INDIAN SCHOLL AL WADI AL KABIR
DEPARTMENT OF SCIENCE 2016-17
CLASS-9-PHYSICS-MOTION
SHORT NOTES
NAME OF THE STUDENT CLASS / SEC
Velocity:-Velocity is defined as the speed of an object moving in a definite direction. It is also defined as the displacement produced per unit time.
Velocity = displacement
Time
V = S
t
SI unit of velocity is m/s.
It is a vector quantity.
Uniform velocity:-If a body covers equal distances in equal intervals of time, in a given direction, its velocity is said to be uniform velocity.
Velocity of a body can be changed in two ways:-
(i)By changing the speed of the body,
(ii)By keeping the speed constant but by changing the direction,
The magnitude of speed and velocity of a moving body is equal only if it moves in a single straight line.
If a body does not move in a single straight line, then the speed and velocity are not equal.
Average velocity:-
When the velocity of a bodychanges at a non-uniform rate, its average velocity is defined as the total displacement divided by the total time taken.
Average velocity = total displacement
Total time taken
When the velocity of a body changes at a uniform rate, then the average velocity is given by the arithmetic mean of initial velocity and final velocity for a given period of time.
Average velocity = initial velocity + final velocity
2
Vav = u + v
2
Differences between speed and velocity
speed / velocity- It is the distance travelled by a body per unit time in any direction
- It is the distance travelled by a body per unit time in a given direction.
- It is a scalar quantity
- It is a vector quantity.
- It is always positive or zero
- It may be positive or negative or zero.
Acceleration: - It is defined as the rate of change of velocity.
If the velocity of a body changes from u to v in time t, then
Acceleration = change in velocity
Time taken
= Final velocity- initial velocity
Time takenPage 1 of 4
a = v-u
t
SI unit of acceleration is m/s2
It is a vector quantity.
Retardation(deceleration):-When the velocity of a body decreases, the acceleration is negative. Negative acceleration is called deceleration.
Uniform acceleration:- A body has a uniform acceleration if its velocity changes at a uniform rate.
[A body has uniform acceleration if it travels in a straight line and its velocity increases by equal amounts in equal intervals of time.]
The v-t graph of a body having uniformly accelerated motion is a straight line.
Non-uniform acceleration:-A body has a non-uniform acceleration if its velocity increases by unequal amounts in equal intervals of time.
The v-t graph for non-uniformly accelerated motion is a curved line.
GRAPHICAL REPRESENTATION OF MOTION
Distance-time graph:-
It is a graph obtained by plotting distance travelled along Y- axis and time taken along X-axis.
From d-t graph, the position of a body at any instant of time can be determined.
The slope of d-t graph gives the speed/velocity of an object.
Figure shows d-t graph for a body moving with uniform velocity. It covers distances s1 and s2at times t1 and t2.Slope of line AB = BC
AC
= s2-s1 = displacement = Velocity.
t2-t1 time
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VELOCITY –TIME GRAPHS
The slope of v-t graph gives the acceleration of a body.
Figure shows v-t graph for a body in uniform acceleration. Body has velocities u and v at times t1and t2 respectively.
Slope of line AE = ED
AD
= v-u = change in velocity = Acceleration of the body.
t2-t1 time taken
Area under v-t graph(uniform velocity) gives distance covered by the object.
EQUATIONS OF MOTION BY GRAPHICAL METHOD
- Equation for velocity –time relation.
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- Equation for position –time relation
- Equation for position –velocity relation
UNIFORM CIRCULAR MOTION
If a body moves with a uniform speed along a circular path, it is said to be in uniform circular motion.
In uniform circular motion, the direction of velocity changes at every point but its magnitude remains constant. Hence it is an example of accelerated motion.
Eg:- Motion of the Moon around the Earth, Motion of a satellite in circular orbit at constant speed.
If a body of mass m rotates in a circle of radius r with velocity v. It completes one revolution in time T. Then
Velocity = Distance = Circumference
Time Time period
V = 2∏r
T
Prepared by Asha JohnPage 4 of 4