# What is Power Class 9

Last updated on May 6th, 2023 at 06:18 am

In this post, we will define Power (in physics), and list down different formulas of power for class 9. We will also solve a sample numerical using the power formula and concept.

## What is Power Class 9 – Physics

**Power is the rate at which work is done or the rate at which energy is supplied or used. Like work and energy, power is also a scalar quantity.**

The** SI unit** for power is Joules per second (**Js**^{-1}), which is defined to be a Watt (W).

So 1 W = 1 J/s. There are other popular units like Kilowatt and horsepower. 1 kilowatt = 1000 Watt and 1 Horsepower = 746 Watt.

### Power formula class 9

When we define power as the rate of work done or rate of energy supplied (or used) we get the following formulas:

**Average Power formula**

Average Power = Work done during some time interval / Duration of time interval ……………… (1)

Average Power = Total Energy supplied or used during some time interval / Duration of time interval ………… (2)

**Instantaneous Power formula**

The rate at which work is done might not be constant. We define instantaneous power P as the quotient in Eq. (1) as Δt approaches zero:

### Numerical problem solved using the power formula

A box with a mass of 30 kg can be lifted by a light rope threaded through a smooth pulley.

(a) If the box is lifted at a constant speed from the ground to a height of 2.0 m in 4.0 s, what is the power required?

(b) If the box is lifted with a constant acceleration of 1.5 m/s2 from rest to a height of 3.0 m above the floor, what is the power required? Take g, the gravitational field strength as 10 N/kg.

Solution: