# Physics Formulas for Class 9

The physics formulas for class 9 are based on NCERT science textbook. These formulas are in tune with CBSE syllabus. You can download these formulas from the link given above. You may also like physics formulas for class 11 and 12.

## Formulas on Motion

The speed of an object travelling a distace $d$ in time $t$ is given by \begin{align} v=\frac{d}{t} \end{align}

The average speed of an object is the ratio of total distance travelled to the total time.

The velocity of an object is given by \begin{align} \text{velocity}=\frac{\text{displacement}}{\text{time}}=\frac{s}{t} \end{align}

The acceleration of an object whose velocity changes from $v_1$ to $v_2$ in time $t$ is \begin{align} a=\frac{v_2-v_1}{t}. \end{align}

An object starts its motion with initial velocity $u$ and moves with a constant acceleration $a$. Its velocity $v$ and displacement $s$ at time $t$ are given by \begin{align} v=u+at\\ s=ut+\frac{1}{2}at^2 \\ v^2=u^2+2as \end{align}

## Formulas on force and laws of motion

The linear momentum of an object of mass $m$ moving with a velocity $v$ is given by \begin{align} p=mv \end{align}

The force on an object of mass $m$ accelerating with an acceleration $a$ is given by \begin{align} F=ma \end{align}

The linear momentum of system is conserved if external force on the system is zero. It is very useful in collisions. \begin{align} m_1u_1+m_2u_2=m_1v_1+m_2v_2 \end{align}

## Formulas on Gravitation

The attractive force between two particles of masses $M$ and $m$ separated by a distance $d$ is given by \begin{align} F=\frac{GMm}{d^2}, \end{align} where $G=6.67\times10^{-11}\;\mathrm{N\,m^2\,kg^{-2}}$.

Let $M$ be the mass and $R$ be the radius of the earth. The acceleration due to gravity at a distance $d\,(>R)$ from the centre of the earth is given by \begin{align} g=\frac{GM}{d^2}. \end{align} The acceleration due to gravity on the surface of the earth is \begin{align} g=\frac{GM}{R^2}=9.8\,\mathrm{m/s^2}. \end{align}

The weight $W$ of an object of mass $m$ is equal to gravitaional force on the object \begin{align} W=mg \end{align}

The pressure $p$ when a thrust (force) $F$ acts perpendicular to an area $A$ is given by \begin{align} p=\frac{F}{A} \end{align} The SI unit of pressure is N/m2.

The relative density is of a substance is the ratio of its density to the density of the water \begin{align} \rho_\text{relative}=\frac{\rho_\text{substance}}{\rho_\text{water}} \end{align}

## Formulas on Work and Energy

The work done by a force $F$min displacing and object by displacement $s$ in the direction of force is given by \begin{align} W=Fs \end{align}

The kinetic energy of an object of mass $m$ moving with a velocity $v$ is given by \begin{align} K=\frac{1}{2}mv^2 \end{align}

The gravitational potential energy of an object of mass $m$ placed at a height $h$ (above the earth's surface) is given by \begin{align} U=mgh \end{align}

## Formulas on Sound

The frequency $\nu$ and time period $T$ are related by \begin{align} \nu=\frac{1}{T}. \end{align}

The velocity $v$ of a wave of frequency $\nu$ and wavelength $\lambda$ is given by \begin{align} v=\nu\lambda. \end{align}