Class 9 Science Note


Force is an external agent that changes or tends to change the state of the body.


Amount of matter contained in the body is called mass.

Inertia: It is the tendency of a body to maintain its state of rest or a uniform motion unless it is acted upon by some external force. It only depends on mass.

i.e.     Larger the mass large will be the inertia.



The distance travelled by the body in a unit time is called speed. Speed is a scalar quantity. It has only magnitude but no direction. . Its unit is m/s


Speed        =   ${\rm{\: \: \: }}\frac{{{\rm{distance\: travelled\: }}}}{{{\rm{time\: taken\: to\: travel\: that\: distance\: }}}}$




The total displacement covered by the body in unit time is called velocity. Velocity is a vector quantity. It has both magnitude and direction. Its unit is m/s 

Velocity =      $\frac{{{\rm{\: total\: displacement\: }}}}{{{\rm{time\: taken\: to\: travel\: that\: distance\: }}}}$




The rate of change of velocity with time is called acceleration. Its unit is m/s2. It is vector quantity. It may be positive or negative .Negative acceleration is called retardation.



Equations of motion

The equations of motion describe the motion of the body at a particular time.

v = u + at              

s = $\frac{{{\rm{u\: }} + {\rm{v\: }}}}{2}$t

  S = u t + $\frac{1}{2}$at2

v2 = u2 + 2aS

  Where, v=final velocity

u =initial velocity

a = acceleration of the body

t= time taken by the body 

    S = distanced travelled


Newton‘s   laws of motion


Newton‘s   First law of motionstates that “Everybody continues in its state of rest or uniform motion in a straight line unless it is acted upon by the external force”.  Newton’s first law is also called law of inertia.


Newton‘s second law of motionstates that “Acceleration produced on a body is directly proportional to applied force and inversely proportional to the mass”.

acceleration (a) $ \propto $ force (F)…………….1

acceleration (a) $ \propto $  mass (m)………………2

From 1 and 2

F$ \propto $ ma

 F = kma

 (Where k is the constant of proportionality and k = 1) 
F = ma


So, the second law of the motion gives the measure of the force.

Newton’s third law of motion states that “to the every action there is an equal and opposite reaction”.

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