# Minimun Power needed

arpsachin2

## Homework Statement

A fan accelerates 2 m^3/min of air from rest to a velocity of 15 m/s. If the density of air is 1.25 kg/m^3, determine the minimum power that must be supplied to the fan?

## The Attempt at a Solution

Need some hints or something.

## Answers and Replies

Homework Helper
welcome to pf!

hi arpsachin2! welcome to pf! (try using the X2 icon just above the Reply box )

hint: momentum arpsachin2
Thank you.

Okay a bit more I guess. :(

Homework Helper
Gold Member
You can convert the volume of air to mass using density.

Homework Helper
(just got up :zzz: …)

force = rate of change of momentum

power = force times … ? arpsachin2
Power = force * velocity

Am I heading the right way?

Fan accelerates at 2m^3/60 secs to 15 m/s

And density is 1.25kg/m^3.

So is it Power = 1.25kg/m^3 * Acceleration?

How do I get the acceleration?

Homework Helper
hi arpsachin2! Power = force * velocity

Am I heading the right way?

yes Fan accelerates at 2m^3/60 secs to 15 m/s

And density is 1.25kg/m^3.

So is it Power = 1.25kg/m^3 * Acceleration?

and now you've totally lost me! you don't need the acceleration!

you only need the force, and you can get that from the change in momentum arpsachin2
if possible could you show me an example or something?

Thanks

arpsachin2
hi arpsachin2! yes and now you've totally lost me! you don't need the acceleration!

you only need the force, and you can get that from the change in momentum Hey,

would it be possible to chat on Gmail or yahoo or something?

arpsachin2
hi arpsachin2! yes and now you've totally lost me! you don't need the acceleration!

you only need the force, and you can get that from the change in momentum So change in momentum is

Final momentum - Initial Momentum right

So, would it be 15 m/s - xxxx?

Homework Helper
hi arpsachin2! (just got up :zzz: …)
So change in momentum is

Final momentum - Initial Momentum right

So, would it be 15 m/s - xxxx?

15 m/s is a speed, you need a momentum (= mass times speed), in kg m/s,

and then you need a rate of change of momentum, in kg m/s2