Calculating Photon Momentum & Force

Click For Summary

Homework Help Overview

The discussion revolves around calculating the momentum of a photon and determining the number of photons required to support a given mass using light. The subject area includes concepts from optics and mechanics, particularly focusing on photon momentum and force calculations.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • The original poster attempts to calculate the momentum of a photon and the force needed to support a mass using light. Some participants suggest applying Newton's second law in its original form and discuss the relationship between force and momentum change.

Discussion Status

Participants are exploring different approaches to relate the momentum of photons to the force required to support the mass. Some guidance has been offered regarding the application of the formula for force as the rate of change of momentum, but there is no explicit consensus on the next steps.

Contextual Notes

The original poster expresses confusion about the relationship between momentum and force in this context, indicating a potential gap in understanding how to connect the calculations for photons to the required force.

ride4life
Messages
33
Reaction score
0

Homework Statement


a) Determine the momentum of a photon of light of wavelength 400nm.
b) A mass of 1mg is being held up by a beam of 400nm light. How many photons per second would have to hit the mass (from below) to do this, assuming the photons bounce back with the same momentum as they hit.


Homework Equations


a) p=h/(wavelength)
b) F=mg

The Attempt at a Solution


a) p=6.64x10^-34/400x10^-9=1.66x10^27 kgm/s
b) F=10^-3x9.81=9.81x10^-3N
I can only figure out how much force is needed to keep the mass still, I don't see how p and F are related :(
 
Physics news on Phys.org
Try using Newtons second law in its "original" form.
 
yes i think it will solve the problem!

Its F = dP/dt
 
Don't be confused, i am sure you know the formula cupid.callin mentioned, F = dP/dt,
the problem asks you to find photons per second so you found one of the photons momentum, you try to reach dP/dt so you have to product p with dn/dt what is asked, then reach dP/dt which is force
consider P is the total momentum and p is the momentum of one photon
 

Similar threads

Compton Scattering - Find x component of electron momentum
  • · Replies 3 ·
Replies
3
Views
2K
Finding the momentum of a photon
  • · Replies 11 ·
Replies
11
Views
3K
Compare the wavelengths of a photon and an electron
  • · Replies 2 ·
Replies
2
Views
4K
Finding momentum of a photon/ Finding the mass of a object
  • · Replies 8 ·
Replies
8
Views
3K
Momentum in a photon -> cathode collision
  • · Replies 5 ·
Replies
5
Views
1K
Compare and contrast a 2.2 eV photon with a 2.2 eV electron
  • · Replies 2 ·
Replies
2
Views
4K
Electron vs Photon question? Significance of momentum?
  • · Replies 31 ·
2
Replies
31
Views
6K
What is the final velocity of a sail after being hit by photons from a star?
  • · Replies 9 ·
Replies
9
Views
2K
What are the Units for the momentum of a Photon?
  • · Replies 6 ·
Replies
6
Views
8K
Why is the thrust equation same under gravitational force?
  • · Replies 10 ·
Replies
10
Views
2K