## Class 9 Science Note

# Force

**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.

**Speed **

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

**Velocity **

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

**Acceleration **

The rate of change of velocity with time is called acceleration. Its unit is m/s^{2}. 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 + $\frac{1}{2}$at^{2}

v^{2} = u^{2} + 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 motion**states 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 motion**states that “Acceleration produced on a body is directly proportional to applied force and inversely proportional to the mass”.

Mathematically,

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”.