# Find force from flux

## Homework Statement

What is the flux on a solar sail per square meter, if it orbits the Sun at 3 AU. Flux at 1 AU is 1.36 KW/square meter

## Homework Equations

Flux goes as 1/r^2

## The Attempt at a Solution

Flux at 3 AU will be 1.36/3^2 = 1.36/9 = 0.15 KW/square meter

I can't figure out how to relate this to the force though?

Any help would be appreciated.

*This is practise for a test not an assignment queston

gneill
Mentor

## Homework Statement

What is the flux on a solar sail per square meter, if it orbits the Sun at 3 AU. Flux at 1 AU is 1.36 KW/square meter

## Homework Equations

Flux goes as 1/r^2

## The Attempt at a Solution

Flux at 3 AU will be 1.36/3^2 = 1.36/9 = 0.15 KW/square meter

I can't figure out how to relate this to the force though?

Any help would be appreciated.

*This is practise for a test not an assignment queston

The force will depend upon the angle of incidence of the light (assuming we are talking photon flux here!) and how much of the light is absorbed versus reflected. For perfect reflection and an angle of incidence of zero degrees from the normal the pressure exerted will be 2Ef/c. If the angle is not zero degrees then there's a cos2 term. Look up "radiation pressure".

The information I gave in the question is all I have.

Oops just realized I put the wrong thing in the first post, it should say "What is the force..."

I think I got it. Force per square meter = Pressure and (Radiative) Pressure = Flux/c