ACCELERATION  When a body describes unequal changes in equal intervals of time, the body is said to be moving with non-uniform acceleration.  For Example – A car moving on a busy road has a non-uniform acceleration.